| Message ID | 20220628161948.475097-6-kpsingh@kernel.org (mailing list archive) |
|---|---|
| State | Handled Elsewhere |
| Headers | show |
| Series | Add bpf_getxattr | expand |
On Tue, Jun 28, 2022 at 04:19:48PM +0000, KP Singh wrote: > A simple test that adds an xattr on a copied /bin/ls and reads it back > when the copied ls is executed. > > Signed-off-by: KP Singh <kpsingh@kernel.org> > --- > .../testing/selftests/bpf/prog_tests/xattr.c | 54 +++++++++++++++++++ > tools/testing/selftests/bpf/progs/xattr.c | 37 +++++++++++++ > 2 files changed, 91 insertions(+) > create mode 100644 tools/testing/selftests/bpf/prog_tests/xattr.c > create mode 100644 tools/testing/selftests/bpf/progs/xattr.c > > diff --git a/tools/testing/selftests/bpf/prog_tests/xattr.c b/tools/testing/selftests/bpf/prog_tests/xattr.c > new file mode 100644 > index 000000000000..ef07fa8a1763 > --- /dev/null > +++ b/tools/testing/selftests/bpf/prog_tests/xattr.c > @@ -0,0 +1,54 @@ > +// SPDX-License-Identifier: GPL-2.0 > + > +/* > + * Copyright 2022 Google LLC. > + */ > + > +#include <test_progs.h> > +#include <sys/xattr.h> > +#include "xattr.skel.h" > + > +#define XATTR_NAME "security.bpf" > +#define XATTR_VALUE "test_progs" > + > +void test_xattr(void) > +{ > + struct xattr *skel = NULL; > + char tmp_dir_path[] = "/tmp/xattrXXXXXX"; > + char tmp_exec_path[64]; > + char cmd[256]; > + int err; > + > + if (CHECK_FAIL(!mkdtemp(tmp_dir_path))) > + goto close_prog; > + > + snprintf(tmp_exec_path, sizeof(tmp_exec_path), "%s/copy_of_ls", > + tmp_dir_path); > + snprintf(cmd, sizeof(cmd), "cp /bin/ls %s", tmp_exec_path); > + if (CHECK_FAIL(system(cmd))) > + goto close_prog_rmdir; > + > + if (CHECK_FAIL(setxattr(tmp_exec_path, XATTR_NAME, XATTR_VALUE, > + sizeof(XATTR_VALUE), 0))) > + goto close_prog_rmdir; > + > + skel = xattr__open_and_load(); > + if (!ASSERT_OK_PTR(skel, "skel_load")) > + goto close_prog_rmdir; > + > + err = xattr__attach(skel); > + if (!ASSERT_OK(err, "xattr__attach failed")) > + goto close_prog_rmdir; > + > + snprintf(cmd, sizeof(cmd), "%s -l", tmp_exec_path); > + if (CHECK_FAIL(system(cmd))) > + goto close_prog_rmdir; > + > + ASSERT_EQ(skel->bss->result, 1, "xattr result"); > + > +close_prog_rmdir: > + snprintf(cmd, sizeof(cmd), "rm -rf %s", tmp_dir_path); > + system(cmd); > +close_prog: > + xattr__destroy(skel); > +} > diff --git a/tools/testing/selftests/bpf/progs/xattr.c b/tools/testing/selftests/bpf/progs/xattr.c > new file mode 100644 > index 000000000000..ccc078fb8ebd > --- /dev/null > +++ b/tools/testing/selftests/bpf/progs/xattr.c > @@ -0,0 +1,37 @@ > +// SPDX-License-Identifier: GPL-2.0 > + > +/* > + * Copyright 2022 Google LLC. > + */ > + > +#include "vmlinux.h" > +#include <bpf/bpf_helpers.h> > +#include <bpf/bpf_tracing.h> > + > +char _license[] SEC("license") = "GPL"; > + > +#define XATTR_NAME "security.bpf" > +#define XATTR_VALUE "test_progs" > + > +__u64 result = 0; > + > +extern ssize_t bpf_getxattr(struct dentry *dentry, struct inode *inode, > + const char *name, void *value, int size) __ksym; > + > +SEC("lsm.s/bprm_committed_creds") > +void BPF_PROG(bprm_cc, struct linux_binprm *bprm) > +{ > + struct task_struct *current = bpf_get_current_task_btf(); > + char dir_xattr_value[64] = {0}; > + int xattr_sz = 0; > + > + xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, > + bprm->file->f_path.dentry->d_inode, XATTR_NAME, > + dir_xattr_value, 64); Yeah, this isn't right. You're not accounting for the caller's userns nor for the idmapped mount. If this is supposed to work you will need a variant of vfs_getxattr() that takes the mount's idmapping into account afaict. See what needs to happen after do_getxattr().
On Tue, Jun 28, 2022 at 7:33 PM Christian Brauner <brauner@kernel.org> wrote: > > On Tue, Jun 28, 2022 at 04:19:48PM +0000, KP Singh wrote: > > A simple test that adds an xattr on a copied /bin/ls and reads it back > > when the copied ls is executed. > > > > Signed-off-by: KP Singh <kpsingh@kernel.org> > > --- > > .../testing/selftests/bpf/prog_tests/xattr.c | 54 +++++++++++++++++++ [...] > > +SEC("lsm.s/bprm_committed_creds") > > +void BPF_PROG(bprm_cc, struct linux_binprm *bprm) > > +{ > > + struct task_struct *current = bpf_get_current_task_btf(); > > + char dir_xattr_value[64] = {0}; > > + int xattr_sz = 0; > > + > > + xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, > > + bprm->file->f_path.dentry->d_inode, XATTR_NAME, > > + dir_xattr_value, 64); > > Yeah, this isn't right. You're not accounting for the caller's userns > nor for the idmapped mount. If this is supposed to work you will need a > variant of vfs_getxattr() that takes the mount's idmapping into account > afaict. See what needs to happen after do_getxattr(). Thanks for taking a look. So, If I understand correctly, we don't need xattr_permission (and other checks in vfs_getxattr) here as the BPF programs run as CAP_SYS_ADMIN. but... So, Is this bit what's missing then? error = vfs_getxattr(mnt_userns, d, kname, ctx->kvalue, ctx->size); if (error > 0) { if ((strcmp(kname, XATTR_NAME_POSIX_ACL_ACCESS) == 0) || (strcmp(kname, XATTR_NAME_POSIX_ACL_DEFAULT) == 0)) posix_acl_fix_xattr_to_user(mnt_userns, d_inode(d), ctx->kvalue, error); if (ctx->size && copy_to_user(ctx->value, ctx->kvalue, error)) error = -EFAULT; } else if (error == -ERANGE && ctx->size >= XATTR_SIZE_MAX) { /* The file system tried to returned a value bigger than XATTR_SIZE_MAX bytes. Not possible. */ error = -E2BIG; }
On Tue, Jun 28, 2022 at 10:52 AM KP Singh <kpsingh@kernel.org> wrote: > > On Tue, Jun 28, 2022 at 7:33 PM Christian Brauner <brauner@kernel.org> wrote: > > > > On Tue, Jun 28, 2022 at 04:19:48PM +0000, KP Singh wrote: > > > A simple test that adds an xattr on a copied /bin/ls and reads it back > > > when the copied ls is executed. > > > > > > Signed-off-by: KP Singh <kpsingh@kernel.org> > > > --- > > > .../testing/selftests/bpf/prog_tests/xattr.c | 54 +++++++++++++++++++ > > [...] > > > > +SEC("lsm.s/bprm_committed_creds") > > > +void BPF_PROG(bprm_cc, struct linux_binprm *bprm) > > > +{ > > > + struct task_struct *current = bpf_get_current_task_btf(); > > > + char dir_xattr_value[64] = {0}; > > > + int xattr_sz = 0; > > > + > > > + xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, > > > + bprm->file->f_path.dentry->d_inode, XATTR_NAME, > > > + dir_xattr_value, 64); > > > > Yeah, this isn't right. You're not accounting for the caller's userns > > nor for the idmapped mount. If this is supposed to work you will need a > > variant of vfs_getxattr() that takes the mount's idmapping into account > > afaict. See what needs to happen after do_getxattr(). > > Thanks for taking a look. > > So, If I understand correctly, we don't need xattr_permission (and > other checks in > vfs_getxattr) here as the BPF programs run as CAP_SYS_ADMIN. > > but... > > So, Is this bit what's missing then? > > error = vfs_getxattr(mnt_userns, d, kname, ctx->kvalue, ctx->size); > if (error > 0) { > if ((strcmp(kname, XATTR_NAME_POSIX_ACL_ACCESS) == 0) || > (strcmp(kname, XATTR_NAME_POSIX_ACL_DEFAULT) == 0)) > posix_acl_fix_xattr_to_user(mnt_userns, d_inode(d), > ctx->kvalue, error); That will not be correct. posix_acl_fix_xattr_to_user checking current_user_ns() is checking random tasks that happen to be running when lsm hook got invoked. KP, we probably have to document clearly that neither 'current*' should not be used here. xattr_permission also makes little sense in this context. If anything it can be a different kfunc if there is a use case, but I don't see it yet. bpf-lsm prog calling __vfs_getxattr is just like other lsm-s that call it directly. It's the kernel that is doing its security thing.
On Tue, Jun 28, 2022 at 03:28:42PM -0700, Alexei Starovoitov wrote: > On Tue, Jun 28, 2022 at 10:52 AM KP Singh <kpsingh@kernel.org> wrote: > > > > On Tue, Jun 28, 2022 at 7:33 PM Christian Brauner <brauner@kernel.org> wrote: > > > > > > On Tue, Jun 28, 2022 at 04:19:48PM +0000, KP Singh wrote: > > > > A simple test that adds an xattr on a copied /bin/ls and reads it back > > > > when the copied ls is executed. > > > > > > > > Signed-off-by: KP Singh <kpsingh@kernel.org> > > > > --- > > > > .../testing/selftests/bpf/prog_tests/xattr.c | 54 +++++++++++++++++++ > > > > [...] > > > > > > +SEC("lsm.s/bprm_committed_creds") > > > > +void BPF_PROG(bprm_cc, struct linux_binprm *bprm) > > > > +{ > > > > + struct task_struct *current = bpf_get_current_task_btf(); > > > > + char dir_xattr_value[64] = {0}; > > > > + int xattr_sz = 0; > > > > + > > > > + xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, > > > > + bprm->file->f_path.dentry->d_inode, XATTR_NAME, > > > > + dir_xattr_value, 64); > > > > > > Yeah, this isn't right. You're not accounting for the caller's userns > > > nor for the idmapped mount. If this is supposed to work you will need a > > > variant of vfs_getxattr() that takes the mount's idmapping into account > > > afaict. See what needs to happen after do_getxattr(). > > > > Thanks for taking a look. > > > > So, If I understand correctly, we don't need xattr_permission (and > > other checks in > > vfs_getxattr) here as the BPF programs run as CAP_SYS_ADMIN. > > > > but... > > > > So, Is this bit what's missing then? > > > > error = vfs_getxattr(mnt_userns, d, kname, ctx->kvalue, ctx->size); > > if (error > 0) { > > if ((strcmp(kname, XATTR_NAME_POSIX_ACL_ACCESS) == 0) || > > (strcmp(kname, XATTR_NAME_POSIX_ACL_DEFAULT) == 0)) > > posix_acl_fix_xattr_to_user(mnt_userns, d_inode(d), > > ctx->kvalue, error); > > That will not be correct. > posix_acl_fix_xattr_to_user checking current_user_ns() > is checking random tasks that happen to be running > when lsm hook got invoked. > > KP, > we probably have to document clearly that neither 'current*' > should not be used here. > xattr_permission also makes little sense in this context. > If anything it can be a different kfunc if there is a use case, > but I don't see it yet. > bpf-lsm prog calling __vfs_getxattr is just like other lsm-s that > call it directly. It's the kernel that is doing its security thing. Right, but LSMs usually only retrieve their own xattr namespace (ima, selinux, smack) or they calculate hashes for xattrs based on the raw filesystem xattr values (evm). But this new bpf_getxattr() is different. It allows to retrieve _any_ xattr in any security hook it can be attached to. So someone can write a bpf program that retrieves filesystem capabilites or posix acls. And these are xattrs that require higher-level vfs involvement to be sensible in most contexts. So looking at: SEC("lsm.s/bprm_committed_creds") void BPF_PROG(bprm_cc, struct linux_binprm *bprm) { struct task_struct *current = bpf_get_current_task_btf(); char dir_xattr_value[64] = {0}; int xattr_sz = 0; xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, bprm->file->f_path.dentry->d_inode, XATTR_NAME, dir_xattr_value, 64); if (xattr_sz <= 0) return; if (!bpf_strncmp(dir_xattr_value, sizeof(XATTR_VALUE), XATTR_VALUE)) result = 1; } This hooks a bpf-lsm program to the security_bprm_committed_creds() hook. It then retrieves the extended attributes of the file to be executed. The hook currently always retrieves the raw filesystem values. But for example any XATTR_NAME_CAPS filesystem capabilities that might've been stored will be taken into account during exec. And both the idmapping of the mount and the caller matter when determing whether they are used or not. But the current implementation of bpf_getxattr() just ignores both. It will always retrieve the raw filesystem values. So if one invokes this hook they're not actually retrieving the values as they are seen by fs/exec.c. And I'm wondering why that is ok? And even if this is ok for some use-cases it might very well become a security issue in others if access decisions are always based on the raw values. I'm not well-versed in this so bear with me, please.
On Wed, Jun 29, 2022 at 10:11:19AM +0200, Christian Brauner wrote: > On Tue, Jun 28, 2022 at 03:28:42PM -0700, Alexei Starovoitov wrote: > > On Tue, Jun 28, 2022 at 10:52 AM KP Singh <kpsingh@kernel.org> wrote: > > > > > > On Tue, Jun 28, 2022 at 7:33 PM Christian Brauner <brauner@kernel.org> wrote: > > > > > > > > On Tue, Jun 28, 2022 at 04:19:48PM +0000, KP Singh wrote: > > > > > A simple test that adds an xattr on a copied /bin/ls and reads it back > > > > > when the copied ls is executed. > > > > > > > > > > Signed-off-by: KP Singh <kpsingh@kernel.org> > > > > > --- > > > > > .../testing/selftests/bpf/prog_tests/xattr.c | 54 +++++++++++++++++++ > > > > > > [...] > > > > > > > > +SEC("lsm.s/bprm_committed_creds") > > > > > +void BPF_PROG(bprm_cc, struct linux_binprm *bprm) > > > > > +{ > > > > > + struct task_struct *current = bpf_get_current_task_btf(); > > > > > + char dir_xattr_value[64] = {0}; > > > > > + int xattr_sz = 0; > > > > > + > > > > > + xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, > > > > > + bprm->file->f_path.dentry->d_inode, XATTR_NAME, > > > > > + dir_xattr_value, 64); > > > > > > > > Yeah, this isn't right. You're not accounting for the caller's userns > > > > nor for the idmapped mount. If this is supposed to work you will need a > > > > variant of vfs_getxattr() that takes the mount's idmapping into account > > > > afaict. See what needs to happen after do_getxattr(). > > > > > > Thanks for taking a look. > > > > > > So, If I understand correctly, we don't need xattr_permission (and > > > other checks in > > > vfs_getxattr) here as the BPF programs run as CAP_SYS_ADMIN. > > > > > > but... > > > > > > So, Is this bit what's missing then? > > > > > > error = vfs_getxattr(mnt_userns, d, kname, ctx->kvalue, ctx->size); > > > if (error > 0) { > > > if ((strcmp(kname, XATTR_NAME_POSIX_ACL_ACCESS) == 0) || > > > (strcmp(kname, XATTR_NAME_POSIX_ACL_DEFAULT) == 0)) > > > posix_acl_fix_xattr_to_user(mnt_userns, d_inode(d), > > > ctx->kvalue, error); > > > > That will not be correct. > > posix_acl_fix_xattr_to_user checking current_user_ns() > > is checking random tasks that happen to be running > > when lsm hook got invoked. > > > > KP, > > we probably have to document clearly that neither 'current*' > > should not be used here. > > xattr_permission also makes little sense in this context. > > If anything it can be a different kfunc if there is a use case, > > but I don't see it yet. > > bpf-lsm prog calling __vfs_getxattr is just like other lsm-s that > > call it directly. It's the kernel that is doing its security thing. > > Right, but LSMs usually only retrieve their own xattr namespace (ima, > selinux, smack) or they calculate hashes for xattrs based on the raw > filesystem xattr values (evm). > > But this new bpf_getxattr() is different. It allows to retrieve _any_ > xattr in any security hook it can be attached to. So someone can write a > bpf program that retrieves filesystem capabilites or posix acls. And > these are xattrs that require higher-level vfs involvement to be > sensible in most contexts. > > So looking at: > > SEC("lsm.s/bprm_committed_creds") > void BPF_PROG(bprm_cc, struct linux_binprm *bprm) > { > struct task_struct *current = bpf_get_current_task_btf(); > char dir_xattr_value[64] = {0}; > int xattr_sz = 0; > > xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, > bprm->file->f_path.dentry->d_inode, XATTR_NAME, > dir_xattr_value, 64); > > if (xattr_sz <= 0) > return; > > if (!bpf_strncmp(dir_xattr_value, sizeof(XATTR_VALUE), XATTR_VALUE)) > result = 1; > } > > This hooks a bpf-lsm program to the security_bprm_committed_creds() > hook. It then retrieves the extended attributes of the file to be > executed. The hook currently always retrieves the raw filesystem values. > > But for example any XATTR_NAME_CAPS filesystem capabilities that > might've been stored will be taken into account during exec. And both > the idmapping of the mount and the caller matter when determing whether > they are used or not. > > But the current implementation of bpf_getxattr() just ignores both. It > will always retrieve the raw filesystem values. So if one invokes this > hook they're not actually retrieving the values as they are seen by > fs/exec.c. And I'm wondering why that is ok? And even if this is ok for > some use-cases it might very well become a security issue in others if > access decisions are always based on the raw values. > > I'm not well-versed in this so bear with me, please. If this is really just about retrieving the "security.bpf" xattr and no other xattr then the bpf_getxattr() variant should somehow hard-code that to ensure that no other xattrs can be retrieved, imho.
On Wed, Jun 29, 2022 at 2:56 AM Christian Brauner <brauner@kernel.org> wrote: > > On Wed, Jun 29, 2022 at 10:11:19AM +0200, Christian Brauner wrote: > > On Tue, Jun 28, 2022 at 03:28:42PM -0700, Alexei Starovoitov wrote: > > > On Tue, Jun 28, 2022 at 10:52 AM KP Singh <kpsingh@kernel.org> wrote: > > > > > > > > On Tue, Jun 28, 2022 at 7:33 PM Christian Brauner <brauner@kernel.org> wrote: > > > > > > > > > > On Tue, Jun 28, 2022 at 04:19:48PM +0000, KP Singh wrote: > > > > > > A simple test that adds an xattr on a copied /bin/ls and reads it back > > > > > > when the copied ls is executed. > > > > > > > > > > > > Signed-off-by: KP Singh <kpsingh@kernel.org> > > > > > > --- > > > > > > .../testing/selftests/bpf/prog_tests/xattr.c | 54 +++++++++++++++++++ > > > > > > > > [...] > > > > > > > > > > +SEC("lsm.s/bprm_committed_creds") > > > > > > +void BPF_PROG(bprm_cc, struct linux_binprm *bprm) > > > > > > +{ > > > > > > + struct task_struct *current = bpf_get_current_task_btf(); > > > > > > + char dir_xattr_value[64] = {0}; > > > > > > + int xattr_sz = 0; > > > > > > + > > > > > > + xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, > > > > > > + bprm->file->f_path.dentry->d_inode, XATTR_NAME, > > > > > > + dir_xattr_value, 64); > > > > > > > > > > Yeah, this isn't right. You're not accounting for the caller's userns > > > > > nor for the idmapped mount. If this is supposed to work you will need a > > > > > variant of vfs_getxattr() that takes the mount's idmapping into account > > > > > afaict. See what needs to happen after do_getxattr(). > > > > > > > > Thanks for taking a look. > > > > > > > > So, If I understand correctly, we don't need xattr_permission (and > > > > other checks in > > > > vfs_getxattr) here as the BPF programs run as CAP_SYS_ADMIN. > > > > > > > > but... > > > > > > > > So, Is this bit what's missing then? > > > > > > > > error = vfs_getxattr(mnt_userns, d, kname, ctx->kvalue, ctx->size); > > > > if (error > 0) { > > > > if ((strcmp(kname, XATTR_NAME_POSIX_ACL_ACCESS) == 0) || > > > > (strcmp(kname, XATTR_NAME_POSIX_ACL_DEFAULT) == 0)) > > > > posix_acl_fix_xattr_to_user(mnt_userns, d_inode(d), > > > > ctx->kvalue, error); > > > > > > That will not be correct. > > > posix_acl_fix_xattr_to_user checking current_user_ns() > > > is checking random tasks that happen to be running > > > when lsm hook got invoked. > > > > > > KP, > > > we probably have to document clearly that neither 'current*' > > > should not be used here. > > > xattr_permission also makes little sense in this context. > > > If anything it can be a different kfunc if there is a use case, > > > but I don't see it yet. > > > bpf-lsm prog calling __vfs_getxattr is just like other lsm-s that > > > call it directly. It's the kernel that is doing its security thing. > > > > Right, but LSMs usually only retrieve their own xattr namespace (ima, > > selinux, smack) or they calculate hashes for xattrs based on the raw > > filesystem xattr values (evm). > > > > But this new bpf_getxattr() is different. It allows to retrieve _any_ > > xattr in any security hook it can be attached to. So someone can write a > > bpf program that retrieves filesystem capabilites or posix acls. And > > these are xattrs that require higher-level vfs involvement to be > > sensible in most contexts. > > > > So looking at: > > > > SEC("lsm.s/bprm_committed_creds") > > void BPF_PROG(bprm_cc, struct linux_binprm *bprm) > > { > > struct task_struct *current = bpf_get_current_task_btf(); > > char dir_xattr_value[64] = {0}; > > int xattr_sz = 0; > > > > xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, > > bprm->file->f_path.dentry->d_inode, XATTR_NAME, > > dir_xattr_value, 64); > > > > if (xattr_sz <= 0) > > return; > > > > if (!bpf_strncmp(dir_xattr_value, sizeof(XATTR_VALUE), XATTR_VALUE)) > > result = 1; > > } > > > > This hooks a bpf-lsm program to the security_bprm_committed_creds() > > hook. It then retrieves the extended attributes of the file to be > > executed. The hook currently always retrieves the raw filesystem values. > > > > But for example any XATTR_NAME_CAPS filesystem capabilities that > > might've been stored will be taken into account during exec. And both > > the idmapping of the mount and the caller matter when determing whether > > they are used or not. > > > > But the current implementation of bpf_getxattr() just ignores both. It > > will always retrieve the raw filesystem values. So if one invokes this > > hook they're not actually retrieving the values as they are seen by > > fs/exec.c. And I'm wondering why that is ok? And even if this is ok for > > some use-cases it might very well become a security issue in others if > > access decisions are always based on the raw values. > > > > I'm not well-versed in this so bear with me, please. > > If this is really just about retrieving the "security.bpf" xattr and no > other xattr then the bpf_getxattr() variant should somehow hard-code > that to ensure that no other xattrs can be retrieved, imho. All of these restrictions look very artificial to me. Especially the part "might very well become a security issue" just doesn't click. We're talking about bpf-lsm progs here that implement security. Can somebody implement a poor bpf-lsm that doesn't enforce any actual security? Sure. It's a code. No one complains about the usage of EXPORT_SYMBOL(__vfs_getxattr) in the existing LSMs like selinux. No one complains about its usage in out of tree LSMs. Is that a security issue? Of course not. __vfs_getxattr is a kernel mechanism that LSMs use to implement the security features they need. __vfs_getxattr as kfunc here is pretty much the same as EXPORT_SYMBOL with a big difference that it's EXPORT_SYMBOL_GPL. BPF land doesn't have an equivalent of non-gpl export and is not going to get one.
On Wed, Jun 29, 2022 at 08:02:50PM -0700, Alexei Starovoitov wrote: > On Wed, Jun 29, 2022 at 2:56 AM Christian Brauner <brauner@kernel.org> wrote: > > > > On Wed, Jun 29, 2022 at 10:11:19AM +0200, Christian Brauner wrote: > > > On Tue, Jun 28, 2022 at 03:28:42PM -0700, Alexei Starovoitov wrote: > > > > On Tue, Jun 28, 2022 at 10:52 AM KP Singh <kpsingh@kernel.org> wrote: > > > > > > > > > > On Tue, Jun 28, 2022 at 7:33 PM Christian Brauner <brauner@kernel.org> wrote: > > > > > > > > > > > > On Tue, Jun 28, 2022 at 04:19:48PM +0000, KP Singh wrote: > > > > > > > A simple test that adds an xattr on a copied /bin/ls and reads it back > > > > > > > when the copied ls is executed. > > > > > > > > > > > > > > Signed-off-by: KP Singh <kpsingh@kernel.org> > > > > > > > --- > > > > > > > .../testing/selftests/bpf/prog_tests/xattr.c | 54 +++++++++++++++++++ > > > > > > > > > > [...] > > > > > > > > > > > > +SEC("lsm.s/bprm_committed_creds") > > > > > > > +void BPF_PROG(bprm_cc, struct linux_binprm *bprm) > > > > > > > +{ > > > > > > > + struct task_struct *current = bpf_get_current_task_btf(); > > > > > > > + char dir_xattr_value[64] = {0}; > > > > > > > + int xattr_sz = 0; > > > > > > > + > > > > > > > + xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, > > > > > > > + bprm->file->f_path.dentry->d_inode, XATTR_NAME, > > > > > > > + dir_xattr_value, 64); > > > > > > > > > > > > Yeah, this isn't right. You're not accounting for the caller's userns > > > > > > nor for the idmapped mount. If this is supposed to work you will need a > > > > > > variant of vfs_getxattr() that takes the mount's idmapping into account > > > > > > afaict. See what needs to happen after do_getxattr(). > > > > > > > > > > Thanks for taking a look. > > > > > > > > > > So, If I understand correctly, we don't need xattr_permission (and > > > > > other checks in > > > > > vfs_getxattr) here as the BPF programs run as CAP_SYS_ADMIN. > > > > > > > > > > but... > > > > > > > > > > So, Is this bit what's missing then? > > > > > > > > > > error = vfs_getxattr(mnt_userns, d, kname, ctx->kvalue, ctx->size); > > > > > if (error > 0) { > > > > > if ((strcmp(kname, XATTR_NAME_POSIX_ACL_ACCESS) == 0) || > > > > > (strcmp(kname, XATTR_NAME_POSIX_ACL_DEFAULT) == 0)) > > > > > posix_acl_fix_xattr_to_user(mnt_userns, d_inode(d), > > > > > ctx->kvalue, error); > > > > > > > > That will not be correct. > > > > posix_acl_fix_xattr_to_user checking current_user_ns() > > > > is checking random tasks that happen to be running > > > > when lsm hook got invoked. > > > > > > > > KP, > > > > we probably have to document clearly that neither 'current*' > > > > should not be used here. > > > > xattr_permission also makes little sense in this context. > > > > If anything it can be a different kfunc if there is a use case, > > > > but I don't see it yet. > > > > bpf-lsm prog calling __vfs_getxattr is just like other lsm-s that > > > > call it directly. It's the kernel that is doing its security thing. > > > > > > Right, but LSMs usually only retrieve their own xattr namespace (ima, > > > selinux, smack) or they calculate hashes for xattrs based on the raw > > > filesystem xattr values (evm). > > > > > > But this new bpf_getxattr() is different. It allows to retrieve _any_ > > > xattr in any security hook it can be attached to. So someone can write a > > > bpf program that retrieves filesystem capabilites or posix acls. And > > > these are xattrs that require higher-level vfs involvement to be > > > sensible in most contexts. > > > > > > So looking at: > > > > > > SEC("lsm.s/bprm_committed_creds") > > > void BPF_PROG(bprm_cc, struct linux_binprm *bprm) > > > { > > > struct task_struct *current = bpf_get_current_task_btf(); > > > char dir_xattr_value[64] = {0}; > > > int xattr_sz = 0; > > > > > > xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, > > > bprm->file->f_path.dentry->d_inode, XATTR_NAME, > > > dir_xattr_value, 64); > > > > > > if (xattr_sz <= 0) > > > return; > > > > > > if (!bpf_strncmp(dir_xattr_value, sizeof(XATTR_VALUE), XATTR_VALUE)) > > > result = 1; > > > } > > > > > > This hooks a bpf-lsm program to the security_bprm_committed_creds() > > > hook. It then retrieves the extended attributes of the file to be > > > executed. The hook currently always retrieves the raw filesystem values. > > > > > > But for example any XATTR_NAME_CAPS filesystem capabilities that > > > might've been stored will be taken into account during exec. And both > > > the idmapping of the mount and the caller matter when determing whether > > > they are used or not. > > > > > > But the current implementation of bpf_getxattr() just ignores both. It > > > will always retrieve the raw filesystem values. So if one invokes this > > > hook they're not actually retrieving the values as they are seen by > > > fs/exec.c. And I'm wondering why that is ok? And even if this is ok for > > > some use-cases it might very well become a security issue in others if > > > access decisions are always based on the raw values. > > > > > > I'm not well-versed in this so bear with me, please. > > > > If this is really just about retrieving the "security.bpf" xattr and no > > other xattr then the bpf_getxattr() variant should somehow hard-code > > that to ensure that no other xattrs can be retrieved, imho. > > All of these restrictions look very artificial to me. > Especially the part "might very well become a security issue" > just doesn't click. > We're talking about bpf-lsm progs here that implement security. > Can somebody implement a poor bpf-lsm that doesn't enforce > any actual security? Sure. It's a code. The point is that with the current implementation of bpf_getxattr() you are able to retrieve any xattrs and we have way less control over a bpf-lsm program than we do over selinux which a simple git grep __vfs_getxattr() is all we need. The thing is that with bpf_getxattr() as it stands it is currently impossible to retrieve xattr values - specifically filesystem capabilities and posix acls - and see them exactly like the code you're trying to supervise is. And that seems very strange from a security perspective. So if someone were to write SEC("lsm.s/bprm_creds_from_file") void BPF_PROG(bprm_cc, struct linux_binprm *bprm) { struct task_struct *current = bpf_get_current_task_btf(); xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, bprm->file->f_path.dentry->d_inode, XATTR_NAME_POSIX_ACL_ACCESS, ..); // or xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, bprm->file->f_path.dentry->d_inode, XATTR_NAME_CAPS, ..); } they'd get the raw nscaps and the raw xattrs back. But now, as just a tiny example, the nscaps->rootuid and the ->e_id fields in the posix ACLs make zero sense in this context. And what's more there's no way for the bpf-lsm program to turn them into something that makes sense in the context of the hook they are retrieved in. It lacks all the necessary helpers to do so afaict. > No one complains about the usage of EXPORT_SYMBOL(__vfs_getxattr) > in the existing LSMs like selinux. Selinux only cares about its own xattr namespace. It doesn't retrieve fscaps or posix acls and it's not possible to write selinux programs that do so. With the bpf-lsm that's very much possible. And if we'd notice selinux would start retrieving random xattrs we'd ask the same questions we do here. > No one complains about its usage in out of tree LSMs. > Is that a security issue? Of course not. > __vfs_getxattr is a kernel mechanism that LSMs use to implement > the security features they need. > __vfs_getxattr as kfunc here is pretty much the same as EXPORT_SYMBOL > with a big difference that it's EXPORT_SYMBOL_GPL. > BPF land doesn't have an equivalent of non-gpl export and is not going > to get one. This discussion would probably be a lot shorter if this series were sent with a proper explanation of how this supposed to work and what it's used for. A series without a cover letter and no detailed explanation in the commit messages makes it quite hard to understand whether what is asked can be acked or not. I'm just adding Serge and Casey to double-check here as the LSM stuff is more up their alley. I can just look at this from the perspective of a vfs person. If you have your eBPF meeting thing I'm also happy to jump on there next week to get more context.
On Thu, Jun 30, 2022 at 1:45 PM Christian Brauner <brauner@kernel.org> wrote: > > On Wed, Jun 29, 2022 at 08:02:50PM -0700, Alexei Starovoitov wrote: > > On Wed, Jun 29, 2022 at 2:56 AM Christian Brauner <brauner@kernel.org> wrote: [...] > > > > > > > > > > > > > > > > Signed-off-by: KP Singh <kpsingh@kernel.org> > > > > > > > > --- > > > > > > > > .../testing/selftests/bpf/prog_tests/xattr.c | 54 +++++++++++++++++++ > > > > > > > > > > > > [...] > > > > > > > > > > > > > > +SEC("lsm.s/bprm_committed_creds") > > > > > > > > +void BPF_PROG(bprm_cc, struct linux_binprm *bprm) > > > > > > > > +{ > > > > > > > > + struct task_struct *current = bpf_get_current_task_btf(); > > > > > > > > + char dir_xattr_value[64] = {0}; > > > > > > > > + int xattr_sz = 0; > > > > > > > > + > > > > > > > > + xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, > > > > > > > > + bprm->file->f_path.dentry->d_inode, XATTR_NAME, > > > > > > > > + dir_xattr_value, 64); > > > > > > > > > > > > > > Yeah, this isn't right. You're not accounting for the caller's userns > > > > > > > nor for the idmapped mount. If this is supposed to work you will need a > > > > > > > variant of vfs_getxattr() that takes the mount's idmapping into account > > > > > > > afaict. See what needs to happen after do_getxattr(). > > > > > > > > > > > > Thanks for taking a look. > > > > > > [...] > > > > > > > > > > That will not be correct. > > > > > posix_acl_fix_xattr_to_user checking current_user_ns() > > > > > is checking random tasks that happen to be running > > > > > when lsm hook got invoked. > > > > > > > > > > KP, > > > > > we probably have to document clearly that neither 'current*' > > > > > should not be used here. > > > > > xattr_permission also makes little sense in this context. > > > > > If anything it can be a different kfunc if there is a use case, > > > > > but I don't see it yet. > > > > > bpf-lsm prog calling __vfs_getxattr is just like other lsm-s that > > > > > call it directly. It's the kernel that is doing its security thing. > > > > > > > > Right, but LSMs usually only retrieve their own xattr namespace (ima, > > > > selinux, smack) or they calculate hashes for xattrs based on the raw > > > > filesystem xattr values (evm). > > > > > > > > But this new bpf_getxattr() is different. It allows to retrieve _any_ > > > > xattr in any security hook it can be attached to. So someone can write a > > > > bpf program that retrieves filesystem capabilites or posix acls. And > > > > these are xattrs that require higher-level vfs involvement to be > > > > sensible in most contexts. > > > > [...] > > > > > > > > This hooks a bpf-lsm program to the security_bprm_committed_creds() > > > > hook. It then retrieves the extended attributes of the file to be > > > > executed. The hook currently always retrieves the raw filesystem values. > > > > > > > > But for example any XATTR_NAME_CAPS filesystem capabilities that > > > > might've been stored will be taken into account during exec. And both > > > > the idmapping of the mount and the caller matter when determing whether > > > > they are used or not. > > > > > > > > But the current implementation of bpf_getxattr() just ignores both. It > > > > will always retrieve the raw filesystem values. So if one invokes this > > > > hook they're not actually retrieving the values as they are seen by > > > > fs/exec.c. And I'm wondering why that is ok? And even if this is ok for > > > > some use-cases it might very well become a security issue in others if > > > > access decisions are always based on the raw values. > > > > > > > > I'm not well-versed in this so bear with me, please. > > > > > > If this is really just about retrieving the "security.bpf" xattr and no > > > other xattr then the bpf_getxattr() variant should somehow hard-code > > > that to ensure that no other xattrs can be retrieved, imho. > > > > All of these restrictions look very artificial to me. > > Especially the part "might very well become a security issue" > > just doesn't click. > > We're talking about bpf-lsm progs here that implement security. > > Can somebody implement a poor bpf-lsm that doesn't enforce > > any actual security? Sure. It's a code. > > The point is that with the current implementation of bpf_getxattr() you > are able to retrieve any xattrs and we have way less control over a > bpf-lsm program than we do over selinux which a simple git grep > __vfs_getxattr() is all we need. > > The thing is that with bpf_getxattr() as it stands it is currently > impossible to retrieve xattr values - specifically filesystem > capabilities and posix acls - and see them exactly like the code you're > trying to supervise is. And that seems very strange from a security > perspective. So if someone were to write > > SEC("lsm.s/bprm_creds_from_file") > void BPF_PROG(bprm_cc, struct linux_binprm *bprm) > { > struct task_struct *current = bpf_get_current_task_btf(); > > xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, > bprm->file->f_path.dentry->d_inode, > XATTR_NAME_POSIX_ACL_ACCESS, ..); > // or > xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, > bprm->file->f_path.dentry->d_inode, > XATTR_NAME_CAPS, ..); > > } > > they'd get the raw nscaps and the raw xattrs back. But now, as just a > tiny example, the nscaps->rootuid and the ->e_id fields in the posix > ACLs make zero sense in this context. > > And what's more there's no way for the bpf-lsm program to turn them into > something that makes sense in the context of the hook they are retrieved > in. It lacks all the necessary helpers to do so afaict. > > > No one complains about the usage of EXPORT_SYMBOL(__vfs_getxattr) > > in the existing LSMs like selinux. > > Selinux only cares about its own xattr namespace. It doesn't retrieve > fscaps or posix acls and it's not possible to write selinux programs > that do so. With the bpf-lsm that's very much possible. > > And if we'd notice selinux would start retrieving random xattrs we'd ask > the same questions we do here. > > > No one complains about its usage in out of tree LSMs. > > Is that a security issue? Of course not. > > __vfs_getxattr is a kernel mechanism that LSMs use to implement > > the security features they need. > > __vfs_getxattr as kfunc here is pretty much the same as EXPORT_SYMBOL > > with a big difference that it's EXPORT_SYMBOL_GPL. > > BPF land doesn't have an equivalent of non-gpl export and is not going > > to get one. I want to reiterate what Alexei is saying here: *Please* consider this as a simple wrapper around __vfs_getxattr with a limited attach surface and extra verification checks and and nothing else. What you are saying is __vfs_getxattr does not make sense in some contexts. But kernel modules can still use it right? The user is implementing an LSM, if they chose to do things that don't make sense, then they can surely cause a lot more harm: SEC("lsm/bprm_check_security") int BPF_PROG(bprm_check, struct linux_binprm *bprm) { return -EPERM; } > > This discussion would probably be a lot shorter if this series were sent > with a proper explanation of how this supposed to work and what it's > used for. It's currently scoped to BPF LSM (albeit limited to LSM for now) but it won't just be used in LSM programs but some (allow-listed) tracing programs too. We want to leave the flexibility to the implementer of the LSM hooks. If the implementer choses to retrieve posix_acl_* we can also expose posix_acl_fix_xattr_to_user or a different kfunc that adds this logic too but that would be a separate kfunc (and a separate use-case). > > A series without a cover letter and no detailed explanation in the > commit messages makes it quite hard to understand whether what is asked > can be acked or not. As I mentioned in https://lore.kernel.org/bpf/CACYkzJ70uqVJr5EnM0i03Lu+zkuSsXOXcOLQoUS6HZPqH=skpQ@mail.gmail.com/T/#m74f32bae800a97d5c2caf08cee4199d3ba48d76c I will resend with a cover letter that has more details. > > I'm just adding Serge and Casey to double-check here as the LSM stuff is > more up their alley. I can just look at this from the perspective of a > vfs person. > > If you have your eBPF meeting thing I'm also happy to jump on there next Sure, we can discuss this during BPF office hours next week. > week to get more context.
On Thu, Jun 30, 2022 at 2:21 PM KP Singh <kpsingh@kernel.org> wrote: > > On Thu, Jun 30, 2022 at 1:45 PM Christian Brauner <brauner@kernel.org> wrote: > > > > On Wed, Jun 29, 2022 at 08:02:50PM -0700, Alexei Starovoitov wrote: > > > On Wed, Jun 29, 2022 at 2:56 AM Christian Brauner <brauner@kernel.org> wrote: > > [...] > > > > > > > > > > > > > > > > > > > Signed-off-by: KP Singh <kpsingh@kernel.org> > > > > > > > > > --- > > > > > > > > > .../testing/selftests/bpf/prog_tests/xattr.c | 54 +++++++++++++++++++ > > > > > > > > > > > > > > [...] > > > > > > > > > > > > > > > > +SEC("lsm.s/bprm_committed_creds") > > > > > > > > > +void BPF_PROG(bprm_cc, struct linux_binprm *bprm) > > > > > > > > > +{ > > > > > > > > > + struct task_struct *current = bpf_get_current_task_btf(); > > > > > > > > > + char dir_xattr_value[64] = {0}; > > > > > > > > > + int xattr_sz = 0; > > > > > > > > > + > > > > > > > > > + xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, > > > > > > > > > + bprm->file->f_path.dentry->d_inode, XATTR_NAME, > > > > > > > > > + dir_xattr_value, 64); > > > > > > > > > > > > > > > > Yeah, this isn't right. You're not accounting for the caller's userns > > > > > > > > nor for the idmapped mount. If this is supposed to work you will need a > > > > > > > > variant of vfs_getxattr() that takes the mount's idmapping into account > > > > > > > > afaict. See what needs to happen after do_getxattr(). > > > > > > > > > > > > > > Thanks for taking a look. > > > > > > > > > [...] > > > > > > > > > > > > > That will not be correct. > > > > > > posix_acl_fix_xattr_to_user checking current_user_ns() > > > > > > is checking random tasks that happen to be running > > > > > > when lsm hook got invoked. > > > > > > > > > > > > KP, > > > > > > we probably have to document clearly that neither 'current*' > > > > > > should not be used here. > > > > > > xattr_permission also makes little sense in this context. > > > > > > If anything it can be a different kfunc if there is a use case, > > > > > > but I don't see it yet. > > > > > > bpf-lsm prog calling __vfs_getxattr is just like other lsm-s that > > > > > > call it directly. It's the kernel that is doing its security thing. > > > > > > > > > > Right, but LSMs usually only retrieve their own xattr namespace (ima, > > > > > selinux, smack) or they calculate hashes for xattrs based on the raw > > > > > filesystem xattr values (evm). > > > > > > > > > > But this new bpf_getxattr() is different. It allows to retrieve _any_ > > > > > xattr in any security hook it can be attached to. So someone can write a > > > > > bpf program that retrieves filesystem capabilites or posix acls. And > > > > > these are xattrs that require higher-level vfs involvement to be > > > > > sensible in most contexts. > > > > > > > [...] > > > > > > > > > > > This hooks a bpf-lsm program to the security_bprm_committed_creds() > > > > > hook. It then retrieves the extended attributes of the file to be > > > > > executed. The hook currently always retrieves the raw filesystem values. > > > > > > > > > > But for example any XATTR_NAME_CAPS filesystem capabilities that > > > > > might've been stored will be taken into account during exec. And both > > > > > the idmapping of the mount and the caller matter when determing whether > > > > > they are used or not. > > > > > > > > > > But the current implementation of bpf_getxattr() just ignores both. It > > > > > will always retrieve the raw filesystem values. So if one invokes this > > > > > hook they're not actually retrieving the values as they are seen by > > > > > fs/exec.c. And I'm wondering why that is ok? And even if this is ok for > > > > > some use-cases it might very well become a security issue in others if > > > > > access decisions are always based on the raw values. > > > > > > > > > > I'm not well-versed in this so bear with me, please. > > > > > > > > If this is really just about retrieving the "security.bpf" xattr and no > > > > other xattr then the bpf_getxattr() variant should somehow hard-code > > > > that to ensure that no other xattrs can be retrieved, imho. > > > > > > All of these restrictions look very artificial to me. > > > Especially the part "might very well become a security issue" > > > just doesn't click. > > > We're talking about bpf-lsm progs here that implement security. > > > Can somebody implement a poor bpf-lsm that doesn't enforce > > > any actual security? Sure. It's a code. > > > > The point is that with the current implementation of bpf_getxattr() you > > are able to retrieve any xattrs and we have way less control over a > > bpf-lsm program than we do over selinux which a simple git grep > > __vfs_getxattr() is all we need. > > > > The thing is that with bpf_getxattr() as it stands it is currently > > impossible to retrieve xattr values - specifically filesystem > > capabilities and posix acls - and see them exactly like the code you're > > trying to supervise is. And that seems very strange from a security > > perspective. So if someone were to write > > > > SEC("lsm.s/bprm_creds_from_file") > > void BPF_PROG(bprm_cc, struct linux_binprm *bprm) > > { > > struct task_struct *current = bpf_get_current_task_btf(); > > > > xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, > > bprm->file->f_path.dentry->d_inode, > > XATTR_NAME_POSIX_ACL_ACCESS, ..); > > // or > > xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, > > bprm->file->f_path.dentry->d_inode, > > XATTR_NAME_CAPS, ..); > > > > } > > > > they'd get the raw nscaps and the raw xattrs back. But now, as just a > > tiny example, the nscaps->rootuid and the ->e_id fields in the posix > > ACLs make zero sense in this context. > > > > And what's more there's no way for the bpf-lsm program to turn them into > > something that makes sense in the context of the hook they are retrieved > > in. It lacks all the necessary helpers to do so afaict. > > > > > No one complains about the usage of EXPORT_SYMBOL(__vfs_getxattr) > > > in the existing LSMs like selinux. > > > > Selinux only cares about its own xattr namespace. It doesn't retrieve > > fscaps or posix acls and it's not possible to write selinux programs > > that do so. With the bpf-lsm that's very much possible. > > > > And if we'd notice selinux would start retrieving random xattrs we'd ask > > the same questions we do here. > > > > > No one complains about its usage in out of tree LSMs. > > > Is that a security issue? Of course not. > > > __vfs_getxattr is a kernel mechanism that LSMs use to implement > > > the security features they need. > > > __vfs_getxattr as kfunc here is pretty much the same as EXPORT_SYMBOL Alexei, should we consider renaming it to bpf__vfs_getxattr to emphasize the fact that this is just a simple wrapper around __vfs_getxattr? > > > with a big difference that it's EXPORT_SYMBOL_GPL. > > > BPF land doesn't have an equivalent of non-gpl export and is not going > > > to get one. > > I want to reiterate what Alexei is saying here: > > *Please* consider this as a simple wrapper around __vfs_getxattr > with a limited attach surface and extra verification checks and > and nothing else. > > What you are saying is __vfs_getxattr does not make sense in some > contexts. But kernel modules can still use it right? > > The user is implementing an LSM, if they chose to do things that don't make > sense, then they can surely cause a lot more harm: > > SEC("lsm/bprm_check_security") > int BPF_PROG(bprm_check, struct linux_binprm *bprm) > { > return -EPERM; > } > > > > > This discussion would probably be a lot shorter if this series were sent > > with a proper explanation of how this supposed to work and what it's > > used for. > > It's currently scoped to BPF LSM (albeit limited to LSM for now) > but it won't just be used in LSM programs but some (allow-listed) > tracing programs too. > > We want to leave the flexibility to the implementer of the LSM hooks. If the > implementer choses to retrieve posix_acl_* we can also expose > posix_acl_fix_xattr_to_user or a different kfunc that adds this logic too > but that would be a separate kfunc (and a separate use-case). > > > > > A series without a cover letter and no detailed explanation in the > > commit messages makes it quite hard to understand whether what is asked > > can be acked or not. > > As I mentioned in > > https://lore.kernel.org/bpf/CACYkzJ70uqVJr5EnM0i03Lu+zkuSsXOXcOLQoUS6HZPqH=skpQ@mail.gmail.com/T/#m74f32bae800a97d5c2caf08cee4199d3ba48d76c > > I will resend with a cover letter that has more details. > > > > > I'm just adding Serge and Casey to double-check here as the LSM stuff is > > more up their alley. I can just look at this from the perspective of a > > vfs person. > > > > If you have your eBPF meeting thing I'm also happy to jump on there next > > Sure, we can discuss this during BPF office hours next week. > > > > week to get more context.
On Thu, Jun 30, 2022 at 02:21:56PM +0200, KP Singh wrote: > On Thu, Jun 30, 2022 at 1:45 PM Christian Brauner <brauner@kernel.org> wrote: > > > > On Wed, Jun 29, 2022 at 08:02:50PM -0700, Alexei Starovoitov wrote: > > > On Wed, Jun 29, 2022 at 2:56 AM Christian Brauner <brauner@kernel.org> wrote: > > [...] > > > > > > > > > > > > > > > > > > > Signed-off-by: KP Singh <kpsingh@kernel.org> > > > > > > > > > --- > > > > > > > > > .../testing/selftests/bpf/prog_tests/xattr.c | 54 +++++++++++++++++++ > > > > > > > > > > > > > > [...] > > > > > > > > > > > > > > > > +SEC("lsm.s/bprm_committed_creds") > > > > > > > > > +void BPF_PROG(bprm_cc, struct linux_binprm *bprm) > > > > > > > > > +{ > > > > > > > > > + struct task_struct *current = bpf_get_current_task_btf(); > > > > > > > > > + char dir_xattr_value[64] = {0}; > > > > > > > > > + int xattr_sz = 0; > > > > > > > > > + > > > > > > > > > + xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, > > > > > > > > > + bprm->file->f_path.dentry->d_inode, XATTR_NAME, > > > > > > > > > + dir_xattr_value, 64); > > > > > > > > > > > > > > > > Yeah, this isn't right. You're not accounting for the caller's userns > > > > > > > > nor for the idmapped mount. If this is supposed to work you will need a > > > > > > > > variant of vfs_getxattr() that takes the mount's idmapping into account > > > > > > > > afaict. See what needs to happen after do_getxattr(). > > > > > > > > > > > > > > Thanks for taking a look. > > > > > > > > > [...] > > > > > > > > > > > > > That will not be correct. > > > > > > posix_acl_fix_xattr_to_user checking current_user_ns() > > > > > > is checking random tasks that happen to be running > > > > > > when lsm hook got invoked. > > > > > > > > > > > > KP, > > > > > > we probably have to document clearly that neither 'current*' > > > > > > should not be used here. > > > > > > xattr_permission also makes little sense in this context. > > > > > > If anything it can be a different kfunc if there is a use case, > > > > > > but I don't see it yet. > > > > > > bpf-lsm prog calling __vfs_getxattr is just like other lsm-s that > > > > > > call it directly. It's the kernel that is doing its security thing. > > > > > > > > > > Right, but LSMs usually only retrieve their own xattr namespace (ima, > > > > > selinux, smack) or they calculate hashes for xattrs based on the raw > > > > > filesystem xattr values (evm). > > > > > > > > > > But this new bpf_getxattr() is different. It allows to retrieve _any_ > > > > > xattr in any security hook it can be attached to. So someone can write a > > > > > bpf program that retrieves filesystem capabilites or posix acls. And > > > > > these are xattrs that require higher-level vfs involvement to be > > > > > sensible in most contexts. > > > > > > > [...] > > > > > > > > > > > This hooks a bpf-lsm program to the security_bprm_committed_creds() > > > > > hook. It then retrieves the extended attributes of the file to be > > > > > executed. The hook currently always retrieves the raw filesystem values. > > > > > > > > > > But for example any XATTR_NAME_CAPS filesystem capabilities that > > > > > might've been stored will be taken into account during exec. And both > > > > > the idmapping of the mount and the caller matter when determing whether > > > > > they are used or not. > > > > > > > > > > But the current implementation of bpf_getxattr() just ignores both. It > > > > > will always retrieve the raw filesystem values. So if one invokes this > > > > > hook they're not actually retrieving the values as they are seen by > > > > > fs/exec.c. And I'm wondering why that is ok? And even if this is ok for > > > > > some use-cases it might very well become a security issue in others if > > > > > access decisions are always based on the raw values. > > > > > > > > > > I'm not well-versed in this so bear with me, please. > > > > > > > > If this is really just about retrieving the "security.bpf" xattr and no > > > > other xattr then the bpf_getxattr() variant should somehow hard-code > > > > that to ensure that no other xattrs can be retrieved, imho. > > > > > > All of these restrictions look very artificial to me. > > > Especially the part "might very well become a security issue" > > > just doesn't click. > > > We're talking about bpf-lsm progs here that implement security. > > > Can somebody implement a poor bpf-lsm that doesn't enforce > > > any actual security? Sure. It's a code. > > > > The point is that with the current implementation of bpf_getxattr() you > > are able to retrieve any xattrs and we have way less control over a > > bpf-lsm program than we do over selinux which a simple git grep > > __vfs_getxattr() is all we need. > > > > The thing is that with bpf_getxattr() as it stands it is currently > > impossible to retrieve xattr values - specifically filesystem > > capabilities and posix acls - and see them exactly like the code you're > > trying to supervise is. And that seems very strange from a security > > perspective. So if someone were to write > > > > SEC("lsm.s/bprm_creds_from_file") > > void BPF_PROG(bprm_cc, struct linux_binprm *bprm) > > { > > struct task_struct *current = bpf_get_current_task_btf(); > > > > xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, > > bprm->file->f_path.dentry->d_inode, > > XATTR_NAME_POSIX_ACL_ACCESS, ..); > > // or > > xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, > > bprm->file->f_path.dentry->d_inode, > > XATTR_NAME_CAPS, ..); > > > > } > > > > they'd get the raw nscaps and the raw xattrs back. But now, as just a > > tiny example, the nscaps->rootuid and the ->e_id fields in the posix > > ACLs make zero sense in this context. > > > > And what's more there's no way for the bpf-lsm program to turn them into > > something that makes sense in the context of the hook they are retrieved > > in. It lacks all the necessary helpers to do so afaict. > > > > > No one complains about the usage of EXPORT_SYMBOL(__vfs_getxattr) > > > in the existing LSMs like selinux. > > > > Selinux only cares about its own xattr namespace. It doesn't retrieve > > fscaps or posix acls and it's not possible to write selinux programs > > that do so. With the bpf-lsm that's very much possible. > > > > And if we'd notice selinux would start retrieving random xattrs we'd ask > > the same questions we do here. > > > > > No one complains about its usage in out of tree LSMs. > > > Is that a security issue? Of course not. > > > __vfs_getxattr is a kernel mechanism that LSMs use to implement > > > the security features they need. > > > __vfs_getxattr as kfunc here is pretty much the same as EXPORT_SYMBOL > > > with a big difference that it's EXPORT_SYMBOL_GPL. > > > BPF land doesn't have an equivalent of non-gpl export and is not going > > > to get one. > > I want to reiterate what Alexei is saying here: > > *Please* consider this as a simple wrapper around __vfs_getxattr > with a limited attach surface and extra verification checks and > and nothing else. > > What you are saying is __vfs_getxattr does not make sense in some > contexts. But kernel modules can still use it right? > > The user is implementing an LSM, if they chose to do things that don't make > sense, then they can surely cause a lot more harm: > > SEC("lsm/bprm_check_security") > int BPF_PROG(bprm_check, struct linux_binprm *bprm) > { > return -EPERM; > } > > > > > This discussion would probably be a lot shorter if this series were sent > > with a proper explanation of how this supposed to work and what it's > > used for. > > It's currently scoped to BPF LSM (albeit limited to LSM for now) > but it won't just be used in LSM programs but some (allow-listed) > tracing programs too. > > We want to leave the flexibility to the implementer of the LSM hooks. If the > implementer choses to retrieve posix_acl_* we can also expose > posix_acl_fix_xattr_to_user or a different kfunc that adds this logic too > but that would be a separate kfunc (and a separate use-case). No, sorry. That's what I feared and that's why I think this low-level exposure of __vfs_getxattr() is wrong: The posix_acl_fix_xattr_*() helpers, as well as the helpers like get_file_caps() will not be exported. We're not going to export that deeply internal vfs machinery. So I would NACK that. If you want that - and that's what I'm saying here - you need to encapsulate this into your vfs_*xattr() helper that you can call from your kfuncs. > > > > > A series without a cover letter and no detailed explanation in the > > commit messages makes it quite hard to understand whether what is asked > > can be acked or not. > > As I mentioned in > > https://lore.kernel.org/bpf/CACYkzJ70uqVJr5EnM0i03Lu+zkuSsXOXcOLQoUS6HZPqH=skpQ@mail.gmail.com/T/#m74f32bae800a97d5c2caf08cee4199d3ba48d76c > > I will resend with a cover letter that has more details. Thank you! > > > > > I'm just adding Serge and Casey to double-check here as the LSM stuff is > > more up their alley. I can just look at this from the perspective of a > > vfs person. > > > > If you have your eBPF meeting thing I'm also happy to jump on there next > > Sure, we can discuss this during BPF office hours next week. Sounds good.
On Thu, Jun 30, 2022 at 3:26 PM Christian Brauner <brauner@kernel.org> wrote: > > On Thu, Jun 30, 2022 at 02:21:56PM +0200, KP Singh wrote: > > On Thu, Jun 30, 2022 at 1:45 PM Christian Brauner <brauner@kernel.org> wrote: > > > > > > On Wed, Jun 29, 2022 at 08:02:50PM -0700, Alexei Starovoitov wrote: > > > > On Wed, Jun 29, 2022 at 2:56 AM Christian Brauner <brauner@kernel.org> wrote: > > > > [...] > > > > > > > > > > > > > > > > > > > > > > Signed-off-by: KP Singh <kpsingh@kernel.org> > > > > > > > > > > --- > > > > > > > > > > .../testing/selftests/bpf/prog_tests/xattr.c | 54 +++++++++++++++++++ > > > > > > > > > > > > > > > > [...] > > > > > > > > > > > > > > > > > > +SEC("lsm.s/bprm_committed_creds") > > > > > > > > > > +void BPF_PROG(bprm_cc, struct linux_binprm *bprm) > > > > > > > > > > +{ > > > > > > > > > > + struct task_struct *current = bpf_get_current_task_btf(); > > > > > > > > > > + char dir_xattr_value[64] = {0}; > > > > > > > > > > + int xattr_sz = 0; > > > > > > > > > > + > > > > > > > > > > + xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, > > > > > > > > > > + bprm->file->f_path.dentry->d_inode, XATTR_NAME, > > > > > > > > > > + dir_xattr_value, 64); > > > > > > > > > > > > > > > > > > Yeah, this isn't right. You're not accounting for the caller's userns > > > > > > > > > nor for the idmapped mount. If this is supposed to work you will need a > > > > > > > > > variant of vfs_getxattr() that takes the mount's idmapping into account > > > > > > > > > afaict. See what needs to happen after do_getxattr(). > > > > > > > > > > > > > > > > Thanks for taking a look. > > > > > > > > > > > > [...] > > > > > > > > > > > > > > > > That will not be correct. > > > > > > > posix_acl_fix_xattr_to_user checking current_user_ns() > > > > > > > is checking random tasks that happen to be running > > > > > > > when lsm hook got invoked. > > > > > > > > > > > > > > KP, > > > > > > > we probably have to document clearly that neither 'current*' > > > > > > > should not be used here. > > > > > > > xattr_permission also makes little sense in this context. > > > > > > > If anything it can be a different kfunc if there is a use case, > > > > > > > but I don't see it yet. > > > > > > > bpf-lsm prog calling __vfs_getxattr is just like other lsm-s that > > > > > > > call it directly. It's the kernel that is doing its security thing. > > > > > > > > > > > > Right, but LSMs usually only retrieve their own xattr namespace (ima, > > > > > > selinux, smack) or they calculate hashes for xattrs based on the raw > > > > > > filesystem xattr values (evm). > > > > > > > > > > > > But this new bpf_getxattr() is different. It allows to retrieve _any_ > > > > > > xattr in any security hook it can be attached to. So someone can write a > > > > > > bpf program that retrieves filesystem capabilites or posix acls. And > > > > > > these are xattrs that require higher-level vfs involvement to be > > > > > > sensible in most contexts. > > > > > > > > > > [...] > > > > > > > > > > > > > > This hooks a bpf-lsm program to the security_bprm_committed_creds() > > > > > > hook. It then retrieves the extended attributes of the file to be > > > > > > executed. The hook currently always retrieves the raw filesystem values. > > > > > > > > > > > > But for example any XATTR_NAME_CAPS filesystem capabilities that > > > > > > might've been stored will be taken into account during exec. And both > > > > > > the idmapping of the mount and the caller matter when determing whether > > > > > > they are used or not. > > > > > > > > > > > > But the current implementation of bpf_getxattr() just ignores both. It > > > > > > will always retrieve the raw filesystem values. So if one invokes this > > > > > > hook they're not actually retrieving the values as they are seen by > > > > > > fs/exec.c. And I'm wondering why that is ok? And even if this is ok for > > > > > > some use-cases it might very well become a security issue in others if > > > > > > access decisions are always based on the raw values. > > > > > > > > > > > > I'm not well-versed in this so bear with me, please. > > > > > > > > > > If this is really just about retrieving the "security.bpf" xattr and no > > > > > other xattr then the bpf_getxattr() variant should somehow hard-code > > > > > that to ensure that no other xattrs can be retrieved, imho. > > > > > > > > All of these restrictions look very artificial to me. > > > > Especially the part "might very well become a security issue" > > > > just doesn't click. > > > > We're talking about bpf-lsm progs here that implement security. > > > > Can somebody implement a poor bpf-lsm that doesn't enforce > > > > any actual security? Sure. It's a code. > > > > > > The point is that with the current implementation of bpf_getxattr() you > > > are able to retrieve any xattrs and we have way less control over a > > > bpf-lsm program than we do over selinux which a simple git grep > > > __vfs_getxattr() is all we need. > > > > > > The thing is that with bpf_getxattr() as it stands it is currently > > > impossible to retrieve xattr values - specifically filesystem > > > capabilities and posix acls - and see them exactly like the code you're > > > trying to supervise is. And that seems very strange from a security > > > perspective. So if someone were to write > > > > > > SEC("lsm.s/bprm_creds_from_file") > > > void BPF_PROG(bprm_cc, struct linux_binprm *bprm) > > > { > > > struct task_struct *current = bpf_get_current_task_btf(); > > > > > > xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, > > > bprm->file->f_path.dentry->d_inode, > > > XATTR_NAME_POSIX_ACL_ACCESS, ..); > > > // or > > > xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, > > > bprm->file->f_path.dentry->d_inode, > > > XATTR_NAME_CAPS, ..); > > > > > > } > > > > > > they'd get the raw nscaps and the raw xattrs back. But now, as just a > > > tiny example, the nscaps->rootuid and the ->e_id fields in the posix > > > ACLs make zero sense in this context. > > > > > > And what's more there's no way for the bpf-lsm program to turn them into > > > something that makes sense in the context of the hook they are retrieved > > > in. It lacks all the necessary helpers to do so afaict. > > > > > > > No one complains about the usage of EXPORT_SYMBOL(__vfs_getxattr) > > > > in the existing LSMs like selinux. > > > > > > Selinux only cares about its own xattr namespace. It doesn't retrieve > > > fscaps or posix acls and it's not possible to write selinux programs > > > that do so. With the bpf-lsm that's very much possible. > > > > > > And if we'd notice selinux would start retrieving random xattrs we'd ask > > > the same questions we do here. > > > > > > > No one complains about its usage in out of tree LSMs. > > > > Is that a security issue? Of course not. > > > > __vfs_getxattr is a kernel mechanism that LSMs use to implement > > > > the security features they need. > > > > __vfs_getxattr as kfunc here is pretty much the same as EXPORT_SYMBOL > > > > with a big difference that it's EXPORT_SYMBOL_GPL. > > > > BPF land doesn't have an equivalent of non-gpl export and is not going > > > > to get one. > > > > I want to reiterate what Alexei is saying here: > > > > *Please* consider this as a simple wrapper around __vfs_getxattr > > with a limited attach surface and extra verification checks and > > and nothing else. > > > > What you are saying is __vfs_getxattr does not make sense in some > > contexts. But kernel modules can still use it right? > > > > The user is implementing an LSM, if they chose to do things that don't make > > sense, then they can surely cause a lot more harm: > > > > SEC("lsm/bprm_check_security") > > int BPF_PROG(bprm_check, struct linux_binprm *bprm) > > { > > return -EPERM; > > } > > > > > > > > This discussion would probably be a lot shorter if this series were sent > > > with a proper explanation of how this supposed to work and what it's > > > used for. > > > > It's currently scoped to BPF LSM (albeit limited to LSM for now) > > but it won't just be used in LSM programs but some (allow-listed) > > tracing programs too. > > > > We want to leave the flexibility to the implementer of the LSM hooks. If the > > implementer choses to retrieve posix_acl_* we can also expose > > posix_acl_fix_xattr_to_user or a different kfunc that adds this logic too > > but that would be a separate kfunc (and a separate use-case). > > No, sorry. That's what I feared and that's why I think this low-level > exposure of __vfs_getxattr() is wrong: > The posix_acl_fix_xattr_*() helpers, as well as the helpers like > get_file_caps() will not be exported. We're not going to export that I don't want to expose them and I don't want any others to be exposed either. > deeply internal vfs machinery. So I would NACK that. If you want that - > and that's what I'm saying here - you need to encapsulate this into your > vfs_*xattr() helper that you can call from your kfuncs. It seems like __vfs_getxattr is already exposed and does the wrong thing in some contexts, why can't we just "fix" __vfs_getxattr then? - KP > > > > > > > > > A series without a cover letter and no detailed explanation in the > > > commit messages makes it quite hard to understand whether what is asked > > > can be acked or not. > > > > As I mentioned in > > > > https://lore.kernel.org/bpf/CACYkzJ70uqVJr5EnM0i03Lu+zkuSsXOXcOLQoUS6HZPqH=skpQ@mail.gmail.com/T/#m74f32bae800a97d5c2caf08cee4199d3ba48d76c > > > > I will resend with a cover letter that has more details. > > Thank you! > > > > > > > > > I'm just adding Serge and Casey to double-check here as the LSM stuff is > > > more up their alley. I can just look at this from the perspective of a > > > vfs person. > > > > > > If you have your eBPF meeting thing I'm also happy to jump on there next > > > > Sure, we can discuss this during BPF office hours next week. > > Sounds good.
On Thu, Jun 30, 2022 at 03:29:53PM +0200, KP Singh wrote: > On Thu, Jun 30, 2022 at 3:26 PM Christian Brauner <brauner@kernel.org> wrote: > > > > On Thu, Jun 30, 2022 at 02:21:56PM +0200, KP Singh wrote: > > > On Thu, Jun 30, 2022 at 1:45 PM Christian Brauner <brauner@kernel.org> wrote: > > > > > > > > On Wed, Jun 29, 2022 at 08:02:50PM -0700, Alexei Starovoitov wrote: > > > > > On Wed, Jun 29, 2022 at 2:56 AM Christian Brauner <brauner@kernel.org> wrote: > > > > > > [...] > > > > > > > > > > > > > > > > > > > > > > > > > Signed-off-by: KP Singh <kpsingh@kernel.org> > > > > > > > > > > > --- > > > > > > > > > > > .../testing/selftests/bpf/prog_tests/xattr.c | 54 +++++++++++++++++++ > > > > > > > > > > > > > > > > > > [...] > > > > > > > > > > > > > > > > > > > > +SEC("lsm.s/bprm_committed_creds") > > > > > > > > > > > +void BPF_PROG(bprm_cc, struct linux_binprm *bprm) > > > > > > > > > > > +{ > > > > > > > > > > > + struct task_struct *current = bpf_get_current_task_btf(); > > > > > > > > > > > + char dir_xattr_value[64] = {0}; > > > > > > > > > > > + int xattr_sz = 0; > > > > > > > > > > > + > > > > > > > > > > > + xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, > > > > > > > > > > > + bprm->file->f_path.dentry->d_inode, XATTR_NAME, > > > > > > > > > > > + dir_xattr_value, 64); > > > > > > > > > > > > > > > > > > > > Yeah, this isn't right. You're not accounting for the caller's userns > > > > > > > > > > nor for the idmapped mount. If this is supposed to work you will need a > > > > > > > > > > variant of vfs_getxattr() that takes the mount's idmapping into account > > > > > > > > > > afaict. See what needs to happen after do_getxattr(). > > > > > > > > > > > > > > > > > > Thanks for taking a look. > > > > > > > > > > > > > > > [...] > > > > > > > > > > > > > > > > > > > That will not be correct. > > > > > > > > posix_acl_fix_xattr_to_user checking current_user_ns() > > > > > > > > is checking random tasks that happen to be running > > > > > > > > when lsm hook got invoked. > > > > > > > > > > > > > > > > KP, > > > > > > > > we probably have to document clearly that neither 'current*' > > > > > > > > should not be used here. > > > > > > > > xattr_permission also makes little sense in this context. > > > > > > > > If anything it can be a different kfunc if there is a use case, > > > > > > > > but I don't see it yet. > > > > > > > > bpf-lsm prog calling __vfs_getxattr is just like other lsm-s that > > > > > > > > call it directly. It's the kernel that is doing its security thing. > > > > > > > > > > > > > > Right, but LSMs usually only retrieve their own xattr namespace (ima, > > > > > > > selinux, smack) or they calculate hashes for xattrs based on the raw > > > > > > > filesystem xattr values (evm). > > > > > > > > > > > > > > But this new bpf_getxattr() is different. It allows to retrieve _any_ > > > > > > > xattr in any security hook it can be attached to. So someone can write a > > > > > > > bpf program that retrieves filesystem capabilites or posix acls. And > > > > > > > these are xattrs that require higher-level vfs involvement to be > > > > > > > sensible in most contexts. > > > > > > > > > > > > > [...] > > > > > > > > > > > > > > > > > This hooks a bpf-lsm program to the security_bprm_committed_creds() > > > > > > > hook. It then retrieves the extended attributes of the file to be > > > > > > > executed. The hook currently always retrieves the raw filesystem values. > > > > > > > > > > > > > > But for example any XATTR_NAME_CAPS filesystem capabilities that > > > > > > > might've been stored will be taken into account during exec. And both > > > > > > > the idmapping of the mount and the caller matter when determing whether > > > > > > > they are used or not. > > > > > > > > > > > > > > But the current implementation of bpf_getxattr() just ignores both. It > > > > > > > will always retrieve the raw filesystem values. So if one invokes this > > > > > > > hook they're not actually retrieving the values as they are seen by > > > > > > > fs/exec.c. And I'm wondering why that is ok? And even if this is ok for > > > > > > > some use-cases it might very well become a security issue in others if > > > > > > > access decisions are always based on the raw values. > > > > > > > > > > > > > > I'm not well-versed in this so bear with me, please. > > > > > > > > > > > > If this is really just about retrieving the "security.bpf" xattr and no > > > > > > other xattr then the bpf_getxattr() variant should somehow hard-code > > > > > > that to ensure that no other xattrs can be retrieved, imho. > > > > > > > > > > All of these restrictions look very artificial to me. > > > > > Especially the part "might very well become a security issue" > > > > > just doesn't click. > > > > > We're talking about bpf-lsm progs here that implement security. > > > > > Can somebody implement a poor bpf-lsm that doesn't enforce > > > > > any actual security? Sure. It's a code. > > > > > > > > The point is that with the current implementation of bpf_getxattr() you > > > > are able to retrieve any xattrs and we have way less control over a > > > > bpf-lsm program than we do over selinux which a simple git grep > > > > __vfs_getxattr() is all we need. > > > > > > > > The thing is that with bpf_getxattr() as it stands it is currently > > > > impossible to retrieve xattr values - specifically filesystem > > > > capabilities and posix acls - and see them exactly like the code you're > > > > trying to supervise is. And that seems very strange from a security > > > > perspective. So if someone were to write > > > > > > > > SEC("lsm.s/bprm_creds_from_file") > > > > void BPF_PROG(bprm_cc, struct linux_binprm *bprm) > > > > { > > > > struct task_struct *current = bpf_get_current_task_btf(); > > > > > > > > xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, > > > > bprm->file->f_path.dentry->d_inode, > > > > XATTR_NAME_POSIX_ACL_ACCESS, ..); > > > > // or > > > > xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, > > > > bprm->file->f_path.dentry->d_inode, > > > > XATTR_NAME_CAPS, ..); > > > > > > > > } > > > > > > > > they'd get the raw nscaps and the raw xattrs back. But now, as just a > > > > tiny example, the nscaps->rootuid and the ->e_id fields in the posix > > > > ACLs make zero sense in this context. > > > > > > > > And what's more there's no way for the bpf-lsm program to turn them into > > > > something that makes sense in the context of the hook they are retrieved > > > > in. It lacks all the necessary helpers to do so afaict. > > > > > > > > > No one complains about the usage of EXPORT_SYMBOL(__vfs_getxattr) > > > > > in the existing LSMs like selinux. > > > > > > > > Selinux only cares about its own xattr namespace. It doesn't retrieve > > > > fscaps or posix acls and it's not possible to write selinux programs > > > > that do so. With the bpf-lsm that's very much possible. > > > > > > > > And if we'd notice selinux would start retrieving random xattrs we'd ask > > > > the same questions we do here. > > > > > > > > > No one complains about its usage in out of tree LSMs. > > > > > Is that a security issue? Of course not. > > > > > __vfs_getxattr is a kernel mechanism that LSMs use to implement > > > > > the security features they need. > > > > > __vfs_getxattr as kfunc here is pretty much the same as EXPORT_SYMBOL > > > > > with a big difference that it's EXPORT_SYMBOL_GPL. > > > > > BPF land doesn't have an equivalent of non-gpl export and is not going > > > > > to get one. > > > > > > I want to reiterate what Alexei is saying here: > > > > > > *Please* consider this as a simple wrapper around __vfs_getxattr > > > with a limited attach surface and extra verification checks and > > > and nothing else. > > > > > > What you are saying is __vfs_getxattr does not make sense in some > > > contexts. But kernel modules can still use it right? > > > > > > The user is implementing an LSM, if they chose to do things that don't make > > > sense, then they can surely cause a lot more harm: > > > > > > SEC("lsm/bprm_check_security") > > > int BPF_PROG(bprm_check, struct linux_binprm *bprm) > > > { > > > return -EPERM; > > > } > > > > > > > > > > > This discussion would probably be a lot shorter if this series were sent > > > > with a proper explanation of how this supposed to work and what it's > > > > used for. > > > > > > It's currently scoped to BPF LSM (albeit limited to LSM for now) > > > but it won't just be used in LSM programs but some (allow-listed) > > > tracing programs too. > > > > > > We want to leave the flexibility to the implementer of the LSM hooks. If the > > > implementer choses to retrieve posix_acl_* we can also expose > > > posix_acl_fix_xattr_to_user or a different kfunc that adds this logic too > > > but that would be a separate kfunc (and a separate use-case). > > > > No, sorry. That's what I feared and that's why I think this low-level > > exposure of __vfs_getxattr() is wrong: > > The posix_acl_fix_xattr_*() helpers, as well as the helpers like > > get_file_caps() will not be exported. We're not going to export that > > I don't want to expose them and I don't want any others to be > exposed either. > > > deeply internal vfs machinery. So I would NACK that. If you want that - > > and that's what I'm saying here - you need to encapsulate this into your > > vfs_*xattr() helper that you can call from your kfuncs. > > It seems like __vfs_getxattr is already exposed and does the wrong thing in > some contexts, why can't we just "fix" __vfs_getxattr then? To me having either a version of bpf_getxattr() that restricts access to certain xattrs or a version that takes care to perform the neccesary translations is what seems to make the most sense. I suggested that in one of my first mails. The one thing where the way the xattrs are retrieved really matters is for vfscaps (see get_vfs_caps_from_disk()) you really need something like that function in order for vfs caps to make any sense and be interpretable by the user of the hook. But again, I might just misunderstand the context here and for the bpf-lsm all of this isn't really a concern. If your new series comes out I'll try to get more into the wider context. If the security folks are happy with this then I won't argue.
On Thu, Jun 30, 2022 at 03:47:02PM +0200, Christian Brauner wrote: > On Thu, Jun 30, 2022 at 03:29:53PM +0200, KP Singh wrote: > > On Thu, Jun 30, 2022 at 3:26 PM Christian Brauner <brauner@kernel.org> wrote: > > > > > > On Thu, Jun 30, 2022 at 02:21:56PM +0200, KP Singh wrote: > > > > On Thu, Jun 30, 2022 at 1:45 PM Christian Brauner <brauner@kernel.org> wrote: > > > > > > > > > > On Wed, Jun 29, 2022 at 08:02:50PM -0700, Alexei Starovoitov wrote: > > > > > > On Wed, Jun 29, 2022 at 2:56 AM Christian Brauner <brauner@kernel.org> wrote: > > > > > > > > [...] > > > > > > > > > > > > > > > > > > > > > > > > > > > > Signed-off-by: KP Singh <kpsingh@kernel.org> > > > > > > > > > > > > --- > > > > > > > > > > > > .../testing/selftests/bpf/prog_tests/xattr.c | 54 +++++++++++++++++++ > > > > > > > > > > > > > > > > > > > > [...] > > > > > > > > > > > > > > > > > > > > > > +SEC("lsm.s/bprm_committed_creds") > > > > > > > > > > > > +void BPF_PROG(bprm_cc, struct linux_binprm *bprm) > > > > > > > > > > > > +{ > > > > > > > > > > > > + struct task_struct *current = bpf_get_current_task_btf(); > > > > > > > > > > > > + char dir_xattr_value[64] = {0}; > > > > > > > > > > > > + int xattr_sz = 0; > > > > > > > > > > > > + > > > > > > > > > > > > + xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, > > > > > > > > > > > > + bprm->file->f_path.dentry->d_inode, XATTR_NAME, > > > > > > > > > > > > + dir_xattr_value, 64); > > > > > > > > > > > > > > > > > > > > > > Yeah, this isn't right. You're not accounting for the caller's userns > > > > > > > > > > > nor for the idmapped mount. If this is supposed to work you will need a > > > > > > > > > > > variant of vfs_getxattr() that takes the mount's idmapping into account > > > > > > > > > > > afaict. See what needs to happen after do_getxattr(). > > > > > > > > > > > > > > > > > > > > Thanks for taking a look. > > > > > > > > > > > > > > > > > > [...] > > > > > > > > > > > > > > > > > > > > > > That will not be correct. > > > > > > > > > posix_acl_fix_xattr_to_user checking current_user_ns() > > > > > > > > > is checking random tasks that happen to be running > > > > > > > > > when lsm hook got invoked. > > > > > > > > > > > > > > > > > > KP, > > > > > > > > > we probably have to document clearly that neither 'current*' > > > > > > > > > should not be used here. > > > > > > > > > xattr_permission also makes little sense in this context. > > > > > > > > > If anything it can be a different kfunc if there is a use case, > > > > > > > > > but I don't see it yet. > > > > > > > > > bpf-lsm prog calling __vfs_getxattr is just like other lsm-s that > > > > > > > > > call it directly. It's the kernel that is doing its security thing. > > > > > > > > > > > > > > > > Right, but LSMs usually only retrieve their own xattr namespace (ima, > > > > > > > > selinux, smack) or they calculate hashes for xattrs based on the raw > > > > > > > > filesystem xattr values (evm). > > > > > > > > > > > > > > > > But this new bpf_getxattr() is different. It allows to retrieve _any_ > > > > > > > > xattr in any security hook it can be attached to. So someone can write a > > > > > > > > bpf program that retrieves filesystem capabilites or posix acls. And > > > > > > > > these are xattrs that require higher-level vfs involvement to be > > > > > > > > sensible in most contexts. > > > > > > > > > > > > > > > > [...] > > > > > > > > > > > > > > > > > > > > This hooks a bpf-lsm program to the security_bprm_committed_creds() > > > > > > > > hook. It then retrieves the extended attributes of the file to be > > > > > > > > executed. The hook currently always retrieves the raw filesystem values. > > > > > > > > > > > > > > > > But for example any XATTR_NAME_CAPS filesystem capabilities that > > > > > > > > might've been stored will be taken into account during exec. And both > > > > > > > > the idmapping of the mount and the caller matter when determing whether > > > > > > > > they are used or not. > > > > > > > > > > > > > > > > But the current implementation of bpf_getxattr() just ignores both. It > > > > > > > > will always retrieve the raw filesystem values. So if one invokes this > > > > > > > > hook they're not actually retrieving the values as they are seen by > > > > > > > > fs/exec.c. And I'm wondering why that is ok? And even if this is ok for > > > > > > > > some use-cases it might very well become a security issue in others if > > > > > > > > access decisions are always based on the raw values. > > > > > > > > > > > > > > > > I'm not well-versed in this so bear with me, please. > > > > > > > > > > > > > > If this is really just about retrieving the "security.bpf" xattr and no > > > > > > > other xattr then the bpf_getxattr() variant should somehow hard-code > > > > > > > that to ensure that no other xattrs can be retrieved, imho. > > > > > > > > > > > > All of these restrictions look very artificial to me. > > > > > > Especially the part "might very well become a security issue" > > > > > > just doesn't click. > > > > > > We're talking about bpf-lsm progs here that implement security. > > > > > > Can somebody implement a poor bpf-lsm that doesn't enforce > > > > > > any actual security? Sure. It's a code. > > > > > > > > > > The point is that with the current implementation of bpf_getxattr() you > > > > > are able to retrieve any xattrs and we have way less control over a > > > > > bpf-lsm program than we do over selinux which a simple git grep > > > > > __vfs_getxattr() is all we need. > > > > > > > > > > The thing is that with bpf_getxattr() as it stands it is currently > > > > > impossible to retrieve xattr values - specifically filesystem > > > > > capabilities and posix acls - and see them exactly like the code you're > > > > > trying to supervise is. And that seems very strange from a security > > > > > perspective. So if someone were to write > > > > > > > > > > SEC("lsm.s/bprm_creds_from_file") > > > > > void BPF_PROG(bprm_cc, struct linux_binprm *bprm) > > > > > { > > > > > struct task_struct *current = bpf_get_current_task_btf(); > > > > > > > > > > xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, > > > > > bprm->file->f_path.dentry->d_inode, > > > > > XATTR_NAME_POSIX_ACL_ACCESS, ..); > > > > > // or > > > > > xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, > > > > > bprm->file->f_path.dentry->d_inode, > > > > > XATTR_NAME_CAPS, ..); > > > > > > > > > > } > > > > > > > > > > they'd get the raw nscaps and the raw xattrs back. But now, as just a > > > > > tiny example, the nscaps->rootuid and the ->e_id fields in the posix > > > > > ACLs make zero sense in this context. > > > > > > > > > > And what's more there's no way for the bpf-lsm program to turn them into > > > > > something that makes sense in the context of the hook they are retrieved > > > > > in. It lacks all the necessary helpers to do so afaict. > > > > > > > > > > > No one complains about the usage of EXPORT_SYMBOL(__vfs_getxattr) > > > > > > in the existing LSMs like selinux. > > > > > > > > > > Selinux only cares about its own xattr namespace. It doesn't retrieve > > > > > fscaps or posix acls and it's not possible to write selinux programs > > > > > that do so. With the bpf-lsm that's very much possible. > > > > > > > > > > And if we'd notice selinux would start retrieving random xattrs we'd ask > > > > > the same questions we do here. > > > > > > > > > > > No one complains about its usage in out of tree LSMs. > > > > > > Is that a security issue? Of course not. > > > > > > __vfs_getxattr is a kernel mechanism that LSMs use to implement > > > > > > the security features they need. > > > > > > __vfs_getxattr as kfunc here is pretty much the same as EXPORT_SYMBOL > > > > > > with a big difference that it's EXPORT_SYMBOL_GPL. > > > > > > BPF land doesn't have an equivalent of non-gpl export and is not going > > > > > > to get one. > > > > > > > > I want to reiterate what Alexei is saying here: > > > > > > > > *Please* consider this as a simple wrapper around __vfs_getxattr > > > > with a limited attach surface and extra verification checks and > > > > and nothing else. > > > > > > > > What you are saying is __vfs_getxattr does not make sense in some > > > > contexts. But kernel modules can still use it right? > > > > > > > > The user is implementing an LSM, if they chose to do things that don't make > > > > sense, then they can surely cause a lot more harm: > > > > > > > > SEC("lsm/bprm_check_security") > > > > int BPF_PROG(bprm_check, struct linux_binprm *bprm) > > > > { > > > > return -EPERM; > > > > } > > > > > > > > > > > > > > This discussion would probably be a lot shorter if this series were sent > > > > > with a proper explanation of how this supposed to work and what it's > > > > > used for. > > > > > > > > It's currently scoped to BPF LSM (albeit limited to LSM for now) > > > > but it won't just be used in LSM programs but some (allow-listed) > > > > tracing programs too. > > > > > > > > We want to leave the flexibility to the implementer of the LSM hooks. If the > > > > implementer choses to retrieve posix_acl_* we can also expose > > > > posix_acl_fix_xattr_to_user or a different kfunc that adds this logic too > > > > but that would be a separate kfunc (and a separate use-case). > > > > > > No, sorry. That's what I feared and that's why I think this low-level > > > exposure of __vfs_getxattr() is wrong: > > > The posix_acl_fix_xattr_*() helpers, as well as the helpers like > > > get_file_caps() will not be exported. We're not going to export that > > > > I don't want to expose them and I don't want any others to be > > exposed either. > > > > > deeply internal vfs machinery. So I would NACK that. If you want that - > > > and that's what I'm saying here - you need to encapsulate this into your > > > vfs_*xattr() helper that you can call from your kfuncs. > > > > It seems like __vfs_getxattr is already exposed and does the wrong thing in > > some contexts, why can't we just "fix" __vfs_getxattr then? > > To me having either a version of bpf_getxattr() that restricts access to > certain xattrs or a version that takes care to perform the neccesary > translations is what seems to make the most sense. I suggested that in > one of my first mails. > > The one thing where the way the xattrs are retrieved really matters is > for vfscaps (see get_vfs_caps_from_disk()) you really need something > like that function in order for vfs caps to make any sense and be > interpretable by the user of the hook. > > But again, I might just misunderstand the context here and for the > bpf-lsm all of this isn't really a concern. If your new series comes out > I'll try to get more into the wider context. > If the security folks are happy with this then I won't argue. I think for posix acls you're actually fine since you never report anything to userspace. So one of your bpf-lsms might reasonably interpret this. But for vfscaps you need them fixed up. That's what's done in vfs_getxattr() via xattr_getsecurity() which calls into security_inode_getsecurity() and then into cap_inode_getsecurity() which does the conversion from the on-disk into the proper in-memory representation. And that's nasty because fscaps are versioned depending on whether they are namespaced or not.
On 6/30/2022 6:47 AM, Christian Brauner wrote: > On Thu, Jun 30, 2022 at 03:29:53PM +0200, KP Singh wrote: >> On Thu, Jun 30, 2022 at 3:26 PM Christian Brauner <brauner@kernel.org> wrote: >>> On Thu, Jun 30, 2022 at 02:21:56PM +0200, KP Singh wrote: >>>> On Thu, Jun 30, 2022 at 1:45 PM Christian Brauner <brauner@kernel.org> wrote: >>>>> On Wed, Jun 29, 2022 at 08:02:50PM -0700, Alexei Starovoitov wrote: >>>>>> On Wed, Jun 29, 2022 at 2:56 AM Christian Brauner <brauner@kernel.org> wrote: >>>> [...] >>>> >>>>>>>>>>>> Signed-off-by: KP Singh <kpsingh@kernel.org> >>>>>>>>>>>> --- >>>>>>>>>>>> .../testing/selftests/bpf/prog_tests/xattr.c | 54 +++++++++++++++++++ >>>>>>>>>> [...] >>>>>>>>>> >>>>>>>>>>>> +SEC("lsm.s/bprm_committed_creds") >>>>>>>>>>>> +void BPF_PROG(bprm_cc, struct linux_binprm *bprm) >>>>>>>>>>>> +{ >>>>>>>>>>>> + struct task_struct *current = bpf_get_current_task_btf(); >>>>>>>>>>>> + char dir_xattr_value[64] = {0}; >>>>>>>>>>>> + int xattr_sz = 0; >>>>>>>>>>>> + >>>>>>>>>>>> + xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, >>>>>>>>>>>> + bprm->file->f_path.dentry->d_inode, XATTR_NAME, >>>>>>>>>>>> + dir_xattr_value, 64); >>>>>>>>>>> Yeah, this isn't right. You're not accounting for the caller's userns >>>>>>>>>>> nor for the idmapped mount. If this is supposed to work you will need a >>>>>>>>>>> variant of vfs_getxattr() that takes the mount's idmapping into account >>>>>>>>>>> afaict. See what needs to happen after do_getxattr(). >>>>>>>>>> Thanks for taking a look. >>>>>>>>>> >>>> [...] >>>> >>>>>>>>> That will not be correct. >>>>>>>>> posix_acl_fix_xattr_to_user checking current_user_ns() >>>>>>>>> is checking random tasks that happen to be running >>>>>>>>> when lsm hook got invoked. >>>>>>>>> >>>>>>>>> KP, >>>>>>>>> we probably have to document clearly that neither 'current*' >>>>>>>>> should not be used here. >>>>>>>>> xattr_permission also makes little sense in this context. >>>>>>>>> If anything it can be a different kfunc if there is a use case, >>>>>>>>> but I don't see it yet. >>>>>>>>> bpf-lsm prog calling __vfs_getxattr is just like other lsm-s that >>>>>>>>> call it directly. It's the kernel that is doing its security thing. >>>>>>>> Right, but LSMs usually only retrieve their own xattr namespace (ima, >>>>>>>> selinux, smack) or they calculate hashes for xattrs based on the raw >>>>>>>> filesystem xattr values (evm). >>>>>>>> >>>>>>>> But this new bpf_getxattr() is different. It allows to retrieve _any_ >>>>>>>> xattr in any security hook it can be attached to. So someone can write a >>>>>>>> bpf program that retrieves filesystem capabilites or posix acls. And >>>>>>>> these are xattrs that require higher-level vfs involvement to be >>>>>>>> sensible in most contexts. >>>>>>>> >>>> [...] >>>> >>>>>>>> This hooks a bpf-lsm program to the security_bprm_committed_creds() >>>>>>>> hook. It then retrieves the extended attributes of the file to be >>>>>>>> executed. The hook currently always retrieves the raw filesystem values. >>>>>>>> >>>>>>>> But for example any XATTR_NAME_CAPS filesystem capabilities that >>>>>>>> might've been stored will be taken into account during exec. And both >>>>>>>> the idmapping of the mount and the caller matter when determing whether >>>>>>>> they are used or not. >>>>>>>> >>>>>>>> But the current implementation of bpf_getxattr() just ignores both. It >>>>>>>> will always retrieve the raw filesystem values. So if one invokes this >>>>>>>> hook they're not actually retrieving the values as they are seen by >>>>>>>> fs/exec.c. And I'm wondering why that is ok? And even if this is ok for >>>>>>>> some use-cases it might very well become a security issue in others if >>>>>>>> access decisions are always based on the raw values. >>>>>>>> >>>>>>>> I'm not well-versed in this so bear with me, please. >>>>>>> If this is really just about retrieving the "security.bpf" xattr and no >>>>>>> other xattr then the bpf_getxattr() variant should somehow hard-code >>>>>>> that to ensure that no other xattrs can be retrieved, imho. >>>>>> All of these restrictions look very artificial to me. >>>>>> Especially the part "might very well become a security issue" >>>>>> just doesn't click. >>>>>> We're talking about bpf-lsm progs here that implement security. >>>>>> Can somebody implement a poor bpf-lsm that doesn't enforce >>>>>> any actual security? Sure. It's a code. >>>>> The point is that with the current implementation of bpf_getxattr() you >>>>> are able to retrieve any xattrs and we have way less control over a >>>>> bpf-lsm program than we do over selinux which a simple git grep >>>>> __vfs_getxattr() is all we need. >>>>> >>>>> The thing is that with bpf_getxattr() as it stands it is currently >>>>> impossible to retrieve xattr values - specifically filesystem >>>>> capabilities and posix acls - and see them exactly like the code you're >>>>> trying to supervise is. And that seems very strange from a security >>>>> perspective. So if someone were to write >>>>> >>>>> SEC("lsm.s/bprm_creds_from_file") >>>>> void BPF_PROG(bprm_cc, struct linux_binprm *bprm) >>>>> { >>>>> struct task_struct *current = bpf_get_current_task_btf(); >>>>> >>>>> xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, >>>>> bprm->file->f_path.dentry->d_inode, >>>>> XATTR_NAME_POSIX_ACL_ACCESS, ..); >>>>> // or >>>>> xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, >>>>> bprm->file->f_path.dentry->d_inode, >>>>> XATTR_NAME_CAPS, ..); >>>>> >>>>> } >>>>> >>>>> they'd get the raw nscaps and the raw xattrs back. But now, as just a >>>>> tiny example, the nscaps->rootuid and the ->e_id fields in the posix >>>>> ACLs make zero sense in this context. >>>>> >>>>> And what's more there's no way for the bpf-lsm program to turn them into >>>>> something that makes sense in the context of the hook they are retrieved >>>>> in. It lacks all the necessary helpers to do so afaict. >>>>> >>>>>> No one complains about the usage of EXPORT_SYMBOL(__vfs_getxattr) >>>>>> in the existing LSMs like selinux. >>>>> Selinux only cares about its own xattr namespace. It doesn't retrieve >>>>> fscaps or posix acls and it's not possible to write selinux programs >>>>> that do so. With the bpf-lsm that's very much possible. >>>>> >>>>> And if we'd notice selinux would start retrieving random xattrs we'd ask >>>>> the same questions we do here. >>>>> >>>>>> No one complains about its usage in out of tree LSMs. >>>>>> Is that a security issue? Of course not. >>>>>> __vfs_getxattr is a kernel mechanism that LSMs use to implement >>>>>> the security features they need. >>>>>> __vfs_getxattr as kfunc here is pretty much the same as EXPORT_SYMBOL >>>>>> with a big difference that it's EXPORT_SYMBOL_GPL. >>>>>> BPF land doesn't have an equivalent of non-gpl export and is not going >>>>>> to get one. >>>> I want to reiterate what Alexei is saying here: >>>> >>>> *Please* consider this as a simple wrapper around __vfs_getxattr >>>> with a limited attach surface and extra verification checks and >>>> and nothing else. >>>> >>>> What you are saying is __vfs_getxattr does not make sense in some >>>> contexts. But kernel modules can still use it right? >>>> >>>> The user is implementing an LSM, if they chose to do things that don't make >>>> sense, then they can surely cause a lot more harm: >>>> >>>> SEC("lsm/bprm_check_security") >>>> int BPF_PROG(bprm_check, struct linux_binprm *bprm) >>>> { >>>> return -EPERM; >>>> } >>>> >>>>> This discussion would probably be a lot shorter if this series were sent >>>>> with a proper explanation of how this supposed to work and what it's >>>>> used for. >>>> It's currently scoped to BPF LSM (albeit limited to LSM for now) >>>> but it won't just be used in LSM programs but some (allow-listed) >>>> tracing programs too. >>>> >>>> We want to leave the flexibility to the implementer of the LSM hooks. If the >>>> implementer choses to retrieve posix_acl_* we can also expose >>>> posix_acl_fix_xattr_to_user or a different kfunc that adds this logic too >>>> but that would be a separate kfunc (and a separate use-case). >>> No, sorry. That's what I feared and that's why I think this low-level >>> exposure of __vfs_getxattr() is wrong: >>> The posix_acl_fix_xattr_*() helpers, as well as the helpers like >>> get_file_caps() will not be exported. We're not going to export that >> I don't want to expose them and I don't want any others to be >> exposed either. >> >>> deeply internal vfs machinery. So I would NACK that. If you want that - >>> and that's what I'm saying here - you need to encapsulate this into your >>> vfs_*xattr() helper that you can call from your kfuncs. >> It seems like __vfs_getxattr is already exposed and does the wrong thing in >> some contexts, why can't we just "fix" __vfs_getxattr then? > To me having either a version of bpf_getxattr() that restricts access to > certain xattrs or a version that takes care to perform the neccesary > translations is what seems to make the most sense. I suggested that in > one of my first mails. > > The one thing where the way the xattrs are retrieved really matters is > for vfscaps (see get_vfs_caps_from_disk()) you really need something > like that function in order for vfs caps to make any sense and be > interpretable by the user of the hook. > > But again, I might just misunderstand the context here and for the > bpf-lsm all of this isn't really a concern. If your new series comes out > I'll try to get more into the wider context. > If the security folks are happy with this then I won't argue. A security module (BPF) using another security module's (Smack) xattrs without that module's (Smack) explicit approval would be considered extremely rude. Smack and SELinux use published interfaces of the capability security module, but never access the capability attributes directly. The details of a security module's implementation are not a factor. The fact that BPF uses loadable programs as opposed to loadable policy is not relevant. The only security.xattr values that the BPF security module should allow the programs it runs to access are the ones it is managing. If you decided to create an eBPF implementation of SELinux you would still have to use attributes specific to the BPF security module. If, on the other hand, you wanted to extend Smack using eBPF programs, and the Smack maintainer liked the idea, it would be OK for the BPF security module to access some of the security.SMACK64 attributes. I want it to be clear that BPF is a Linux Security Module (LSM) and a collection of eBPF programs is *not* an LSM. BPF is responsible for being a good kernel citizen, and must ensure that it does not allow a set of configuration data that violates proper behavior. You can't write an SELinux policy that monster-mashes an ACL. You can't allow BPF to permit that either. You can't count on the good intentions, wisdom or skill of the author of an unreviewed, out of tree, eBPF program. I believe that this was understood during the review process of the BPF LSM.
> > > > This discussion would probably be a lot shorter if this series were sent > > with a proper explanation of how this supposed to work and what it's > > used for. > > It's currently scoped to BPF LSM (albeit limited to LSM for now) > but it won't just be used in LSM programs but some (allow-listed) > tracing programs too. > KP, Without taking sides in the discussion about the security aspect of bpf_getxattr(), I wanted to say that we have plans to add BPF hooks for fanotify event filters and AFAIK Alessio's team is working on adding BPF hooks for FUSE bypass decisions. In both those cases, being able to tag files with some xattr and use that as part of criteria in the hook would be very useful IMO, but I don't think that it should be a problem to limit the scope of the allowed namespace to security.bpf.* for these use cases. Thanks, Amir.
On Thu, Jun 30, 2022 at 6:10 PM Casey Schaufler <casey@schaufler-ca.com> wrote: > > On 6/30/2022 6:47 AM, Christian Brauner wrote: > > On Thu, Jun 30, 2022 at 03:29:53PM +0200, KP Singh wrote: > >> On Thu, Jun 30, 2022 at 3:26 PM Christian Brauner <brauner@kernel.org> wrote: > >>> On Thu, Jun 30, 2022 at 02:21:56PM +0200, KP Singh wrote: > >>>> On Thu, Jun 30, 2022 at 1:45 PM Christian Brauner <brauner@kernel.org> wrote: > >>>>> On Wed, Jun 29, 2022 at 08:02:50PM -0700, Alexei Starovoitov wrote: > >>>>>> On Wed, Jun 29, 2022 at 2:56 AM Christian Brauner <brauner@kernel.org> wrote: > >>>> [...] > >>>> > >>>>>>>>>>>> Signed-off-by: KP Singh <kpsingh@kernel.org> > >>>>>>>>>>>> --- > >>>>>>>>>>>> .../testing/selftests/bpf/prog_tests/xattr.c | 54 +++++++++++++++++++ > >>>>>>>>>> [...] > >>>>>>>>>> > >>>>>>>>>>>> +SEC("lsm.s/bprm_committed_creds") > >>>>>>>>>>>> +void BPF_PROG(bprm_cc, struct linux_binprm *bprm) > >>>>>>>>>>>> +{ > >>>>>>>>>>>> + struct task_struct *current = bpf_get_current_task_btf(); > >>>>>>>>>>>> + char dir_xattr_value[64] = {0}; > >>>>>>>>>>>> + int xattr_sz = 0; > >>>>>>>>>>>> + > >>>>>>>>>>>> + xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, > >>>>>>>>>>>> + bprm->file->f_path.dentry->d_inode, XATTR_NAME, > >>>>>>>>>>>> + dir_xattr_value, 64); > >>>>>>>>>>> Yeah, this isn't right. You're not accounting for the caller's userns > >>>>>>>>>>> nor for the idmapped mount. If this is supposed to work you will need a > >>>>>>>>>>> variant of vfs_getxattr() that takes the mount's idmapping into account > >>>>>>>>>>> afaict. See what needs to happen after do_getxattr(). > >>>>>>>>>> Thanks for taking a look. > >>>>>>>>>> > >>>> [...] > >>>> > >>>>>>>>> That will not be correct. > >>>>>>>>> posix_acl_fix_xattr_to_user checking current_user_ns() > >>>>>>>>> is checking random tasks that happen to be running > >>>>>>>>> when lsm hook got invoked. > >>>>>>>>> > >>>>>>>>> KP, > >>>>>>>>> we probably have to document clearly that neither 'current*' > >>>>>>>>> should not be used here. > >>>>>>>>> xattr_permission also makes little sense in this context. > >>>>>>>>> If anything it can be a different kfunc if there is a use case, > >>>>>>>>> but I don't see it yet. > >>>>>>>>> bpf-lsm prog calling __vfs_getxattr is just like other lsm-s that > >>>>>>>>> call it directly. It's the kernel that is doing its security thing. > >>>>>>>> Right, but LSMs usually only retrieve their own xattr namespace (ima, > >>>>>>>> selinux, smack) or they calculate hashes for xattrs based on the raw > >>>>>>>> filesystem xattr values (evm). > >>>>>>>> > >>>>>>>> But this new bpf_getxattr() is different. It allows to retrieve _any_ > >>>>>>>> xattr in any security hook it can be attached to. So someone can write a > >>>>>>>> bpf program that retrieves filesystem capabilites or posix acls. And > >>>>>>>> these are xattrs that require higher-level vfs involvement to be > >>>>>>>> sensible in most contexts. > >>>>>>>> > >>>> [...] > >>>> > >>>>>>>> This hooks a bpf-lsm program to the security_bprm_committed_creds() > >>>>>>>> hook. It then retrieves the extended attributes of the file to be > >>>>>>>> executed. The hook currently always retrieves the raw filesystem values. > >>>>>>>> > >>>>>>>> But for example any XATTR_NAME_CAPS filesystem capabilities that > >>>>>>>> might've been stored will be taken into account during exec. And both > >>>>>>>> the idmapping of the mount and the caller matter when determing whether > >>>>>>>> they are used or not. > >>>>>>>> > >>>>>>>> But the current implementation of bpf_getxattr() just ignores both. It > >>>>>>>> will always retrieve the raw filesystem values. So if one invokes this > >>>>>>>> hook they're not actually retrieving the values as they are seen by > >>>>>>>> fs/exec.c. And I'm wondering why that is ok? And even if this is ok for > >>>>>>>> some use-cases it might very well become a security issue in others if > >>>>>>>> access decisions are always based on the raw values. > >>>>>>>> > >>>>>>>> I'm not well-versed in this so bear with me, please. > >>>>>>> If this is really just about retrieving the "security.bpf" xattr and no > >>>>>>> other xattr then the bpf_getxattr() variant should somehow hard-code > >>>>>>> that to ensure that no other xattrs can be retrieved, imho. > >>>>>> All of these restrictions look very artificial to me. > >>>>>> Especially the part "might very well become a security issue" > >>>>>> just doesn't click. > >>>>>> We're talking about bpf-lsm progs here that implement security. > >>>>>> Can somebody implement a poor bpf-lsm that doesn't enforce > >>>>>> any actual security? Sure. It's a code. > >>>>> The point is that with the current implementation of bpf_getxattr() you > >>>>> are able to retrieve any xattrs and we have way less control over a > >>>>> bpf-lsm program than we do over selinux which a simple git grep > >>>>> __vfs_getxattr() is all we need. > >>>>> > >>>>> The thing is that with bpf_getxattr() as it stands it is currently > >>>>> impossible to retrieve xattr values - specifically filesystem > >>>>> capabilities and posix acls - and see them exactly like the code you're > >>>>> trying to supervise is. And that seems very strange from a security > >>>>> perspective. So if someone were to write > >>>>> > >>>>> SEC("lsm.s/bprm_creds_from_file") > >>>>> void BPF_PROG(bprm_cc, struct linux_binprm *bprm) > >>>>> { > >>>>> struct task_struct *current = bpf_get_current_task_btf(); > >>>>> > >>>>> xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, > >>>>> bprm->file->f_path.dentry->d_inode, > >>>>> XATTR_NAME_POSIX_ACL_ACCESS, ..); > >>>>> // or > >>>>> xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, > >>>>> bprm->file->f_path.dentry->d_inode, > >>>>> XATTR_NAME_CAPS, ..); > >>>>> > >>>>> } > >>>>> > >>>>> they'd get the raw nscaps and the raw xattrs back. But now, as just a > >>>>> tiny example, the nscaps->rootuid and the ->e_id fields in the posix > >>>>> ACLs make zero sense in this context. > >>>>> > >>>>> And what's more there's no way for the bpf-lsm program to turn them into > >>>>> something that makes sense in the context of the hook they are retrieved > >>>>> in. It lacks all the necessary helpers to do so afaict. > >>>>> > >>>>>> No one complains about the usage of EXPORT_SYMBOL(__vfs_getxattr) > >>>>>> in the existing LSMs like selinux. > >>>>> Selinux only cares about its own xattr namespace. It doesn't retrieve > >>>>> fscaps or posix acls and it's not possible to write selinux programs > >>>>> that do so. With the bpf-lsm that's very much possible. > >>>>> > >>>>> And if we'd notice selinux would start retrieving random xattrs we'd ask > >>>>> the same questions we do here. > >>>>> > >>>>>> No one complains about its usage in out of tree LSMs. > >>>>>> Is that a security issue? Of course not. > >>>>>> __vfs_getxattr is a kernel mechanism that LSMs use to implement > >>>>>> the security features they need. > >>>>>> __vfs_getxattr as kfunc here is pretty much the same as EXPORT_SYMBOL > >>>>>> with a big difference that it's EXPORT_SYMBOL_GPL. > >>>>>> BPF land doesn't have an equivalent of non-gpl export and is not going > >>>>>> to get one. > >>>> I want to reiterate what Alexei is saying here: > >>>> > >>>> *Please* consider this as a simple wrapper around __vfs_getxattr > >>>> with a limited attach surface and extra verification checks and > >>>> and nothing else. > >>>> > >>>> What you are saying is __vfs_getxattr does not make sense in some > >>>> contexts. But kernel modules can still use it right? > >>>> > >>>> The user is implementing an LSM, if they chose to do things that don't make > >>>> sense, then they can surely cause a lot more harm: > >>>> > >>>> SEC("lsm/bprm_check_security") > >>>> int BPF_PROG(bprm_check, struct linux_binprm *bprm) > >>>> { > >>>> return -EPERM; > >>>> } > >>>> > >>>>> This discussion would probably be a lot shorter if this series were sent > >>>>> with a proper explanation of how this supposed to work and what it's > >>>>> used for. > >>>> It's currently scoped to BPF LSM (albeit limited to LSM for now) > >>>> but it won't just be used in LSM programs but some (allow-listed) > >>>> tracing programs too. > >>>> > >>>> We want to leave the flexibility to the implementer of the LSM hooks. If the > >>>> implementer choses to retrieve posix_acl_* we can also expose > >>>> posix_acl_fix_xattr_to_user or a different kfunc that adds this logic too > >>>> but that would be a separate kfunc (and a separate use-case). > >>> No, sorry. That's what I feared and that's why I think this low-level > >>> exposure of __vfs_getxattr() is wrong: > >>> The posix_acl_fix_xattr_*() helpers, as well as the helpers like > >>> get_file_caps() will not be exported. We're not going to export that > >> I don't want to expose them and I don't want any others to be > >> exposed either. > >> > >>> deeply internal vfs machinery. So I would NACK that. If you want that - > >>> and that's what I'm saying here - you need to encapsulate this into your > >>> vfs_*xattr() helper that you can call from your kfuncs. > >> It seems like __vfs_getxattr is already exposed and does the wrong thing in > >> some contexts, why can't we just "fix" __vfs_getxattr then? > > To me having either a version of bpf_getxattr() that restricts access to > > certain xattrs or a version that takes care to perform the neccesary > > translations is what seems to make the most sense. I suggested that in > > one of my first mails. > > > > The one thing where the way the xattrs are retrieved really matters is > > for vfscaps (see get_vfs_caps_from_disk()) you really need something > > like that function in order for vfs caps to make any sense and be > > interpretable by the user of the hook. > > > > But again, I might just misunderstand the context here and for the > > bpf-lsm all of this isn't really a concern. If your new series comes out > > I'll try to get more into the wider context. > > If the security folks are happy with this then I won't argue. > > A security module (BPF) using another security module's (Smack) > xattrs without that module's (Smack) explicit approval would be > considered extremely rude. Smack and SELinux use published interfaces > of the capability security module, but never access the capability > attributes directly. The details of a security module's implementation > are not a factor. The fact that BPF uses loadable programs as opposed > to loadable policy is not relevant. The only security.xattr values > that the BPF security module should allow the programs it runs to > access are the ones it is managing. If you decided to create an eBPF What about kernel modules who can use __vfs_getxattr already as it's an exported symbol? This can still end up influencing security policy or using them in any way they like. Anyways, I think, for now, for the use case we have, it can work with a restriction to security.bpf xattrs. > implementation of SELinux you would still have to use attributes > specific to the BPF security module. If, on the other hand, you wanted > to extend Smack using eBPF programs, and the Smack maintainer liked > the idea, it would be OK for the BPF security module to access some > of the security.SMACK64 attributes. > > I want it to be clear that BPF is a Linux Security Module (LSM) and > a collection of eBPF programs is *not* an LSM. BPF is responsible > for being a good kernel citizen, and must ensure that it does not > allow a set of configuration data that violates proper behavior. > You can't write an SELinux policy that monster-mashes an ACL. > You can't allow BPF to permit that either. You can't count on the > good intentions, wisdom or skill of the author of an unreviewed, > out of tree, eBPF program. I believe that this was understood during > the review process of the BPF LSM. > >
On Thu, Jun 30, 2022 at 6:29 PM Amir Goldstein <amir73il@gmail.com> wrote: > > > > > > > This discussion would probably be a lot shorter if this series were sent > > > with a proper explanation of how this supposed to work and what it's > > > used for. > > > > It's currently scoped to BPF LSM (albeit limited to LSM for now) > > but it won't just be used in LSM programs but some (allow-listed) > > tracing programs too. > > > > KP, > > Without taking sides in the discussion about the security aspect of > bpf_getxattr(), > I wanted to say that we have plans to add BPF hooks for fanotify event > filters and > AFAIK Alessio's team is working on adding BPF hooks for FUSE bypass decisions. > > In both those cases, being able to tag files with some xattr and use > that as part of > criteria in the hook would be very useful IMO, but I don't think that > it should be a > problem to limit the scope of the allowed namespace to security.bpf.* for these > use cases. Thanks Amir, I agree, this does seem like a practical way to move forward. Cheers, - KP > > Thanks, > Amir.
On 6/30/2022 3:23 PM, KP Singh wrote: > On Thu, Jun 30, 2022 at 6:10 PM Casey Schaufler <casey@schaufler-ca.com> wrote: >> On 6/30/2022 6:47 AM, Christian Brauner wrote: >>> On Thu, Jun 30, 2022 at 03:29:53PM +0200, KP Singh wrote: >>>> On Thu, Jun 30, 2022 at 3:26 PM Christian Brauner <brauner@kernel.org> wrote: >>>>> On Thu, Jun 30, 2022 at 02:21:56PM +0200, KP Singh wrote: >>>>>> On Thu, Jun 30, 2022 at 1:45 PM Christian Brauner <brauner@kernel.org> wrote: >>>>>>> On Wed, Jun 29, 2022 at 08:02:50PM -0700, Alexei Starovoitov wrote: >>>>>>>> On Wed, Jun 29, 2022 at 2:56 AM Christian Brauner <brauner@kernel.org> wrote: >>>>>> [...] >>>>>> >>>>>>>>>>>>>> Signed-off-by: KP Singh <kpsingh@kernel.org> >>>>>>>>>>>>>> --- >>>>>>>>>>>>>> .../testing/selftests/bpf/prog_tests/xattr.c | 54 +++++++++++++++++++ >>>>>>>>>>>> [...] >>>>>>>>>>>> >>>>>>>>>>>>>> +SEC("lsm.s/bprm_committed_creds") >>>>>>>>>>>>>> +void BPF_PROG(bprm_cc, struct linux_binprm *bprm) >>>>>>>>>>>>>> +{ >>>>>>>>>>>>>> + struct task_struct *current = bpf_get_current_task_btf(); >>>>>>>>>>>>>> + char dir_xattr_value[64] = {0}; >>>>>>>>>>>>>> + int xattr_sz = 0; >>>>>>>>>>>>>> + >>>>>>>>>>>>>> + xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, >>>>>>>>>>>>>> + bprm->file->f_path.dentry->d_inode, XATTR_NAME, >>>>>>>>>>>>>> + dir_xattr_value, 64); >>>>>>>>>>>>> Yeah, this isn't right. You're not accounting for the caller's userns >>>>>>>>>>>>> nor for the idmapped mount. If this is supposed to work you will need a >>>>>>>>>>>>> variant of vfs_getxattr() that takes the mount's idmapping into account >>>>>>>>>>>>> afaict. See what needs to happen after do_getxattr(). >>>>>>>>>>>> Thanks for taking a look. >>>>>>>>>>>> >>>>>> [...] >>>>>> >>>>>>>>>>> That will not be correct. >>>>>>>>>>> posix_acl_fix_xattr_to_user checking current_user_ns() >>>>>>>>>>> is checking random tasks that happen to be running >>>>>>>>>>> when lsm hook got invoked. >>>>>>>>>>> >>>>>>>>>>> KP, >>>>>>>>>>> we probably have to document clearly that neither 'current*' >>>>>>>>>>> should not be used here. >>>>>>>>>>> xattr_permission also makes little sense in this context. >>>>>>>>>>> If anything it can be a different kfunc if there is a use case, >>>>>>>>>>> but I don't see it yet. >>>>>>>>>>> bpf-lsm prog calling __vfs_getxattr is just like other lsm-s that >>>>>>>>>>> call it directly. It's the kernel that is doing its security thing. >>>>>>>>>> Right, but LSMs usually only retrieve their own xattr namespace (ima, >>>>>>>>>> selinux, smack) or they calculate hashes for xattrs based on the raw >>>>>>>>>> filesystem xattr values (evm). >>>>>>>>>> >>>>>>>>>> But this new bpf_getxattr() is different. It allows to retrieve _any_ >>>>>>>>>> xattr in any security hook it can be attached to. So someone can write a >>>>>>>>>> bpf program that retrieves filesystem capabilites or posix acls. And >>>>>>>>>> these are xattrs that require higher-level vfs involvement to be >>>>>>>>>> sensible in most contexts. >>>>>>>>>> >>>>>> [...] >>>>>> >>>>>>>>>> This hooks a bpf-lsm program to the security_bprm_committed_creds() >>>>>>>>>> hook. It then retrieves the extended attributes of the file to be >>>>>>>>>> executed. The hook currently always retrieves the raw filesystem values. >>>>>>>>>> >>>>>>>>>> But for example any XATTR_NAME_CAPS filesystem capabilities that >>>>>>>>>> might've been stored will be taken into account during exec. And both >>>>>>>>>> the idmapping of the mount and the caller matter when determing whether >>>>>>>>>> they are used or not. >>>>>>>>>> >>>>>>>>>> But the current implementation of bpf_getxattr() just ignores both. It >>>>>>>>>> will always retrieve the raw filesystem values. So if one invokes this >>>>>>>>>> hook they're not actually retrieving the values as they are seen by >>>>>>>>>> fs/exec.c. And I'm wondering why that is ok? And even if this is ok for >>>>>>>>>> some use-cases it might very well become a security issue in others if >>>>>>>>>> access decisions are always based on the raw values. >>>>>>>>>> >>>>>>>>>> I'm not well-versed in this so bear with me, please. >>>>>>>>> If this is really just about retrieving the "security.bpf" xattr and no >>>>>>>>> other xattr then the bpf_getxattr() variant should somehow hard-code >>>>>>>>> that to ensure that no other xattrs can be retrieved, imho. >>>>>>>> All of these restrictions look very artificial to me. >>>>>>>> Especially the part "might very well become a security issue" >>>>>>>> just doesn't click. >>>>>>>> We're talking about bpf-lsm progs here that implement security. >>>>>>>> Can somebody implement a poor bpf-lsm that doesn't enforce >>>>>>>> any actual security? Sure. It's a code. >>>>>>> The point is that with the current implementation of bpf_getxattr() you >>>>>>> are able to retrieve any xattrs and we have way less control over a >>>>>>> bpf-lsm program than we do over selinux which a simple git grep >>>>>>> __vfs_getxattr() is all we need. >>>>>>> >>>>>>> The thing is that with bpf_getxattr() as it stands it is currently >>>>>>> impossible to retrieve xattr values - specifically filesystem >>>>>>> capabilities and posix acls - and see them exactly like the code you're >>>>>>> trying to supervise is. And that seems very strange from a security >>>>>>> perspective. So if someone were to write >>>>>>> >>>>>>> SEC("lsm.s/bprm_creds_from_file") >>>>>>> void BPF_PROG(bprm_cc, struct linux_binprm *bprm) >>>>>>> { >>>>>>> struct task_struct *current = bpf_get_current_task_btf(); >>>>>>> >>>>>>> xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, >>>>>>> bprm->file->f_path.dentry->d_inode, >>>>>>> XATTR_NAME_POSIX_ACL_ACCESS, ..); >>>>>>> // or >>>>>>> xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, >>>>>>> bprm->file->f_path.dentry->d_inode, >>>>>>> XATTR_NAME_CAPS, ..); >>>>>>> >>>>>>> } >>>>>>> >>>>>>> they'd get the raw nscaps and the raw xattrs back. But now, as just a >>>>>>> tiny example, the nscaps->rootuid and the ->e_id fields in the posix >>>>>>> ACLs make zero sense in this context. >>>>>>> >>>>>>> And what's more there's no way for the bpf-lsm program to turn them into >>>>>>> something that makes sense in the context of the hook they are retrieved >>>>>>> in. It lacks all the necessary helpers to do so afaict. >>>>>>> >>>>>>>> No one complains about the usage of EXPORT_SYMBOL(__vfs_getxattr) >>>>>>>> in the existing LSMs like selinux. >>>>>>> Selinux only cares about its own xattr namespace. It doesn't retrieve >>>>>>> fscaps or posix acls and it's not possible to write selinux programs >>>>>>> that do so. With the bpf-lsm that's very much possible. >>>>>>> >>>>>>> And if we'd notice selinux would start retrieving random xattrs we'd ask >>>>>>> the same questions we do here. >>>>>>> >>>>>>>> No one complains about its usage in out of tree LSMs. >>>>>>>> Is that a security issue? Of course not. >>>>>>>> __vfs_getxattr is a kernel mechanism that LSMs use to implement >>>>>>>> the security features they need. >>>>>>>> __vfs_getxattr as kfunc here is pretty much the same as EXPORT_SYMBOL >>>>>>>> with a big difference that it's EXPORT_SYMBOL_GPL. >>>>>>>> BPF land doesn't have an equivalent of non-gpl export and is not going >>>>>>>> to get one. >>>>>> I want to reiterate what Alexei is saying here: >>>>>> >>>>>> *Please* consider this as a simple wrapper around __vfs_getxattr >>>>>> with a limited attach surface and extra verification checks and >>>>>> and nothing else. >>>>>> >>>>>> What you are saying is __vfs_getxattr does not make sense in some >>>>>> contexts. But kernel modules can still use it right? >>>>>> >>>>>> The user is implementing an LSM, if they chose to do things that don't make >>>>>> sense, then they can surely cause a lot more harm: >>>>>> >>>>>> SEC("lsm/bprm_check_security") >>>>>> int BPF_PROG(bprm_check, struct linux_binprm *bprm) >>>>>> { >>>>>> return -EPERM; >>>>>> } >>>>>> >>>>>>> This discussion would probably be a lot shorter if this series were sent >>>>>>> with a proper explanation of how this supposed to work and what it's >>>>>>> used for. >>>>>> It's currently scoped to BPF LSM (albeit limited to LSM for now) >>>>>> but it won't just be used in LSM programs but some (allow-listed) >>>>>> tracing programs too. >>>>>> >>>>>> We want to leave the flexibility to the implementer of the LSM hooks. If the >>>>>> implementer choses to retrieve posix_acl_* we can also expose >>>>>> posix_acl_fix_xattr_to_user or a different kfunc that adds this logic too >>>>>> but that would be a separate kfunc (and a separate use-case). >>>>> No, sorry. That's what I feared and that's why I think this low-level >>>>> exposure of __vfs_getxattr() is wrong: >>>>> The posix_acl_fix_xattr_*() helpers, as well as the helpers like >>>>> get_file_caps() will not be exported. We're not going to export that >>>> I don't want to expose them and I don't want any others to be >>>> exposed either. >>>> >>>>> deeply internal vfs machinery. So I would NACK that. If you want that - >>>>> and that's what I'm saying here - you need to encapsulate this into your >>>>> vfs_*xattr() helper that you can call from your kfuncs. >>>> It seems like __vfs_getxattr is already exposed and does the wrong thing in >>>> some contexts, why can't we just "fix" __vfs_getxattr then? >>> To me having either a version of bpf_getxattr() that restricts access to >>> certain xattrs or a version that takes care to perform the neccesary >>> translations is what seems to make the most sense. I suggested that in >>> one of my first mails. >>> >>> The one thing where the way the xattrs are retrieved really matters is >>> for vfscaps (see get_vfs_caps_from_disk()) you really need something >>> like that function in order for vfs caps to make any sense and be >>> interpretable by the user of the hook. >>> >>> But again, I might just misunderstand the context here and for the >>> bpf-lsm all of this isn't really a concern. If your new series comes out >>> I'll try to get more into the wider context. >>> If the security folks are happy with this then I won't argue. >> A security module (BPF) using another security module's (Smack) >> xattrs without that module's (Smack) explicit approval would be >> considered extremely rude. Smack and SELinux use published interfaces >> of the capability security module, but never access the capability >> attributes directly. The details of a security module's implementation >> are not a factor. The fact that BPF uses loadable programs as opposed >> to loadable policy is not relevant. The only security.xattr values >> that the BPF security module should allow the programs it runs to >> access are the ones it is managing. If you decided to create an eBPF > What about kernel modules who can use __vfs_getxattr already as > it's an exported symbol? This can still end up influencing > security policy or using them in any way they like. If I put code in Smack to read SELinux attributes I would expect to get a possibly polite but definitely strongly worded email from Paul Moore regarding that behavior. The integrity subsystem looks at Smack and SELinux attributes, but that's upstream and we can see what nefarious things are being done with them. Because I can see the upstream kernel code I can convince myself that regardless of the SELinux policy loaded SELinux isn't going to muck with the Smack attributes. I can't say the same for eBPF programs that aren't going to be in Linus' tree. > Anyways, I think, for now, for the use case we have, it can work with > a restriction to security.bpf xattrs. I can't say that this whole discussion is making me feel better about the BPF LSM concept. The approval was based on the notion that eBPF programs were restricted to "safe" behavior. It's hard to see how allowing access to security.selinux could be guaranteed to be in support of safe behavior.
On Fri, Jul 1, 2022 at 2:39 AM Casey Schaufler <casey@schaufler-ca.com> wrote: > > On 6/30/2022 3:23 PM, KP Singh wrote: > > On Thu, Jun 30, 2022 at 6:10 PM Casey Schaufler <casey@schaufler-ca.com> wrote: > >> On 6/30/2022 6:47 AM, Christian Brauner wrote: > >>> On Thu, Jun 30, 2022 at 03:29:53PM +0200, KP Singh wrote: > >>>> On Thu, Jun 30, 2022 at 3:26 PM Christian Brauner <brauner@kernel.org> wrote: > >>>>> On Thu, Jun 30, 2022 at 02:21:56PM +0200, KP Singh wrote: > >>>>>> On Thu, Jun 30, 2022 at 1:45 PM Christian Brauner <brauner@kernel.org> wrote: > >>>>>>> On Wed, Jun 29, 2022 at 08:02:50PM -0700, Alexei Starovoitov wrote: > >>>>>>>> On Wed, Jun 29, 2022 at 2:56 AM Christian Brauner <brauner@kernel.org> wrote: > >>>>>> [...] > >>>>>> > >>>>>>>>>>>>>> Signed-off-by: KP Singh <kpsingh@kernel.org> > >>>>>>>>>>>>>> --- > >>>>>>>>>>>>>> .../testing/selftests/bpf/prog_tests/xattr.c | 54 +++++++++++++++++++ > >>>>>>>>>>>> [...] > >>>>>>>>>>>> > >>>>>>>>>>>>>> +SEC("lsm.s/bprm_committed_creds") > >>>>>>>>>>>>>> +void BPF_PROG(bprm_cc, struct linux_binprm *bprm) > >>>>>>>>>>>>>> +{ > >>>>>>>>>>>>>> + struct task_struct *current = bpf_get_current_task_btf(); > >>>>>>>>>>>>>> + char dir_xattr_value[64] = {0}; > >>>>>>>>>>>>>> + int xattr_sz = 0; > >>>>>>>>>>>>>> + > >>>>>>>>>>>>>> + xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, > >>>>>>>>>>>>>> + bprm->file->f_path.dentry->d_inode, XATTR_NAME, > >>>>>>>>>>>>>> + dir_xattr_value, 64); > >>>>>>>>>>>>> Yeah, this isn't right. You're not accounting for the caller's userns > >>>>>>>>>>>>> nor for the idmapped mount. If this is supposed to work you will need a > >>>>>>>>>>>>> variant of vfs_getxattr() that takes the mount's idmapping into account > >>>>>>>>>>>>> afaict. See what needs to happen after do_getxattr(). > >>>>>>>>>>>> Thanks for taking a look. > >>>>>>>>>>>> > >>>>>> [...] > >>>>>> > >>>>>>>>>>> That will not be correct. > >>>>>>>>>>> posix_acl_fix_xattr_to_user checking current_user_ns() > >>>>>>>>>>> is checking random tasks that happen to be running > >>>>>>>>>>> when lsm hook got invoked. > >>>>>>>>>>> > >>>>>>>>>>> KP, > >>>>>>>>>>> we probably have to document clearly that neither 'current*' > >>>>>>>>>>> should not be used here. > >>>>>>>>>>> xattr_permission also makes little sense in this context. > >>>>>>>>>>> If anything it can be a different kfunc if there is a use case, > >>>>>>>>>>> but I don't see it yet. > >>>>>>>>>>> bpf-lsm prog calling __vfs_getxattr is just like other lsm-s that > >>>>>>>>>>> call it directly. It's the kernel that is doing its security thing. > >>>>>>>>>> Right, but LSMs usually only retrieve their own xattr namespace (ima, > >>>>>>>>>> selinux, smack) or they calculate hashes for xattrs based on the raw > >>>>>>>>>> filesystem xattr values (evm). > >>>>>>>>>> > >>>>>>>>>> But this new bpf_getxattr() is different. It allows to retrieve _any_ > >>>>>>>>>> xattr in any security hook it can be attached to. So someone can write a > >>>>>>>>>> bpf program that retrieves filesystem capabilites or posix acls. And > >>>>>>>>>> these are xattrs that require higher-level vfs involvement to be > >>>>>>>>>> sensible in most contexts. > >>>>>>>>>> > >>>>>> [...] > >>>>>> > >>>>>>>>>> This hooks a bpf-lsm program to the security_bprm_committed_creds() > >>>>>>>>>> hook. It then retrieves the extended attributes of the file to be > >>>>>>>>>> executed. The hook currently always retrieves the raw filesystem values. > >>>>>>>>>> > >>>>>>>>>> But for example any XATTR_NAME_CAPS filesystem capabilities that > >>>>>>>>>> might've been stored will be taken into account during exec. And both > >>>>>>>>>> the idmapping of the mount and the caller matter when determing whether > >>>>>>>>>> they are used or not. > >>>>>>>>>> > >>>>>>>>>> But the current implementation of bpf_getxattr() just ignores both. It > >>>>>>>>>> will always retrieve the raw filesystem values. So if one invokes this > >>>>>>>>>> hook they're not actually retrieving the values as they are seen by > >>>>>>>>>> fs/exec.c. And I'm wondering why that is ok? And even if this is ok for > >>>>>>>>>> some use-cases it might very well become a security issue in others if > >>>>>>>>>> access decisions are always based on the raw values. > >>>>>>>>>> > >>>>>>>>>> I'm not well-versed in this so bear with me, please. > >>>>>>>>> If this is really just about retrieving the "security.bpf" xattr and no > >>>>>>>>> other xattr then the bpf_getxattr() variant should somehow hard-code > >>>>>>>>> that to ensure that no other xattrs can be retrieved, imho. > >>>>>>>> All of these restrictions look very artificial to me. > >>>>>>>> Especially the part "might very well become a security issue" > >>>>>>>> just doesn't click. > >>>>>>>> We're talking about bpf-lsm progs here that implement security. > >>>>>>>> Can somebody implement a poor bpf-lsm that doesn't enforce > >>>>>>>> any actual security? Sure. It's a code. > >>>>>>> The point is that with the current implementation of bpf_getxattr() you > >>>>>>> are able to retrieve any xattrs and we have way less control over a > >>>>>>> bpf-lsm program than we do over selinux which a simple git grep > >>>>>>> __vfs_getxattr() is all we need. > >>>>>>> > >>>>>>> The thing is that with bpf_getxattr() as it stands it is currently > >>>>>>> impossible to retrieve xattr values - specifically filesystem > >>>>>>> capabilities and posix acls - and see them exactly like the code you're > >>>>>>> trying to supervise is. And that seems very strange from a security > >>>>>>> perspective. So if someone were to write > >>>>>>> > >>>>>>> SEC("lsm.s/bprm_creds_from_file") > >>>>>>> void BPF_PROG(bprm_cc, struct linux_binprm *bprm) > >>>>>>> { > >>>>>>> struct task_struct *current = bpf_get_current_task_btf(); > >>>>>>> > >>>>>>> xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, > >>>>>>> bprm->file->f_path.dentry->d_inode, > >>>>>>> XATTR_NAME_POSIX_ACL_ACCESS, ..); > >>>>>>> // or > >>>>>>> xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, > >>>>>>> bprm->file->f_path.dentry->d_inode, > >>>>>>> XATTR_NAME_CAPS, ..); > >>>>>>> > >>>>>>> } > >>>>>>> > >>>>>>> they'd get the raw nscaps and the raw xattrs back. But now, as just a > >>>>>>> tiny example, the nscaps->rootuid and the ->e_id fields in the posix > >>>>>>> ACLs make zero sense in this context. > >>>>>>> > >>>>>>> And what's more there's no way for the bpf-lsm program to turn them into > >>>>>>> something that makes sense in the context of the hook they are retrieved > >>>>>>> in. It lacks all the necessary helpers to do so afaict. > >>>>>>> > >>>>>>>> No one complains about the usage of EXPORT_SYMBOL(__vfs_getxattr) > >>>>>>>> in the existing LSMs like selinux. > >>>>>>> Selinux only cares about its own xattr namespace. It doesn't retrieve > >>>>>>> fscaps or posix acls and it's not possible to write selinux programs > >>>>>>> that do so. With the bpf-lsm that's very much possible. > >>>>>>> > >>>>>>> And if we'd notice selinux would start retrieving random xattrs we'd ask > >>>>>>> the same questions we do here. > >>>>>>> > >>>>>>>> No one complains about its usage in out of tree LSMs. > >>>>>>>> Is that a security issue? Of course not. > >>>>>>>> __vfs_getxattr is a kernel mechanism that LSMs use to implement > >>>>>>>> the security features they need. > >>>>>>>> __vfs_getxattr as kfunc here is pretty much the same as EXPORT_SYMBOL > >>>>>>>> with a big difference that it's EXPORT_SYMBOL_GPL. > >>>>>>>> BPF land doesn't have an equivalent of non-gpl export and is not going > >>>>>>>> to get one. > >>>>>> I want to reiterate what Alexei is saying here: > >>>>>> > >>>>>> *Please* consider this as a simple wrapper around __vfs_getxattr > >>>>>> with a limited attach surface and extra verification checks and > >>>>>> and nothing else. > >>>>>> > >>>>>> What you are saying is __vfs_getxattr does not make sense in some > >>>>>> contexts. But kernel modules can still use it right? > >>>>>> > >>>>>> The user is implementing an LSM, if they chose to do things that don't make > >>>>>> sense, then they can surely cause a lot more harm: > >>>>>> > >>>>>> SEC("lsm/bprm_check_security") > >>>>>> int BPF_PROG(bprm_check, struct linux_binprm *bprm) > >>>>>> { > >>>>>> return -EPERM; > >>>>>> } > >>>>>> > >>>>>>> This discussion would probably be a lot shorter if this series were sent > >>>>>>> with a proper explanation of how this supposed to work and what it's > >>>>>>> used for. > >>>>>> It's currently scoped to BPF LSM (albeit limited to LSM for now) > >>>>>> but it won't just be used in LSM programs but some (allow-listed) > >>>>>> tracing programs too. > >>>>>> > >>>>>> We want to leave the flexibility to the implementer of the LSM hooks. If the > >>>>>> implementer choses to retrieve posix_acl_* we can also expose > >>>>>> posix_acl_fix_xattr_to_user or a different kfunc that adds this logic too > >>>>>> but that would be a separate kfunc (and a separate use-case). > >>>>> No, sorry. That's what I feared and that's why I think this low-level > >>>>> exposure of __vfs_getxattr() is wrong: > >>>>> The posix_acl_fix_xattr_*() helpers, as well as the helpers like > >>>>> get_file_caps() will not be exported. We're not going to export that > >>>> I don't want to expose them and I don't want any others to be > >>>> exposed either. > >>>> > >>>>> deeply internal vfs machinery. So I would NACK that. If you want that - > >>>>> and that's what I'm saying here - you need to encapsulate this into your > >>>>> vfs_*xattr() helper that you can call from your kfuncs. > >>>> It seems like __vfs_getxattr is already exposed and does the wrong thing in > >>>> some contexts, why can't we just "fix" __vfs_getxattr then? > >>> To me having either a version of bpf_getxattr() that restricts access to > >>> certain xattrs or a version that takes care to perform the neccesary > >>> translations is what seems to make the most sense. I suggested that in > >>> one of my first mails. > >>> > >>> The one thing where the way the xattrs are retrieved really matters is > >>> for vfscaps (see get_vfs_caps_from_disk()) you really need something > >>> like that function in order for vfs caps to make any sense and be > >>> interpretable by the user of the hook. > >>> > >>> But again, I might just misunderstand the context here and for the > >>> bpf-lsm all of this isn't really a concern. If your new series comes out > >>> I'll try to get more into the wider context. > >>> If the security folks are happy with this then I won't argue. > >> A security module (BPF) using another security module's (Smack) > >> xattrs without that module's (Smack) explicit approval would be > >> considered extremely rude. Smack and SELinux use published interfaces > >> of the capability security module, but never access the capability > >> attributes directly. The details of a security module's implementation > >> are not a factor. The fact that BPF uses loadable programs as opposed > >> to loadable policy is not relevant. The only security.xattr values > >> that the BPF security module should allow the programs it runs to > >> access are the ones it is managing. If you decided to create an eBPF > > What about kernel modules who can use __vfs_getxattr already as > > it's an exported symbol? This can still end up influencing > > security policy or using them in any way they like. > > If I put code in Smack to read SELinux attributes I would expect > to get a possibly polite but definitely strongly worded email > from Paul Moore regarding that behavior. The integrity subsystem > looks at Smack and SELinux attributes, but that's upstream and > we can see what nefarious things are being done with them. Because > I can see the upstream kernel code I can convince myself that > regardless of the SELinux policy loaded SELinux isn't going to > muck with the Smack attributes. I can't say the same for eBPF > programs that aren't going to be in Linus' tree. > > > Anyways, I think, for now, for the use case we have, it can work with > > a restriction to security.bpf xattrs. > > I can't say that this whole discussion is making me feel better > about the BPF LSM concept. The approval was based on the notion > that eBPF programs were restricted to "safe" behavior. It's > hard to see how allowing access to security.selinux could be > guaranteed to be in support of safe behavior. > Apropos __vfs_getxattr(), looks like ecryptfs_getxattr_lower() is abusing it. Christian, not sure if you intend to spend time of idmapped mount support of ecryptfs lower layer, but anyway that's that. Thanks, Amir.
On Fri, Jul 01, 2022 at 11:32:55AM +0300, Amir Goldstein wrote: > On Fri, Jul 1, 2022 at 2:39 AM Casey Schaufler <casey@schaufler-ca.com> wrote: > > > > On 6/30/2022 3:23 PM, KP Singh wrote: > > > On Thu, Jun 30, 2022 at 6:10 PM Casey Schaufler <casey@schaufler-ca.com> wrote: > > >> On 6/30/2022 6:47 AM, Christian Brauner wrote: > > >>> On Thu, Jun 30, 2022 at 03:29:53PM +0200, KP Singh wrote: > > >>>> On Thu, Jun 30, 2022 at 3:26 PM Christian Brauner <brauner@kernel.org> wrote: > > >>>>> On Thu, Jun 30, 2022 at 02:21:56PM +0200, KP Singh wrote: > > >>>>>> On Thu, Jun 30, 2022 at 1:45 PM Christian Brauner <brauner@kernel.org> wrote: > > >>>>>>> On Wed, Jun 29, 2022 at 08:02:50PM -0700, Alexei Starovoitov wrote: > > >>>>>>>> On Wed, Jun 29, 2022 at 2:56 AM Christian Brauner <brauner@kernel.org> wrote: > > >>>>>> [...] > > >>>>>> > > >>>>>>>>>>>>>> Signed-off-by: KP Singh <kpsingh@kernel.org> > > >>>>>>>>>>>>>> --- > > >>>>>>>>>>>>>> .../testing/selftests/bpf/prog_tests/xattr.c | 54 +++++++++++++++++++ > > >>>>>>>>>>>> [...] > > >>>>>>>>>>>> > > >>>>>>>>>>>>>> +SEC("lsm.s/bprm_committed_creds") > > >>>>>>>>>>>>>> +void BPF_PROG(bprm_cc, struct linux_binprm *bprm) > > >>>>>>>>>>>>>> +{ > > >>>>>>>>>>>>>> + struct task_struct *current = bpf_get_current_task_btf(); > > >>>>>>>>>>>>>> + char dir_xattr_value[64] = {0}; > > >>>>>>>>>>>>>> + int xattr_sz = 0; > > >>>>>>>>>>>>>> + > > >>>>>>>>>>>>>> + xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, > > >>>>>>>>>>>>>> + bprm->file->f_path.dentry->d_inode, XATTR_NAME, > > >>>>>>>>>>>>>> + dir_xattr_value, 64); > > >>>>>>>>>>>>> Yeah, this isn't right. You're not accounting for the caller's userns > > >>>>>>>>>>>>> nor for the idmapped mount. If this is supposed to work you will need a > > >>>>>>>>>>>>> variant of vfs_getxattr() that takes the mount's idmapping into account > > >>>>>>>>>>>>> afaict. See what needs to happen after do_getxattr(). > > >>>>>>>>>>>> Thanks for taking a look. > > >>>>>>>>>>>> > > >>>>>> [...] > > >>>>>> > > >>>>>>>>>>> That will not be correct. > > >>>>>>>>>>> posix_acl_fix_xattr_to_user checking current_user_ns() > > >>>>>>>>>>> is checking random tasks that happen to be running > > >>>>>>>>>>> when lsm hook got invoked. > > >>>>>>>>>>> > > >>>>>>>>>>> KP, > > >>>>>>>>>>> we probably have to document clearly that neither 'current*' > > >>>>>>>>>>> should not be used here. > > >>>>>>>>>>> xattr_permission also makes little sense in this context. > > >>>>>>>>>>> If anything it can be a different kfunc if there is a use case, > > >>>>>>>>>>> but I don't see it yet. > > >>>>>>>>>>> bpf-lsm prog calling __vfs_getxattr is just like other lsm-s that > > >>>>>>>>>>> call it directly. It's the kernel that is doing its security thing. > > >>>>>>>>>> Right, but LSMs usually only retrieve their own xattr namespace (ima, > > >>>>>>>>>> selinux, smack) or they calculate hashes for xattrs based on the raw > > >>>>>>>>>> filesystem xattr values (evm). > > >>>>>>>>>> > > >>>>>>>>>> But this new bpf_getxattr() is different. It allows to retrieve _any_ > > >>>>>>>>>> xattr in any security hook it can be attached to. So someone can write a > > >>>>>>>>>> bpf program that retrieves filesystem capabilites or posix acls. And > > >>>>>>>>>> these are xattrs that require higher-level vfs involvement to be > > >>>>>>>>>> sensible in most contexts. > > >>>>>>>>>> > > >>>>>> [...] > > >>>>>> > > >>>>>>>>>> This hooks a bpf-lsm program to the security_bprm_committed_creds() > > >>>>>>>>>> hook. It then retrieves the extended attributes of the file to be > > >>>>>>>>>> executed. The hook currently always retrieves the raw filesystem values. > > >>>>>>>>>> > > >>>>>>>>>> But for example any XATTR_NAME_CAPS filesystem capabilities that > > >>>>>>>>>> might've been stored will be taken into account during exec. And both > > >>>>>>>>>> the idmapping of the mount and the caller matter when determing whether > > >>>>>>>>>> they are used or not. > > >>>>>>>>>> > > >>>>>>>>>> But the current implementation of bpf_getxattr() just ignores both. It > > >>>>>>>>>> will always retrieve the raw filesystem values. So if one invokes this > > >>>>>>>>>> hook they're not actually retrieving the values as they are seen by > > >>>>>>>>>> fs/exec.c. And I'm wondering why that is ok? And even if this is ok for > > >>>>>>>>>> some use-cases it might very well become a security issue in others if > > >>>>>>>>>> access decisions are always based on the raw values. > > >>>>>>>>>> > > >>>>>>>>>> I'm not well-versed in this so bear with me, please. > > >>>>>>>>> If this is really just about retrieving the "security.bpf" xattr and no > > >>>>>>>>> other xattr then the bpf_getxattr() variant should somehow hard-code > > >>>>>>>>> that to ensure that no other xattrs can be retrieved, imho. > > >>>>>>>> All of these restrictions look very artificial to me. > > >>>>>>>> Especially the part "might very well become a security issue" > > >>>>>>>> just doesn't click. > > >>>>>>>> We're talking about bpf-lsm progs here that implement security. > > >>>>>>>> Can somebody implement a poor bpf-lsm that doesn't enforce > > >>>>>>>> any actual security? Sure. It's a code. > > >>>>>>> The point is that with the current implementation of bpf_getxattr() you > > >>>>>>> are able to retrieve any xattrs and we have way less control over a > > >>>>>>> bpf-lsm program than we do over selinux which a simple git grep > > >>>>>>> __vfs_getxattr() is all we need. > > >>>>>>> > > >>>>>>> The thing is that with bpf_getxattr() as it stands it is currently > > >>>>>>> impossible to retrieve xattr values - specifically filesystem > > >>>>>>> capabilities and posix acls - and see them exactly like the code you're > > >>>>>>> trying to supervise is. And that seems very strange from a security > > >>>>>>> perspective. So if someone were to write > > >>>>>>> > > >>>>>>> SEC("lsm.s/bprm_creds_from_file") > > >>>>>>> void BPF_PROG(bprm_cc, struct linux_binprm *bprm) > > >>>>>>> { > > >>>>>>> struct task_struct *current = bpf_get_current_task_btf(); > > >>>>>>> > > >>>>>>> xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, > > >>>>>>> bprm->file->f_path.dentry->d_inode, > > >>>>>>> XATTR_NAME_POSIX_ACL_ACCESS, ..); > > >>>>>>> // or > > >>>>>>> xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, > > >>>>>>> bprm->file->f_path.dentry->d_inode, > > >>>>>>> XATTR_NAME_CAPS, ..); > > >>>>>>> > > >>>>>>> } > > >>>>>>> > > >>>>>>> they'd get the raw nscaps and the raw xattrs back. But now, as just a > > >>>>>>> tiny example, the nscaps->rootuid and the ->e_id fields in the posix > > >>>>>>> ACLs make zero sense in this context. > > >>>>>>> > > >>>>>>> And what's more there's no way for the bpf-lsm program to turn them into > > >>>>>>> something that makes sense in the context of the hook they are retrieved > > >>>>>>> in. It lacks all the necessary helpers to do so afaict. > > >>>>>>> > > >>>>>>>> No one complains about the usage of EXPORT_SYMBOL(__vfs_getxattr) > > >>>>>>>> in the existing LSMs like selinux. > > >>>>>>> Selinux only cares about its own xattr namespace. It doesn't retrieve > > >>>>>>> fscaps or posix acls and it's not possible to write selinux programs > > >>>>>>> that do so. With the bpf-lsm that's very much possible. > > >>>>>>> > > >>>>>>> And if we'd notice selinux would start retrieving random xattrs we'd ask > > >>>>>>> the same questions we do here. > > >>>>>>> > > >>>>>>>> No one complains about its usage in out of tree LSMs. > > >>>>>>>> Is that a security issue? Of course not. > > >>>>>>>> __vfs_getxattr is a kernel mechanism that LSMs use to implement > > >>>>>>>> the security features they need. > > >>>>>>>> __vfs_getxattr as kfunc here is pretty much the same as EXPORT_SYMBOL > > >>>>>>>> with a big difference that it's EXPORT_SYMBOL_GPL. > > >>>>>>>> BPF land doesn't have an equivalent of non-gpl export and is not going > > >>>>>>>> to get one. > > >>>>>> I want to reiterate what Alexei is saying here: > > >>>>>> > > >>>>>> *Please* consider this as a simple wrapper around __vfs_getxattr > > >>>>>> with a limited attach surface and extra verification checks and > > >>>>>> and nothing else. > > >>>>>> > > >>>>>> What you are saying is __vfs_getxattr does not make sense in some > > >>>>>> contexts. But kernel modules can still use it right? > > >>>>>> > > >>>>>> The user is implementing an LSM, if they chose to do things that don't make > > >>>>>> sense, then they can surely cause a lot more harm: > > >>>>>> > > >>>>>> SEC("lsm/bprm_check_security") > > >>>>>> int BPF_PROG(bprm_check, struct linux_binprm *bprm) > > >>>>>> { > > >>>>>> return -EPERM; > > >>>>>> } > > >>>>>> > > >>>>>>> This discussion would probably be a lot shorter if this series were sent > > >>>>>>> with a proper explanation of how this supposed to work and what it's > > >>>>>>> used for. > > >>>>>> It's currently scoped to BPF LSM (albeit limited to LSM for now) > > >>>>>> but it won't just be used in LSM programs but some (allow-listed) > > >>>>>> tracing programs too. > > >>>>>> > > >>>>>> We want to leave the flexibility to the implementer of the LSM hooks. If the > > >>>>>> implementer choses to retrieve posix_acl_* we can also expose > > >>>>>> posix_acl_fix_xattr_to_user or a different kfunc that adds this logic too > > >>>>>> but that would be a separate kfunc (and a separate use-case). > > >>>>> No, sorry. That's what I feared and that's why I think this low-level > > >>>>> exposure of __vfs_getxattr() is wrong: > > >>>>> The posix_acl_fix_xattr_*() helpers, as well as the helpers like > > >>>>> get_file_caps() will not be exported. We're not going to export that > > >>>> I don't want to expose them and I don't want any others to be > > >>>> exposed either. > > >>>> > > >>>>> deeply internal vfs machinery. So I would NACK that. If you want that - > > >>>>> and that's what I'm saying here - you need to encapsulate this into your > > >>>>> vfs_*xattr() helper that you can call from your kfuncs. > > >>>> It seems like __vfs_getxattr is already exposed and does the wrong thing in > > >>>> some contexts, why can't we just "fix" __vfs_getxattr then? > > >>> To me having either a version of bpf_getxattr() that restricts access to > > >>> certain xattrs or a version that takes care to perform the neccesary > > >>> translations is what seems to make the most sense. I suggested that in > > >>> one of my first mails. > > >>> > > >>> The one thing where the way the xattrs are retrieved really matters is > > >>> for vfscaps (see get_vfs_caps_from_disk()) you really need something > > >>> like that function in order for vfs caps to make any sense and be > > >>> interpretable by the user of the hook. > > >>> > > >>> But again, I might just misunderstand the context here and for the > > >>> bpf-lsm all of this isn't really a concern. If your new series comes out > > >>> I'll try to get more into the wider context. > > >>> If the security folks are happy with this then I won't argue. > > >> A security module (BPF) using another security module's (Smack) > > >> xattrs without that module's (Smack) explicit approval would be > > >> considered extremely rude. Smack and SELinux use published interfaces > > >> of the capability security module, but never access the capability > > >> attributes directly. The details of a security module's implementation > > >> are not a factor. The fact that BPF uses loadable programs as opposed > > >> to loadable policy is not relevant. The only security.xattr values > > >> that the BPF security module should allow the programs it runs to > > >> access are the ones it is managing. If you decided to create an eBPF > > > What about kernel modules who can use __vfs_getxattr already as > > > it's an exported symbol? This can still end up influencing > > > security policy or using them in any way they like. > > > > If I put code in Smack to read SELinux attributes I would expect > > to get a possibly polite but definitely strongly worded email > > from Paul Moore regarding that behavior. The integrity subsystem > > looks at Smack and SELinux attributes, but that's upstream and > > we can see what nefarious things are being done with them. Because > > I can see the upstream kernel code I can convince myself that > > regardless of the SELinux policy loaded SELinux isn't going to > > muck with the Smack attributes. I can't say the same for eBPF > > programs that aren't going to be in Linus' tree. > > > > > Anyways, I think, for now, for the use case we have, it can work with > > > a restriction to security.bpf xattrs. > > > > I can't say that this whole discussion is making me feel better > > about the BPF LSM concept. The approval was based on the notion > > that eBPF programs were restricted to "safe" behavior. It's > > hard to see how allowing access to security.selinux could be > > guaranteed to be in support of safe behavior. > > > > Apropos __vfs_getxattr(), looks like ecryptfs_getxattr_lower() > is abusing it. Heh, quoting what I wrote to KP yesterday off-list about __vfs_getxattr(): "it's [__vfs_getxattr()] exported but [afaict] it's not used in kernel modules. afaict it's only exposed because of ecryptfs" So right at the beginning I had already pondered whether we should just rip out __vfs_getxattr() from ecryptfs and unexport the helper completely because there's barely a reason to use it. Module/driver code should not use something as low-level as __vfs_getxattr() imho. Overlayfs does it correctly and uses vfs_getxattr() but maybe ecryptfs needs to use it for for some reason?. I haven't looked yet. > Christian, not sure if you intend to spend time of idmapped > mount support of ecryptfs lower layer, but anyway that's that. Not really. Remember the conversation we had with Tyler at LSFMM where he considered marking it deprecated. I don't think it's worth putting in the work.
> > > > Apropos __vfs_getxattr(), looks like ecryptfs_getxattr_lower() > > is abusing it. > > Heh, quoting what I wrote to KP yesterday off-list about > __vfs_getxattr(): > > "it's [__vfs_getxattr()] exported but [afaict] it's not used in kernel > modules. afaict it's only exposed because of ecryptfs" > > So right at the beginning I had already pondered whether we should just > rip out __vfs_getxattr() from ecryptfs and unexport the helper > completely because there's barely a reason to use it. Module/driver code > should not use something as low-level as __vfs_getxattr() imho. > > Overlayfs does it correctly and uses vfs_getxattr() but maybe ecryptfs > needs to use it for for some reason?. I haven't looked yet. > No reason AFAIK (CC Tyler+Miklos) Most lower ecryptfs operations use vfs_XXX() 48b512e68571 ("ecryptfs: call vfs_setxattr() in ecryptfs_setxattr()") fixed vfs_setxattr() which was later changed to __vfs_setxattr_locked(), but left __vfs_getxattr(), __vfs_removexattr() and i_op->listxattr(). > > Christian, not sure if you intend to spend time of idmapped > > mount support of ecryptfs lower layer, but anyway that's that. > > Not really. Remember the conversation we had with Tyler at LSFMM where > he considered marking it deprecated. I don't think it's worth putting in > the work. OK, so just need a volunteer to close the security hole and possibly unexport __vfs_getxattr(). Does anybody know of any out of tree modules that use it for a good reason? Thanks, Amir.
diff --git a/tools/testing/selftests/bpf/prog_tests/xattr.c b/tools/testing/selftests/bpf/prog_tests/xattr.c new file mode 100644 index 000000000000..ef07fa8a1763 --- /dev/null +++ b/tools/testing/selftests/bpf/prog_tests/xattr.c @@ -0,0 +1,54 @@ +// SPDX-License-Identifier: GPL-2.0 + +/* + * Copyright 2022 Google LLC. + */ + +#include <test_progs.h> +#include <sys/xattr.h> +#include "xattr.skel.h" + +#define XATTR_NAME "security.bpf" +#define XATTR_VALUE "test_progs" + +void test_xattr(void) +{ + struct xattr *skel = NULL; + char tmp_dir_path[] = "/tmp/xattrXXXXXX"; + char tmp_exec_path[64]; + char cmd[256]; + int err; + + if (CHECK_FAIL(!mkdtemp(tmp_dir_path))) + goto close_prog; + + snprintf(tmp_exec_path, sizeof(tmp_exec_path), "%s/copy_of_ls", + tmp_dir_path); + snprintf(cmd, sizeof(cmd), "cp /bin/ls %s", tmp_exec_path); + if (CHECK_FAIL(system(cmd))) + goto close_prog_rmdir; + + if (CHECK_FAIL(setxattr(tmp_exec_path, XATTR_NAME, XATTR_VALUE, + sizeof(XATTR_VALUE), 0))) + goto close_prog_rmdir; + + skel = xattr__open_and_load(); + if (!ASSERT_OK_PTR(skel, "skel_load")) + goto close_prog_rmdir; + + err = xattr__attach(skel); + if (!ASSERT_OK(err, "xattr__attach failed")) + goto close_prog_rmdir; + + snprintf(cmd, sizeof(cmd), "%s -l", tmp_exec_path); + if (CHECK_FAIL(system(cmd))) + goto close_prog_rmdir; + + ASSERT_EQ(skel->bss->result, 1, "xattr result"); + +close_prog_rmdir: + snprintf(cmd, sizeof(cmd), "rm -rf %s", tmp_dir_path); + system(cmd); +close_prog: + xattr__destroy(skel); +} diff --git a/tools/testing/selftests/bpf/progs/xattr.c b/tools/testing/selftests/bpf/progs/xattr.c new file mode 100644 index 000000000000..ccc078fb8ebd --- /dev/null +++ b/tools/testing/selftests/bpf/progs/xattr.c @@ -0,0 +1,37 @@ +// SPDX-License-Identifier: GPL-2.0 + +/* + * Copyright 2022 Google LLC. + */ + +#include "vmlinux.h" +#include <bpf/bpf_helpers.h> +#include <bpf/bpf_tracing.h> + +char _license[] SEC("license") = "GPL"; + +#define XATTR_NAME "security.bpf" +#define XATTR_VALUE "test_progs" + +__u64 result = 0; + +extern ssize_t bpf_getxattr(struct dentry *dentry, struct inode *inode, + const char *name, void *value, int size) __ksym; + +SEC("lsm.s/bprm_committed_creds") +void BPF_PROG(bprm_cc, struct linux_binprm *bprm) +{ + struct task_struct *current = bpf_get_current_task_btf(); + char dir_xattr_value[64] = {0}; + int xattr_sz = 0; + + xattr_sz = bpf_getxattr(bprm->file->f_path.dentry, + bprm->file->f_path.dentry->d_inode, XATTR_NAME, + dir_xattr_value, 64); + + if (xattr_sz <= 0) + return; + + if (!bpf_strncmp(dir_xattr_value, sizeof(XATTR_VALUE), XATTR_VALUE)) + result = 1; +}
A simple test that adds an xattr on a copied /bin/ls and reads it back when the copied ls is executed. Signed-off-by: KP Singh <kpsingh@kernel.org> --- .../testing/selftests/bpf/prog_tests/xattr.c | 54 +++++++++++++++++++ tools/testing/selftests/bpf/progs/xattr.c | 37 +++++++++++++ 2 files changed, 91 insertions(+) create mode 100644 tools/testing/selftests/bpf/prog_tests/xattr.c create mode 100644 tools/testing/selftests/bpf/progs/xattr.c