| Message ID | 20190731153443.4984-1-acgoide@tycho.nsa.gov (mailing list archive) |
|---|---|
| State | Superseded |
| Headers | show |
| Series | fanotify, inotify, dnotify, security: add security hook for fs notifications | expand |
On 7/31/2019 8:34 AM, Aaron Goidel wrote: > As of now, setting watches on filesystem objects has, at most, applied a > check for read access to the inode, and in the case of fanotify, requires > CAP_SYS_ADMIN. No specific security hook or permission check has been > provided to control the setting of watches. Using any of inotify, dnotify, > or fanotify, it is possible to observe, not only write-like operations, but > even read access to a file. Modeling the watch as being merely a read from > the file is insufficient for the needs of SELinux. This is due to the fact > that read access should not necessarily imply access to information about > when another process reads from a file. Furthermore, fanotify watches grant > more power to an application in the form of permission events. While > notification events are solely, unidirectional (i.e. they only pass > information to the receiving application), permission events are blocking. > Permission events make a request to the receiving application which will > then reply with a decision as to whether or not that action may be > completed. This causes the issue of the watching application having the > ability to exercise control over the triggering process. Without drawing a > distinction within the permission check, the ability to read would imply > the greater ability to control an application. Additionally, mount and > superblock watches apply to all files within the same mount or superblock. > Read access to one file should not necessarily imply the ability to watch > all files accessed within a given mount or superblock. > > In order to solve these issues, a new LSM hook is implemented and has been > placed within the system calls for marking filesystem objects with inotify, > fanotify, and dnotify watches. These calls to the hook are placed at the > point at which the target path has been resolved and are provided with the > path struct, the mask of requested notification events, and the type of > object on which the mark is being set (inode, superblock, or mount). The > mask and obj_type have already been translated into common FS_* values > shared by the entirety of the fs notification infrastructure. The path > struct is passed rather than just the inode so that the mount is available, > particularly for mount watches. This also allows for use of the hook by > pathname-based security modules. However, since the hook is intended for > use even by inode based security modules, it is not placed under the > CONFIG_SECURITY_PATH conditional. Otherwise, the inode-based security > modules would need to enable all of the path hooks, even though they do not > use any of them. > > This only provides a hook at the point of setting a watch, and presumes > that permission to set a particular watch implies the ability to receive > all notification about that object which match the mask. This is all that > is required for SELinux. If other security modules require additional hooks > or infrastructure to control delivery of notification, these can be added > by them. It does not make sense for us to propose hooks for which we have > no implementation. The understanding that all notifications received by the > requesting application are all strictly of a type for which the application > has been granted permission shows that this implementation is sufficient in > its coverage. > > Security modules wishing to provide complete control over fanotify must > also implement a security_file_open hook that validates that the access > requested by the watching application is authorized. Fanotify has the issue > that it returns a file descriptor with the file mode specified during > fanotify_init() to the watching process on event. This is already covered > by the LSM security_file_open hook if the security module implements > checking of the requested file mode there. Otherwise, a watching process > can obtain escalated access to a file for which it has not been authorized. > > The selinux_path_notify hook implementation works by adding five new file > permissions: watch, watch_mount, watch_sb, watch_reads, and watch_with_perm > (descriptions about which will follow), and one new filesystem permission: > watch (which is applied to superblock checks). The hook then decides which > subset of these permissions must be held by the requesting application > based on the contents of the provided mask and the obj_type. The > selinux_file_open hook already checks the requested file mode and therefore > ensures that a watching process cannot escalate its access through > fanotify. > > The watch, watch_mount, and watch_sb permissions are the baseline > permissions for setting a watch on an object and each are a requirement for > any watch to be set on a file, mount, or superblock respectively. It should > be noted that having either of the other two permissions (watch_reads and > watch_with_perm) does not imply the watch, watch_mount, or watch_sb > permission. Superblock watches further require the filesystem watch > permission to the superblock. As there is no labeled object in view for > mounts, there is no specific check for mount watches beyond watch_mount to > the inode. Such a check could be added in the future, if a suitable labeled > object existed representing the mount. > > The watch_reads permission is required to receive notifications from > read-exclusive events on filesystem objects. These events include accessing > a file for the purpose of reading and closing a file which has been opened > read-only. This distinction has been drawn in order to provide a direct > indication in the policy for this otherwise not obvious capability. Read > access to a file should not necessarily imply the ability to observe read > events on a file. > > Finally, watch_with_perm only applies to fanotify masks since it is the > only way to set a mask which allows for the blocking, permission event. > This permission is needed for any watch which is of this type. Though > fanotify requires CAP_SYS_ADMIN, this is insufficient as it gives implicit > trust to root, which we do not do, and does not support least privilege. > > Signed-off-by: Aaron Goidel <acgoide@tycho.nsa.gov> I can't say that I accept your arguments that this is sufficient, but as you point out, the SELinux team does, and if I want more for Smack that's my fish to fry. Acked-by: Casey Schaufler <casey@schaufler-ca.com> > --- > fs/notify/dnotify/dnotify.c | 15 +++++++-- > fs/notify/fanotify/fanotify_user.c | 27 +++++++++++++++-- > fs/notify/inotify/inotify_user.c | 13 ++++++-- > include/linux/lsm_hooks.h | 9 +++++- > include/linux/security.h | 10 ++++-- > security/security.c | 6 ++++ > security/selinux/hooks.c | 47 +++++++++++++++++++++++++++++ > security/selinux/include/classmap.h | 5 +-- > 8 files changed, 120 insertions(+), 12 deletions(-) > > diff --git a/fs/notify/dnotify/dnotify.c b/fs/notify/dnotify/dnotify.c > index 250369d6901d..87a7f61bc91c 100644 > --- a/fs/notify/dnotify/dnotify.c > +++ b/fs/notify/dnotify/dnotify.c > @@ -22,6 +22,7 @@ > #include <linux/sched/signal.h> > #include <linux/dnotify.h> > #include <linux/init.h> > +#include <linux/security.h> > #include <linux/spinlock.h> > #include <linux/slab.h> > #include <linux/fdtable.h> > @@ -288,6 +289,17 @@ int fcntl_dirnotify(int fd, struct file *filp, unsigned long arg) > goto out_err; > } > > + /* > + * convert the userspace DN_* "arg" to the internal FS_* > + * defined in fsnotify > + */ > + mask = convert_arg(arg); > + > + error = security_path_notify(&filp->f_path, mask, > + FSNOTIFY_OBJ_TYPE_INODE); > + if (error) > + goto out_err; > + > /* expect most fcntl to add new rather than augment old */ > dn = kmem_cache_alloc(dnotify_struct_cache, GFP_KERNEL); > if (!dn) { > @@ -302,9 +314,6 @@ int fcntl_dirnotify(int fd, struct file *filp, unsigned long arg) > goto out_err; > } > > - /* convert the userspace DN_* "arg" to the internal FS_* defines in fsnotify */ > - mask = convert_arg(arg); > - > /* set up the new_fsn_mark and new_dn_mark */ > new_fsn_mark = &new_dn_mark->fsn_mark; > fsnotify_init_mark(new_fsn_mark, dnotify_group); > diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c > index a90bb19dcfa2..b83f27021f54 100644 > --- a/fs/notify/fanotify/fanotify_user.c > +++ b/fs/notify/fanotify/fanotify_user.c > @@ -528,9 +528,10 @@ static const struct file_operations fanotify_fops = { > }; > > static int fanotify_find_path(int dfd, const char __user *filename, > - struct path *path, unsigned int flags) > + struct path *path, unsigned int flags, __u64 mask) > { > int ret; > + unsigned int obj_type; > > pr_debug("%s: dfd=%d filename=%p flags=%x\n", __func__, > dfd, filename, flags); > @@ -567,8 +568,30 @@ static int fanotify_find_path(int dfd, const char __user *filename, > > /* you can only watch an inode if you have read permissions on it */ > ret = inode_permission(path->dentry->d_inode, MAY_READ); > + if (ret) { > + path_put(path); > + goto out; > + } > + > + switch (flags & FANOTIFY_MARK_TYPE_BITS) { > + case FAN_MARK_MOUNT: > + obj_type = FSNOTIFY_OBJ_TYPE_VFSMOUNT; > + break; > + case FAN_MARK_FILESYSTEM: > + obj_type = FSNOTIFY_OBJ_TYPE_SB; > + break; > + case FAN_MARK_INODE: > + obj_type = FSNOTIFY_OBJ_TYPE_INODE; > + break; > + default: > + ret = -EINVAL; > + goto out; > + } > + > + ret = security_path_notify(path, mask, obj_type); > if (ret) > path_put(path); > + > out: > return ret; > } > @@ -1014,7 +1037,7 @@ static int do_fanotify_mark(int fanotify_fd, unsigned int flags, __u64 mask, > goto fput_and_out; > } > > - ret = fanotify_find_path(dfd, pathname, &path, flags); > + ret = fanotify_find_path(dfd, pathname, &path, flags, mask); > if (ret) > goto fput_and_out; > > diff --git a/fs/notify/inotify/inotify_user.c b/fs/notify/inotify/inotify_user.c > index 7b53598c8804..e0d593c2d437 100644 > --- a/fs/notify/inotify/inotify_user.c > +++ b/fs/notify/inotify/inotify_user.c > @@ -39,6 +39,7 @@ > #include <linux/poll.h> > #include <linux/wait.h> > #include <linux/memcontrol.h> > +#include <linux/security.h> > > #include "inotify.h" > #include "../fdinfo.h" > @@ -342,7 +343,8 @@ static const struct file_operations inotify_fops = { > /* > * find_inode - resolve a user-given path to a specific inode > */ > -static int inotify_find_inode(const char __user *dirname, struct path *path, unsigned flags) > +static int inotify_find_inode(const char __user *dirname, struct path *path, > + unsigned int flags, __u64 mask) > { > int error; > > @@ -351,8 +353,15 @@ static int inotify_find_inode(const char __user *dirname, struct path *path, uns > return error; > /* you can only watch an inode if you have read permissions on it */ > error = inode_permission(path->dentry->d_inode, MAY_READ); > + if (error) { > + path_put(path); > + return error; > + } > + error = security_path_notify(path, mask, > + FSNOTIFY_OBJ_TYPE_INODE); > if (error) > path_put(path); > + > return error; > } > > @@ -744,7 +753,7 @@ SYSCALL_DEFINE3(inotify_add_watch, int, fd, const char __user *, pathname, > if (mask & IN_ONLYDIR) > flags |= LOOKUP_DIRECTORY; > > - ret = inotify_find_inode(pathname, &path, flags); > + ret = inotify_find_inode(pathname, &path, flags, mask); > if (ret) > goto fput_and_out; > > diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h > index 47f58cfb6a19..ead98af9c602 100644 > --- a/include/linux/lsm_hooks.h > +++ b/include/linux/lsm_hooks.h > @@ -339,6 +339,9 @@ > * Check for permission to change root directory. > * @path contains the path structure. > * Return 0 if permission is granted. > + * @path_notify: > + * Check permissions before setting a watch on events as defined by @mask, > + * on an object at @path, whose type is defined by @obj_type. > * @inode_readlink: > * Check the permission to read the symbolic link. > * @dentry contains the dentry structure for the file link. > @@ -1535,7 +1538,9 @@ union security_list_options { > int (*path_chown)(const struct path *path, kuid_t uid, kgid_t gid); > int (*path_chroot)(const struct path *path); > #endif > - > + /* Needed for inode based security check */ > + int (*path_notify)(const struct path *path, u64 mask, > + unsigned int obj_type); > int (*inode_alloc_security)(struct inode *inode); > void (*inode_free_security)(struct inode *inode); > int (*inode_init_security)(struct inode *inode, struct inode *dir, > @@ -1860,6 +1865,8 @@ struct security_hook_heads { > struct hlist_head path_chown; > struct hlist_head path_chroot; > #endif > + /* Needed for inode based modules as well */ > + struct hlist_head path_notify; > struct hlist_head inode_alloc_security; > struct hlist_head inode_free_security; > struct hlist_head inode_init_security; > diff --git a/include/linux/security.h b/include/linux/security.h > index 659071c2e57c..7d9c1da1f659 100644 > --- a/include/linux/security.h > +++ b/include/linux/security.h > @@ -259,7 +259,8 @@ int security_dentry_create_files_as(struct dentry *dentry, int mode, > struct qstr *name, > const struct cred *old, > struct cred *new); > - > +int security_path_notify(const struct path *path, u64 mask, > + unsigned int obj_type); > int security_inode_alloc(struct inode *inode); > void security_inode_free(struct inode *inode); > int security_inode_init_security(struct inode *inode, struct inode *dir, > @@ -387,7 +388,6 @@ int security_ismaclabel(const char *name); > int security_secid_to_secctx(u32 secid, char **secdata, u32 *seclen); > int security_secctx_to_secid(const char *secdata, u32 seclen, u32 *secid); > void security_release_secctx(char *secdata, u32 seclen); > - > void security_inode_invalidate_secctx(struct inode *inode); > int security_inode_notifysecctx(struct inode *inode, void *ctx, u32 ctxlen); > int security_inode_setsecctx(struct dentry *dentry, void *ctx, u32 ctxlen); > @@ -621,6 +621,12 @@ static inline int security_move_mount(const struct path *from_path, > return 0; > } > > +static inline int security_path_notify(const struct path *path, u64 mask, > + unsigned int obj_type) > +{ > + return 0; > +} > + > static inline int security_inode_alloc(struct inode *inode) > { > return 0; > diff --git a/security/security.c b/security/security.c > index 613a5c00e602..30687e1366b7 100644 > --- a/security/security.c > +++ b/security/security.c > @@ -871,6 +871,12 @@ int security_move_mount(const struct path *from_path, const struct path *to_path > return call_int_hook(move_mount, 0, from_path, to_path); > } > > +int security_path_notify(const struct path *path, u64 mask, > + unsigned int obj_type) > +{ > + return call_int_hook(path_notify, 0, path, mask, obj_type); > +} > + > int security_inode_alloc(struct inode *inode) > { > int rc = lsm_inode_alloc(inode); > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c > index c61787b15f27..a1aaf79421df 100644 > --- a/security/selinux/hooks.c > +++ b/security/selinux/hooks.c > @@ -92,6 +92,8 @@ > #include <linux/kernfs.h> > #include <linux/stringhash.h> /* for hashlen_string() */ > #include <uapi/linux/mount.h> > +#include <linux/fsnotify.h> > +#include <linux/fanotify.h> > > #include "avc.h" > #include "objsec.h" > @@ -3261,6 +3263,50 @@ static int selinux_inode_removexattr(struct dentry *dentry, const char *name) > return -EACCES; > } > > +static int selinux_path_notify(const struct path *path, u64 mask, > + unsigned int obj_type) > +{ > + int ret; > + u32 perm; > + > + struct common_audit_data ad; > + > + ad.type = LSM_AUDIT_DATA_PATH; > + ad.u.path = *path; > + > + /* > + * Set permission needed based on the type of mark being set. > + * Performs an additional check for sb watches. > + */ > + switch (obj_type) { > + case FSNOTIFY_OBJ_TYPE_VFSMOUNT: > + perm = FILE__WATCH_MOUNT; > + break; > + case FSNOTIFY_OBJ_TYPE_SB: > + perm = FILE__WATCH_SB; > + ret = superblock_has_perm(current_cred(), path->dentry->d_sb, > + FILESYSTEM__WATCH, &ad); > + if (ret) > + return ret; > + break; > + case FSNOTIFY_OBJ_TYPE_INODE: > + perm = FILE__WATCH; > + break; > + default: > + return -EINVAL; > + } > + > + // check if the mask is requesting ability to set a blocking watch > + if (mask & (ALL_FSNOTIFY_PERM_EVENTS)) > + perm |= FILE__WATCH_WITH_PERM; // if so, check that permission > + > + // is the mask asking to watch file reads? > + if (mask & (FS_ACCESS | FS_ACCESS_PERM | FS_CLOSE_NOWRITE)) > + perm |= FILE__WATCH_READS; // check that permission as well > + > + return path_has_perm(current_cred(), path, perm); > +} > + > /* > * Copy the inode security context value to the user. > * > @@ -6797,6 +6843,7 @@ static struct security_hook_list selinux_hooks[] __lsm_ro_after_init = { > LSM_HOOK_INIT(inode_getsecid, selinux_inode_getsecid), > LSM_HOOK_INIT(inode_copy_up, selinux_inode_copy_up), > LSM_HOOK_INIT(inode_copy_up_xattr, selinux_inode_copy_up_xattr), > + LSM_HOOK_INIT(path_notify, selinux_path_notify), > > LSM_HOOK_INIT(kernfs_init_security, selinux_kernfs_init_security), > > diff --git a/security/selinux/include/classmap.h b/security/selinux/include/classmap.h > index 201f7e588a29..32e9b03be3dd 100644 > --- a/security/selinux/include/classmap.h > +++ b/security/selinux/include/classmap.h > @@ -7,7 +7,8 @@ > > #define COMMON_FILE_PERMS COMMON_FILE_SOCK_PERMS, "unlink", "link", \ > "rename", "execute", "quotaon", "mounton", "audit_access", \ > - "open", "execmod" > + "open", "execmod", "watch", "watch_mount", "watch_sb", \ > + "watch_with_perm", "watch_reads" > > #define COMMON_SOCK_PERMS COMMON_FILE_SOCK_PERMS, "bind", "connect", \ > "listen", "accept", "getopt", "setopt", "shutdown", "recvfrom", \ > @@ -60,7 +61,7 @@ struct security_class_mapping secclass_map[] = { > { "filesystem", > { "mount", "remount", "unmount", "getattr", > "relabelfrom", "relabelto", "associate", "quotamod", > - "quotaget", NULL } }, > + "quotaget", "watch", NULL } }, > { "file", > { COMMON_FILE_PERMS, > "execute_no_trans", "entrypoint", NULL } },
On Wed, Jul 31, 2019 at 1:26 PM Casey Schaufler <casey@schaufler-ca.com> wrote: > On 7/31/2019 8:34 AM, Aaron Goidel wrote: > > As of now, setting watches on filesystem objects has, at most, applied a > > check for read access to the inode, and in the case of fanotify, requires > > CAP_SYS_ADMIN. No specific security hook or permission check has been > > provided to control the setting of watches. Using any of inotify, dnotify, > > or fanotify, it is possible to observe, not only write-like operations, but > > even read access to a file. Modeling the watch as being merely a read from > > the file is insufficient for the needs of SELinux. This is due to the fact > > that read access should not necessarily imply access to information about > > when another process reads from a file. Furthermore, fanotify watches grant > > more power to an application in the form of permission events. While > > notification events are solely, unidirectional (i.e. they only pass > > information to the receiving application), permission events are blocking. > > Permission events make a request to the receiving application which will > > then reply with a decision as to whether or not that action may be > > completed. This causes the issue of the watching application having the > > ability to exercise control over the triggering process. Without drawing a > > distinction within the permission check, the ability to read would imply > > the greater ability to control an application. Additionally, mount and > > superblock watches apply to all files within the same mount or superblock. > > Read access to one file should not necessarily imply the ability to watch > > all files accessed within a given mount or superblock. > > > > In order to solve these issues, a new LSM hook is implemented and has been > > placed within the system calls for marking filesystem objects with inotify, > > fanotify, and dnotify watches. These calls to the hook are placed at the > > point at which the target path has been resolved and are provided with the > > path struct, the mask of requested notification events, and the type of > > object on which the mark is being set (inode, superblock, or mount). The > > mask and obj_type have already been translated into common FS_* values > > shared by the entirety of the fs notification infrastructure. The path > > struct is passed rather than just the inode so that the mount is available, > > particularly for mount watches. This also allows for use of the hook by > > pathname-based security modules. However, since the hook is intended for > > use even by inode based security modules, it is not placed under the > > CONFIG_SECURITY_PATH conditional. Otherwise, the inode-based security > > modules would need to enable all of the path hooks, even though they do not > > use any of them. > > > > This only provides a hook at the point of setting a watch, and presumes > > that permission to set a particular watch implies the ability to receive > > all notification about that object which match the mask. This is all that > > is required for SELinux. If other security modules require additional hooks > > or infrastructure to control delivery of notification, these can be added > > by them. It does not make sense for us to propose hooks for which we have > > no implementation. The understanding that all notifications received by the > > requesting application are all strictly of a type for which the application > > has been granted permission shows that this implementation is sufficient in > > its coverage. > > > > Security modules wishing to provide complete control over fanotify must > > also implement a security_file_open hook that validates that the access > > requested by the watching application is authorized. Fanotify has the issue > > that it returns a file descriptor with the file mode specified during > > fanotify_init() to the watching process on event. This is already covered > > by the LSM security_file_open hook if the security module implements > > checking of the requested file mode there. Otherwise, a watching process > > can obtain escalated access to a file for which it has not been authorized. > > > > The selinux_path_notify hook implementation works by adding five new file > > permissions: watch, watch_mount, watch_sb, watch_reads, and watch_with_perm > > (descriptions about which will follow), and one new filesystem permission: > > watch (which is applied to superblock checks). The hook then decides which > > subset of these permissions must be held by the requesting application > > based on the contents of the provided mask and the obj_type. The > > selinux_file_open hook already checks the requested file mode and therefore > > ensures that a watching process cannot escalate its access through > > fanotify. > > > > The watch, watch_mount, and watch_sb permissions are the baseline > > permissions for setting a watch on an object and each are a requirement for > > any watch to be set on a file, mount, or superblock respectively. It should > > be noted that having either of the other two permissions (watch_reads and > > watch_with_perm) does not imply the watch, watch_mount, or watch_sb > > permission. Superblock watches further require the filesystem watch > > permission to the superblock. As there is no labeled object in view for > > mounts, there is no specific check for mount watches beyond watch_mount to > > the inode. Such a check could be added in the future, if a suitable labeled > > object existed representing the mount. > > > > The watch_reads permission is required to receive notifications from > > read-exclusive events on filesystem objects. These events include accessing > > a file for the purpose of reading and closing a file which has been opened > > read-only. This distinction has been drawn in order to provide a direct > > indication in the policy for this otherwise not obvious capability. Read > > access to a file should not necessarily imply the ability to observe read > > events on a file. > > > > Finally, watch_with_perm only applies to fanotify masks since it is the > > only way to set a mask which allows for the blocking, permission event. > > This permission is needed for any watch which is of this type. Though > > fanotify requires CAP_SYS_ADMIN, this is insufficient as it gives implicit > > trust to root, which we do not do, and does not support least privilege. > > > > Signed-off-by: Aaron Goidel <acgoide@tycho.nsa.gov> > > I can't say that I accept your arguments that this is sufficient, > but as you point out, the SELinux team does, and if I want more > for Smack that's my fish to fry. > > Acked-by: Casey Schaufler <casey@schaufler-ca.com> Thanks Aaron. Thanks Casey. I think we also want an ACK from the other LSMs, what say all of you? Can you live with the new security_path_notify() hook? Aaron, you'll also need to put together a test for the selinux-testsuite to exercise this code. If you already sent it to the list, my apologies but I don't see it anywhere. If you get stuck on the test, let me know and I'll try to help out. Oh, one more thing ... > > +static int selinux_path_notify(const struct path *path, u64 mask, > > + unsigned int obj_type) > > +{ > > + int ret; > > + u32 perm; > > + > > + struct common_audit_data ad; > > + > > + ad.type = LSM_AUDIT_DATA_PATH; > > + ad.u.path = *path; > > + > > + /* > > + * Set permission needed based on the type of mark being set. > > + * Performs an additional check for sb watches. > > + */ > > + switch (obj_type) { > > + case FSNOTIFY_OBJ_TYPE_VFSMOUNT: > > + perm = FILE__WATCH_MOUNT; > > + break; > > + case FSNOTIFY_OBJ_TYPE_SB: > > + perm = FILE__WATCH_SB; > > + ret = superblock_has_perm(current_cred(), path->dentry->d_sb, > > + FILESYSTEM__WATCH, &ad); > > + if (ret) > > + return ret; > > + break; > > + case FSNOTIFY_OBJ_TYPE_INODE: > > + perm = FILE__WATCH; > > + break; > > + default: > > + return -EINVAL; > > + } > > + > > + // check if the mask is requesting ability to set a blocking watch ... in the future please don't use "// XXX", use "/* XXX */" instead :) Don't respin the patch just for this, but if you have to do it for some other reason please fix the C++ style comments. Thanks. > > + if (mask & (ALL_FSNOTIFY_PERM_EVENTS)) > > + perm |= FILE__WATCH_WITH_PERM; // if so, check that permission > > + > > + // is the mask asking to watch file reads? > > + if (mask & (FS_ACCESS | FS_ACCESS_PERM | FS_CLOSE_NOWRITE)) > > + perm |= FILE__WATCH_READS; // check that permission as well > > + > > + return path_has_perm(current_cred(), path, perm); > > +}
On 7/31/19 8:27 PM, Paul Moore wrote: > On Wed, Jul 31, 2019 at 1:26 PM Casey Schaufler <casey@schaufler-ca.com> wrote: >> On 7/31/2019 8:34 AM, Aaron Goidel wrote: >>> As of now, setting watches on filesystem objects has, at most, applied a >>> check for read access to the inode, and in the case of fanotify, requires >>> CAP_SYS_ADMIN. No specific security hook or permission check has been >>> provided to control the setting of watches. Using any of inotify, dnotify, >>> or fanotify, it is possible to observe, not only write-like operations, but >>> even read access to a file. Modeling the watch as being merely a read from >>> the file is insufficient for the needs of SELinux. This is due to the fact >>> that read access should not necessarily imply access to information about >>> when another process reads from a file. Furthermore, fanotify watches grant >>> more power to an application in the form of permission events. While >>> notification events are solely, unidirectional (i.e. they only pass >>> information to the receiving application), permission events are blocking. >>> Permission events make a request to the receiving application which will >>> then reply with a decision as to whether or not that action may be >>> completed. This causes the issue of the watching application having the >>> ability to exercise control over the triggering process. Without drawing a >>> distinction within the permission check, the ability to read would imply >>> the greater ability to control an application. Additionally, mount and >>> superblock watches apply to all files within the same mount or superblock. >>> Read access to one file should not necessarily imply the ability to watch >>> all files accessed within a given mount or superblock. >>> >>> In order to solve these issues, a new LSM hook is implemented and has been >>> placed within the system calls for marking filesystem objects with inotify, >>> fanotify, and dnotify watches. These calls to the hook are placed at the >>> point at which the target path has been resolved and are provided with the >>> path struct, the mask of requested notification events, and the type of >>> object on which the mark is being set (inode, superblock, or mount). The >>> mask and obj_type have already been translated into common FS_* values >>> shared by the entirety of the fs notification infrastructure. The path >>> struct is passed rather than just the inode so that the mount is available, >>> particularly for mount watches. This also allows for use of the hook by >>> pathname-based security modules. However, since the hook is intended for >>> use even by inode based security modules, it is not placed under the >>> CONFIG_SECURITY_PATH conditional. Otherwise, the inode-based security >>> modules would need to enable all of the path hooks, even though they do not >>> use any of them. >>> >>> This only provides a hook at the point of setting a watch, and presumes >>> that permission to set a particular watch implies the ability to receive >>> all notification about that object which match the mask. This is all that >>> is required for SELinux. If other security modules require additional hooks >>> or infrastructure to control delivery of notification, these can be added >>> by them. It does not make sense for us to propose hooks for which we have >>> no implementation. The understanding that all notifications received by the >>> requesting application are all strictly of a type for which the application >>> has been granted permission shows that this implementation is sufficient in >>> its coverage. >>> >>> Security modules wishing to provide complete control over fanotify must >>> also implement a security_file_open hook that validates that the access >>> requested by the watching application is authorized. Fanotify has the issue >>> that it returns a file descriptor with the file mode specified during >>> fanotify_init() to the watching process on event. This is already covered >>> by the LSM security_file_open hook if the security module implements >>> checking of the requested file mode there. Otherwise, a watching process >>> can obtain escalated access to a file for which it has not been authorized. >>> >>> The selinux_path_notify hook implementation works by adding five new file >>> permissions: watch, watch_mount, watch_sb, watch_reads, and watch_with_perm >>> (descriptions about which will follow), and one new filesystem permission: >>> watch (which is applied to superblock checks). The hook then decides which >>> subset of these permissions must be held by the requesting application >>> based on the contents of the provided mask and the obj_type. The >>> selinux_file_open hook already checks the requested file mode and therefore >>> ensures that a watching process cannot escalate its access through >>> fanotify. >>> >>> The watch, watch_mount, and watch_sb permissions are the baseline >>> permissions for setting a watch on an object and each are a requirement for >>> any watch to be set on a file, mount, or superblock respectively. It should >>> be noted that having either of the other two permissions (watch_reads and >>> watch_with_perm) does not imply the watch, watch_mount, or watch_sb >>> permission. Superblock watches further require the filesystem watch >>> permission to the superblock. As there is no labeled object in view for >>> mounts, there is no specific check for mount watches beyond watch_mount to >>> the inode. Such a check could be added in the future, if a suitable labeled >>> object existed representing the mount. >>> >>> The watch_reads permission is required to receive notifications from >>> read-exclusive events on filesystem objects. These events include accessing >>> a file for the purpose of reading and closing a file which has been opened >>> read-only. This distinction has been drawn in order to provide a direct >>> indication in the policy for this otherwise not obvious capability. Read >>> access to a file should not necessarily imply the ability to observe read >>> events on a file. >>> >>> Finally, watch_with_perm only applies to fanotify masks since it is the >>> only way to set a mask which allows for the blocking, permission event. >>> This permission is needed for any watch which is of this type. Though >>> fanotify requires CAP_SYS_ADMIN, this is insufficient as it gives implicit >>> trust to root, which we do not do, and does not support least privilege. >>> >>> Signed-off-by: Aaron Goidel <acgoide@tycho.nsa.gov> >> >> I can't say that I accept your arguments that this is sufficient, >> but as you point out, the SELinux team does, and if I want more >> for Smack that's my fish to fry. >> >> Acked-by: Casey Schaufler <casey@schaufler-ca.com> > > Thanks Aaron. Thanks Casey. > > I think we also want an ACK from the other LSMs, what say all of you? > Can you live with the new security_path_notify() hook? > > Aaron, you'll also need to put together a test for the > selinux-testsuite to exercise this code. If you already sent it to > the list, my apologies but I don't see it anywhere. If you get stuck > on the test, let me know and I'll try to help out. > > Oh, one more thing ... > >>> +static int selinux_path_notify(const struct path *path, u64 mask, >>> + unsigned int obj_type) >>> +{ >>> + int ret; >>> + u32 perm; >>> + >>> + struct common_audit_data ad; >>> + >>> + ad.type = LSM_AUDIT_DATA_PATH; >>> + ad.u.path = *path; >>> + >>> + /* >>> + * Set permission needed based on the type of mark being set. >>> + * Performs an additional check for sb watches. >>> + */ >>> + switch (obj_type) { >>> + case FSNOTIFY_OBJ_TYPE_VFSMOUNT: >>> + perm = FILE__WATCH_MOUNT; >>> + break; >>> + case FSNOTIFY_OBJ_TYPE_SB: >>> + perm = FILE__WATCH_SB; >>> + ret = superblock_has_perm(current_cred(), path->dentry->d_sb, >>> + FILESYSTEM__WATCH, &ad); >>> + if (ret) >>> + return ret; >>> + break; >>> + case FSNOTIFY_OBJ_TYPE_INODE: >>> + perm = FILE__WATCH; >>> + break; >>> + default: >>> + return -EINVAL; >>> + } >>> + >>> + // check if the mask is requesting ability to set a blocking watch > > ... in the future please don't use "// XXX", use "/* XXX */" instead :) > > Don't respin the patch just for this, but if you have to do it for > some other reason please fix the C++ style comments. Thanks. This was discussed during the earlier RFC series but ultimately someone pointed to: https://lkml.org/lkml/2016/7/8/625 where Linus blessed the use of C++/C99 style comments. And checkpatch accepts them these days. Obviously if you truly don't want them in the SELinux code, that's your call. But note that all files now have at least one such comment as a result of the mass SPDX license headers that were added throughout the tree using that style. > >>> + if (mask & (ALL_FSNOTIFY_PERM_EVENTS)) >>> + perm |= FILE__WATCH_WITH_PERM; // if so, check that permission >>> + >>> + // is the mask asking to watch file reads? >>> + if (mask & (FS_ACCESS | FS_ACCESS_PERM | FS_CLOSE_NOWRITE)) >>> + perm |= FILE__WATCH_READS; // check that permission as well >>> + >>> + return path_has_perm(current_cred(), path, perm); >>> +} >
On Thu, Aug 1, 2019 at 7:31 AM Stephen Smalley <sds@tycho.nsa.gov> wrote: > On 7/31/19 8:27 PM, Paul Moore wrote: > > On Wed, Jul 31, 2019 at 1:26 PM Casey Schaufler <casey@schaufler-ca.com> wrote: > >> On 7/31/2019 8:34 AM, Aaron Goidel wrote: ... > >>> +static int selinux_path_notify(const struct path *path, u64 mask, > >>> + unsigned int obj_type) > >>> +{ > >>> + int ret; > >>> + u32 perm; > >>> + > >>> + struct common_audit_data ad; > >>> + > >>> + ad.type = LSM_AUDIT_DATA_PATH; > >>> + ad.u.path = *path; > >>> + > >>> + /* > >>> + * Set permission needed based on the type of mark being set. > >>> + * Performs an additional check for sb watches. > >>> + */ > >>> + switch (obj_type) { > >>> + case FSNOTIFY_OBJ_TYPE_VFSMOUNT: > >>> + perm = FILE__WATCH_MOUNT; > >>> + break; > >>> + case FSNOTIFY_OBJ_TYPE_SB: > >>> + perm = FILE__WATCH_SB; > >>> + ret = superblock_has_perm(current_cred(), path->dentry->d_sb, > >>> + FILESYSTEM__WATCH, &ad); > >>> + if (ret) > >>> + return ret; > >>> + break; > >>> + case FSNOTIFY_OBJ_TYPE_INODE: > >>> + perm = FILE__WATCH; > >>> + break; > >>> + default: > >>> + return -EINVAL; > >>> + } > >>> + > >>> + // check if the mask is requesting ability to set a blocking watch > > > > ... in the future please don't use "// XXX", use "/* XXX */" instead :) > > > > Don't respin the patch just for this, but if you have to do it for > > some other reason please fix the C++ style comments. Thanks. > > This was discussed during the earlier RFC series but ultimately someone > pointed to: > https://lkml.org/lkml/2016/7/8/625 > where Linus blessed the use of C++/C99 style comments. And checkpatch > accepts them these days. Yep, I'm aware of both, it is simply a personal preference of mine. I'm not going to reject patches with C++ style comments, but I would ask people to stick to the good ol' fashioned comments for patches they submit. > Obviously if you truly don't want them in the SELinux code, that's your > call. But note that all files now have at least one such comment as a > result of the mass SPDX license headers that were added throughout the > tree using that style. FYI, the sky is blue. It isn't just the license headers either, Al dropped one into hooks.c :). Just like I don't plan to reject patches due only to the comment style, you don't see me pushing patches to change the C++ comments.
On Wed, Jul 31, 2019 at 11:35 AM Aaron Goidel <acgoide@tycho.nsa.gov> wrote: > As of now, setting watches on filesystem objects has, at most, applied a > check for read access to the inode, and in the case of fanotify, requires > CAP_SYS_ADMIN. No specific security hook or permission check has been > provided to control the setting of watches. Using any of inotify, dnotify, > or fanotify, it is possible to observe, not only write-like operations, but > even read access to a file. Modeling the watch as being merely a read from > the file is insufficient for the needs of SELinux. This is due to the fact > that read access should not necessarily imply access to information about > when another process reads from a file. Furthermore, fanotify watches grant > more power to an application in the form of permission events. While > notification events are solely, unidirectional (i.e. they only pass > information to the receiving application), permission events are blocking. > Permission events make a request to the receiving application which will > then reply with a decision as to whether or not that action may be > completed. This causes the issue of the watching application having the > ability to exercise control over the triggering process. Without drawing a > distinction within the permission check, the ability to read would imply > the greater ability to control an application. Additionally, mount and > superblock watches apply to all files within the same mount or superblock. > Read access to one file should not necessarily imply the ability to watch > all files accessed within a given mount or superblock. > > In order to solve these issues, a new LSM hook is implemented and has been > placed within the system calls for marking filesystem objects with inotify, > fanotify, and dnotify watches. These calls to the hook are placed at the > point at which the target path has been resolved and are provided with the > path struct, the mask of requested notification events, and the type of > object on which the mark is being set (inode, superblock, or mount). The > mask and obj_type have already been translated into common FS_* values > shared by the entirety of the fs notification infrastructure. The path > struct is passed rather than just the inode so that the mount is available, > particularly for mount watches. This also allows for use of the hook by > pathname-based security modules. However, since the hook is intended for > use even by inode based security modules, it is not placed under the > CONFIG_SECURITY_PATH conditional. Otherwise, the inode-based security > modules would need to enable all of the path hooks, even though they do not > use any of them. > > This only provides a hook at the point of setting a watch, and presumes > that permission to set a particular watch implies the ability to receive > all notification about that object which match the mask. This is all that > is required for SELinux. If other security modules require additional hooks > or infrastructure to control delivery of notification, these can be added > by them. It does not make sense for us to propose hooks for which we have > no implementation. The understanding that all notifications received by the > requesting application are all strictly of a type for which the application > has been granted permission shows that this implementation is sufficient in > its coverage. > > Security modules wishing to provide complete control over fanotify must > also implement a security_file_open hook that validates that the access > requested by the watching application is authorized. Fanotify has the issue > that it returns a file descriptor with the file mode specified during > fanotify_init() to the watching process on event. This is already covered > by the LSM security_file_open hook if the security module implements > checking of the requested file mode there. Otherwise, a watching process > can obtain escalated access to a file for which it has not been authorized. > > The selinux_path_notify hook implementation works by adding five new file > permissions: watch, watch_mount, watch_sb, watch_reads, and watch_with_perm > (descriptions about which will follow), and one new filesystem permission: > watch (which is applied to superblock checks). The hook then decides which > subset of these permissions must be held by the requesting application > based on the contents of the provided mask and the obj_type. The > selinux_file_open hook already checks the requested file mode and therefore > ensures that a watching process cannot escalate its access through > fanotify. > > The watch, watch_mount, and watch_sb permissions are the baseline > permissions for setting a watch on an object and each are a requirement for > any watch to be set on a file, mount, or superblock respectively. It should > be noted that having either of the other two permissions (watch_reads and > watch_with_perm) does not imply the watch, watch_mount, or watch_sb > permission. Superblock watches further require the filesystem watch > permission to the superblock. As there is no labeled object in view for > mounts, there is no specific check for mount watches beyond watch_mount to > the inode. Such a check could be added in the future, if a suitable labeled > object existed representing the mount. > > The watch_reads permission is required to receive notifications from > read-exclusive events on filesystem objects. These events include accessing > a file for the purpose of reading and closing a file which has been opened > read-only. This distinction has been drawn in order to provide a direct > indication in the policy for this otherwise not obvious capability. Read > access to a file should not necessarily imply the ability to observe read > events on a file. > > Finally, watch_with_perm only applies to fanotify masks since it is the > only way to set a mask which allows for the blocking, permission event. > This permission is needed for any watch which is of this type. Though > fanotify requires CAP_SYS_ADMIN, this is insufficient as it gives implicit > trust to root, which we do not do, and does not support least privilege. > > Signed-off-by: Aaron Goidel <acgoide@tycho.nsa.gov> > --- > fs/notify/dnotify/dnotify.c | 15 +++++++-- > fs/notify/fanotify/fanotify_user.c | 27 +++++++++++++++-- > fs/notify/inotify/inotify_user.c | 13 ++++++-- > include/linux/lsm_hooks.h | 9 +++++- > include/linux/security.h | 10 ++++-- > security/security.c | 6 ++++ > security/selinux/hooks.c | 47 +++++++++++++++++++++++++++++ > security/selinux/include/classmap.h | 5 +-- > 8 files changed, 120 insertions(+), 12 deletions(-) Other than Casey's comments, and ACK, I'm not seeing much commentary on this patch so FS and LSM folks consider this your last chance - if I don't hear any objections by the end of this week I'll plan on merging this into selinux/next next week. > diff --git a/fs/notify/dnotify/dnotify.c b/fs/notify/dnotify/dnotify.c > index 250369d6901d..87a7f61bc91c 100644 > --- a/fs/notify/dnotify/dnotify.c > +++ b/fs/notify/dnotify/dnotify.c > @@ -22,6 +22,7 @@ > #include <linux/sched/signal.h> > #include <linux/dnotify.h> > #include <linux/init.h> > +#include <linux/security.h> > #include <linux/spinlock.h> > #include <linux/slab.h> > #include <linux/fdtable.h> > @@ -288,6 +289,17 @@ int fcntl_dirnotify(int fd, struct file *filp, unsigned long arg) > goto out_err; > } > > + /* > + * convert the userspace DN_* "arg" to the internal FS_* > + * defined in fsnotify > + */ > + mask = convert_arg(arg); > + > + error = security_path_notify(&filp->f_path, mask, > + FSNOTIFY_OBJ_TYPE_INODE); > + if (error) > + goto out_err; > + > /* expect most fcntl to add new rather than augment old */ > dn = kmem_cache_alloc(dnotify_struct_cache, GFP_KERNEL); > if (!dn) { > @@ -302,9 +314,6 @@ int fcntl_dirnotify(int fd, struct file *filp, unsigned long arg) > goto out_err; > } > > - /* convert the userspace DN_* "arg" to the internal FS_* defines in fsnotify */ > - mask = convert_arg(arg); > - > /* set up the new_fsn_mark and new_dn_mark */ > new_fsn_mark = &new_dn_mark->fsn_mark; > fsnotify_init_mark(new_fsn_mark, dnotify_group); > diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c > index a90bb19dcfa2..b83f27021f54 100644 > --- a/fs/notify/fanotify/fanotify_user.c > +++ b/fs/notify/fanotify/fanotify_user.c > @@ -528,9 +528,10 @@ static const struct file_operations fanotify_fops = { > }; > > static int fanotify_find_path(int dfd, const char __user *filename, > - struct path *path, unsigned int flags) > + struct path *path, unsigned int flags, __u64 mask) > { > int ret; > + unsigned int obj_type; > > pr_debug("%s: dfd=%d filename=%p flags=%x\n", __func__, > dfd, filename, flags); > @@ -567,8 +568,30 @@ static int fanotify_find_path(int dfd, const char __user *filename, > > /* you can only watch an inode if you have read permissions on it */ > ret = inode_permission(path->dentry->d_inode, MAY_READ); > + if (ret) { > + path_put(path); > + goto out; > + } > + > + switch (flags & FANOTIFY_MARK_TYPE_BITS) { > + case FAN_MARK_MOUNT: > + obj_type = FSNOTIFY_OBJ_TYPE_VFSMOUNT; > + break; > + case FAN_MARK_FILESYSTEM: > + obj_type = FSNOTIFY_OBJ_TYPE_SB; > + break; > + case FAN_MARK_INODE: > + obj_type = FSNOTIFY_OBJ_TYPE_INODE; > + break; > + default: > + ret = -EINVAL; > + goto out; > + } > + > + ret = security_path_notify(path, mask, obj_type); > if (ret) > path_put(path); > + > out: > return ret; > } > @@ -1014,7 +1037,7 @@ static int do_fanotify_mark(int fanotify_fd, unsigned int flags, __u64 mask, > goto fput_and_out; > } > > - ret = fanotify_find_path(dfd, pathname, &path, flags); > + ret = fanotify_find_path(dfd, pathname, &path, flags, mask); > if (ret) > goto fput_and_out; > > diff --git a/fs/notify/inotify/inotify_user.c b/fs/notify/inotify/inotify_user.c > index 7b53598c8804..e0d593c2d437 100644 > --- a/fs/notify/inotify/inotify_user.c > +++ b/fs/notify/inotify/inotify_user.c > @@ -39,6 +39,7 @@ > #include <linux/poll.h> > #include <linux/wait.h> > #include <linux/memcontrol.h> > +#include <linux/security.h> > > #include "inotify.h" > #include "../fdinfo.h" > @@ -342,7 +343,8 @@ static const struct file_operations inotify_fops = { > /* > * find_inode - resolve a user-given path to a specific inode > */ > -static int inotify_find_inode(const char __user *dirname, struct path *path, unsigned flags) > +static int inotify_find_inode(const char __user *dirname, struct path *path, > + unsigned int flags, __u64 mask) > { > int error; > > @@ -351,8 +353,15 @@ static int inotify_find_inode(const char __user *dirname, struct path *path, uns > return error; > /* you can only watch an inode if you have read permissions on it */ > error = inode_permission(path->dentry->d_inode, MAY_READ); > + if (error) { > + path_put(path); > + return error; > + } > + error = security_path_notify(path, mask, > + FSNOTIFY_OBJ_TYPE_INODE); > if (error) > path_put(path); > + > return error; > } > > @@ -744,7 +753,7 @@ SYSCALL_DEFINE3(inotify_add_watch, int, fd, const char __user *, pathname, > if (mask & IN_ONLYDIR) > flags |= LOOKUP_DIRECTORY; > > - ret = inotify_find_inode(pathname, &path, flags); > + ret = inotify_find_inode(pathname, &path, flags, mask); > if (ret) > goto fput_and_out; > > diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h > index 47f58cfb6a19..ead98af9c602 100644 > --- a/include/linux/lsm_hooks.h > +++ b/include/linux/lsm_hooks.h > @@ -339,6 +339,9 @@ > * Check for permission to change root directory. > * @path contains the path structure. > * Return 0 if permission is granted. > + * @path_notify: > + * Check permissions before setting a watch on events as defined by @mask, > + * on an object at @path, whose type is defined by @obj_type. > * @inode_readlink: > * Check the permission to read the symbolic link. > * @dentry contains the dentry structure for the file link. > @@ -1535,7 +1538,9 @@ union security_list_options { > int (*path_chown)(const struct path *path, kuid_t uid, kgid_t gid); > int (*path_chroot)(const struct path *path); > #endif > - > + /* Needed for inode based security check */ > + int (*path_notify)(const struct path *path, u64 mask, > + unsigned int obj_type); > int (*inode_alloc_security)(struct inode *inode); > void (*inode_free_security)(struct inode *inode); > int (*inode_init_security)(struct inode *inode, struct inode *dir, > @@ -1860,6 +1865,8 @@ struct security_hook_heads { > struct hlist_head path_chown; > struct hlist_head path_chroot; > #endif > + /* Needed for inode based modules as well */ > + struct hlist_head path_notify; > struct hlist_head inode_alloc_security; > struct hlist_head inode_free_security; > struct hlist_head inode_init_security; > diff --git a/include/linux/security.h b/include/linux/security.h > index 659071c2e57c..7d9c1da1f659 100644 > --- a/include/linux/security.h > +++ b/include/linux/security.h > @@ -259,7 +259,8 @@ int security_dentry_create_files_as(struct dentry *dentry, int mode, > struct qstr *name, > const struct cred *old, > struct cred *new); > - > +int security_path_notify(const struct path *path, u64 mask, > + unsigned int obj_type); > int security_inode_alloc(struct inode *inode); > void security_inode_free(struct inode *inode); > int security_inode_init_security(struct inode *inode, struct inode *dir, > @@ -387,7 +388,6 @@ int security_ismaclabel(const char *name); > int security_secid_to_secctx(u32 secid, char **secdata, u32 *seclen); > int security_secctx_to_secid(const char *secdata, u32 seclen, u32 *secid); > void security_release_secctx(char *secdata, u32 seclen); > - > void security_inode_invalidate_secctx(struct inode *inode); > int security_inode_notifysecctx(struct inode *inode, void *ctx, u32 ctxlen); > int security_inode_setsecctx(struct dentry *dentry, void *ctx, u32 ctxlen); > @@ -621,6 +621,12 @@ static inline int security_move_mount(const struct path *from_path, > return 0; > } > > +static inline int security_path_notify(const struct path *path, u64 mask, > + unsigned int obj_type) > +{ > + return 0; > +} > + > static inline int security_inode_alloc(struct inode *inode) > { > return 0; > diff --git a/security/security.c b/security/security.c > index 613a5c00e602..30687e1366b7 100644 > --- a/security/security.c > +++ b/security/security.c > @@ -871,6 +871,12 @@ int security_move_mount(const struct path *from_path, const struct path *to_path > return call_int_hook(move_mount, 0, from_path, to_path); > } > > +int security_path_notify(const struct path *path, u64 mask, > + unsigned int obj_type) > +{ > + return call_int_hook(path_notify, 0, path, mask, obj_type); > +} > + > int security_inode_alloc(struct inode *inode) > { > int rc = lsm_inode_alloc(inode); > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c > index c61787b15f27..a1aaf79421df 100644 > --- a/security/selinux/hooks.c > +++ b/security/selinux/hooks.c > @@ -92,6 +92,8 @@ > #include <linux/kernfs.h> > #include <linux/stringhash.h> /* for hashlen_string() */ > #include <uapi/linux/mount.h> > +#include <linux/fsnotify.h> > +#include <linux/fanotify.h> > > #include "avc.h" > #include "objsec.h" > @@ -3261,6 +3263,50 @@ static int selinux_inode_removexattr(struct dentry *dentry, const char *name) > return -EACCES; > } > > +static int selinux_path_notify(const struct path *path, u64 mask, > + unsigned int obj_type) > +{ > + int ret; > + u32 perm; > + > + struct common_audit_data ad; > + > + ad.type = LSM_AUDIT_DATA_PATH; > + ad.u.path = *path; > + > + /* > + * Set permission needed based on the type of mark being set. > + * Performs an additional check for sb watches. > + */ > + switch (obj_type) { > + case FSNOTIFY_OBJ_TYPE_VFSMOUNT: > + perm = FILE__WATCH_MOUNT; > + break; > + case FSNOTIFY_OBJ_TYPE_SB: > + perm = FILE__WATCH_SB; > + ret = superblock_has_perm(current_cred(), path->dentry->d_sb, > + FILESYSTEM__WATCH, &ad); > + if (ret) > + return ret; > + break; > + case FSNOTIFY_OBJ_TYPE_INODE: > + perm = FILE__WATCH; > + break; > + default: > + return -EINVAL; > + } > + > + // check if the mask is requesting ability to set a blocking watch > + if (mask & (ALL_FSNOTIFY_PERM_EVENTS)) > + perm |= FILE__WATCH_WITH_PERM; // if so, check that permission > + > + // is the mask asking to watch file reads? > + if (mask & (FS_ACCESS | FS_ACCESS_PERM | FS_CLOSE_NOWRITE)) > + perm |= FILE__WATCH_READS; // check that permission as well > + > + return path_has_perm(current_cred(), path, perm); > +} > + > /* > * Copy the inode security context value to the user. > * > @@ -6797,6 +6843,7 @@ static struct security_hook_list selinux_hooks[] __lsm_ro_after_init = { > LSM_HOOK_INIT(inode_getsecid, selinux_inode_getsecid), > LSM_HOOK_INIT(inode_copy_up, selinux_inode_copy_up), > LSM_HOOK_INIT(inode_copy_up_xattr, selinux_inode_copy_up_xattr), > + LSM_HOOK_INIT(path_notify, selinux_path_notify), > > LSM_HOOK_INIT(kernfs_init_security, selinux_kernfs_init_security), > > diff --git a/security/selinux/include/classmap.h b/security/selinux/include/classmap.h > index 201f7e588a29..32e9b03be3dd 100644 > --- a/security/selinux/include/classmap.h > +++ b/security/selinux/include/classmap.h > @@ -7,7 +7,8 @@ > > #define COMMON_FILE_PERMS COMMON_FILE_SOCK_PERMS, "unlink", "link", \ > "rename", "execute", "quotaon", "mounton", "audit_access", \ > - "open", "execmod" > + "open", "execmod", "watch", "watch_mount", "watch_sb", \ > + "watch_with_perm", "watch_reads" > > #define COMMON_SOCK_PERMS COMMON_FILE_SOCK_PERMS, "bind", "connect", \ > "listen", "accept", "getopt", "setopt", "shutdown", "recvfrom", \ > @@ -60,7 +61,7 @@ struct security_class_mapping secclass_map[] = { > { "filesystem", > { "mount", "remount", "unmount", "getattr", > "relabelfrom", "relabelto", "associate", "quotamod", > - "quotaget", NULL } }, > + "quotaget", "watch", NULL } }, > { "file", > { COMMON_FILE_PERMS, > "execute_no_trans", "entrypoint", NULL } }, > -- > 2.21.0 >
On Thu, Aug 8, 2019 at 9:33 PM Paul Moore <paul@paul-moore.com> wrote: > > On Wed, Jul 31, 2019 at 11:35 AM Aaron Goidel <acgoide@tycho.nsa.gov> wrote: > > As of now, setting watches on filesystem objects has, at most, applied a > > check for read access to the inode, and in the case of fanotify, requires > > CAP_SYS_ADMIN. No specific security hook or permission check has been > > provided to control the setting of watches. Using any of inotify, dnotify, > > or fanotify, it is possible to observe, not only write-like operations, but > > even read access to a file. Modeling the watch as being merely a read from > > the file is insufficient for the needs of SELinux. This is due to the fact > > that read access should not necessarily imply access to information about > > when another process reads from a file. Furthermore, fanotify watches grant > > more power to an application in the form of permission events. While > > notification events are solely, unidirectional (i.e. they only pass > > information to the receiving application), permission events are blocking. > > Permission events make a request to the receiving application which will > > then reply with a decision as to whether or not that action may be > > completed. This causes the issue of the watching application having the > > ability to exercise control over the triggering process. Without drawing a > > distinction within the permission check, the ability to read would imply > > the greater ability to control an application. Additionally, mount and > > superblock watches apply to all files within the same mount or superblock. > > Read access to one file should not necessarily imply the ability to watch > > all files accessed within a given mount or superblock. > > > > In order to solve these issues, a new LSM hook is implemented and has been > > placed within the system calls for marking filesystem objects with inotify, > > fanotify, and dnotify watches. These calls to the hook are placed at the > > point at which the target path has been resolved and are provided with the > > path struct, the mask of requested notification events, and the type of > > object on which the mark is being set (inode, superblock, or mount). The > > mask and obj_type have already been translated into common FS_* values > > shared by the entirety of the fs notification infrastructure. The path > > struct is passed rather than just the inode so that the mount is available, > > particularly for mount watches. This also allows for use of the hook by > > pathname-based security modules. However, since the hook is intended for > > use even by inode based security modules, it is not placed under the > > CONFIG_SECURITY_PATH conditional. Otherwise, the inode-based security > > modules would need to enable all of the path hooks, even though they do not > > use any of them. > > > > This only provides a hook at the point of setting a watch, and presumes > > that permission to set a particular watch implies the ability to receive > > all notification about that object which match the mask. This is all that > > is required for SELinux. If other security modules require additional hooks > > or infrastructure to control delivery of notification, these can be added > > by them. It does not make sense for us to propose hooks for which we have > > no implementation. The understanding that all notifications received by the > > requesting application are all strictly of a type for which the application > > has been granted permission shows that this implementation is sufficient in > > its coverage. > > > > Security modules wishing to provide complete control over fanotify must > > also implement a security_file_open hook that validates that the access > > requested by the watching application is authorized. Fanotify has the issue > > that it returns a file descriptor with the file mode specified during > > fanotify_init() to the watching process on event. This is already covered > > by the LSM security_file_open hook if the security module implements > > checking of the requested file mode there. Otherwise, a watching process > > can obtain escalated access to a file for which it has not been authorized. > > > > The selinux_path_notify hook implementation works by adding five new file > > permissions: watch, watch_mount, watch_sb, watch_reads, and watch_with_perm > > (descriptions about which will follow), and one new filesystem permission: > > watch (which is applied to superblock checks). The hook then decides which > > subset of these permissions must be held by the requesting application > > based on the contents of the provided mask and the obj_type. The > > selinux_file_open hook already checks the requested file mode and therefore > > ensures that a watching process cannot escalate its access through > > fanotify. > > > > The watch, watch_mount, and watch_sb permissions are the baseline > > permissions for setting a watch on an object and each are a requirement for > > any watch to be set on a file, mount, or superblock respectively. It should > > be noted that having either of the other two permissions (watch_reads and > > watch_with_perm) does not imply the watch, watch_mount, or watch_sb > > permission. Superblock watches further require the filesystem watch > > permission to the superblock. As there is no labeled object in view for > > mounts, there is no specific check for mount watches beyond watch_mount to > > the inode. Such a check could be added in the future, if a suitable labeled > > object existed representing the mount. > > > > The watch_reads permission is required to receive notifications from > > read-exclusive events on filesystem objects. These events include accessing > > a file for the purpose of reading and closing a file which has been opened > > read-only. This distinction has been drawn in order to provide a direct > > indication in the policy for this otherwise not obvious capability. Read > > access to a file should not necessarily imply the ability to observe read > > events on a file. > > > > Finally, watch_with_perm only applies to fanotify masks since it is the > > only way to set a mask which allows for the blocking, permission event. > > This permission is needed for any watch which is of this type. Though > > fanotify requires CAP_SYS_ADMIN, this is insufficient as it gives implicit > > trust to root, which we do not do, and does not support least privilege. > > > > Signed-off-by: Aaron Goidel <acgoide@tycho.nsa.gov> > > --- > > fs/notify/dnotify/dnotify.c | 15 +++++++-- > > fs/notify/fanotify/fanotify_user.c | 27 +++++++++++++++-- > > fs/notify/inotify/inotify_user.c | 13 ++++++-- > > include/linux/lsm_hooks.h | 9 +++++- > > include/linux/security.h | 10 ++++-- > > security/security.c | 6 ++++ > > security/selinux/hooks.c | 47 +++++++++++++++++++++++++++++ > > security/selinux/include/classmap.h | 5 +-- > > 8 files changed, 120 insertions(+), 12 deletions(-) > > Other than Casey's comments, and ACK, I'm not seeing much commentary > on this patch so FS and LSM folks consider this your last chance - if > I don't hear any objections by the end of this week I'll plan on > merging this into selinux/next next week. Please consider it is summer time so people may be on vacation like I was... First a suggestion, take it or leave it. The name of the hook _notify() seems misleading to me. naming the hook security_path_watch() seems much more appropriate and matching the name of the constants FILE__WATCH used by selinux. Few more comments below.. > > > diff --git a/fs/notify/dnotify/dnotify.c b/fs/notify/dnotify/dnotify.c > > index 250369d6901d..87a7f61bc91c 100644 > > --- a/fs/notify/dnotify/dnotify.c > > +++ b/fs/notify/dnotify/dnotify.c > > @@ -22,6 +22,7 @@ > > #include <linux/sched/signal.h> > > #include <linux/dnotify.h> > > #include <linux/init.h> > > +#include <linux/security.h> > > #include <linux/spinlock.h> > > #include <linux/slab.h> > > #include <linux/fdtable.h> > > @@ -288,6 +289,17 @@ int fcntl_dirnotify(int fd, struct file *filp, unsigned long arg) > > goto out_err; > > } > > > > + /* > > + * convert the userspace DN_* "arg" to the internal FS_* > > + * defined in fsnotify > > + */ > > + mask = convert_arg(arg); > > + > > + error = security_path_notify(&filp->f_path, mask, > > + FSNOTIFY_OBJ_TYPE_INODE); > > + if (error) > > + goto out_err; > > + > > /* expect most fcntl to add new rather than augment old */ > > dn = kmem_cache_alloc(dnotify_struct_cache, GFP_KERNEL); > > if (!dn) { > > @@ -302,9 +314,6 @@ int fcntl_dirnotify(int fd, struct file *filp, unsigned long arg) > > goto out_err; > > } > > > > - /* convert the userspace DN_* "arg" to the internal FS_* defines in fsnotify */ > > - mask = convert_arg(arg); > > - > > /* set up the new_fsn_mark and new_dn_mark */ > > new_fsn_mark = &new_dn_mark->fsn_mark; > > fsnotify_init_mark(new_fsn_mark, dnotify_group); > > diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c > > index a90bb19dcfa2..b83f27021f54 100644 > > --- a/fs/notify/fanotify/fanotify_user.c > > +++ b/fs/notify/fanotify/fanotify_user.c > > @@ -528,9 +528,10 @@ static const struct file_operations fanotify_fops = { > > }; > > > > static int fanotify_find_path(int dfd, const char __user *filename, > > - struct path *path, unsigned int flags) > > + struct path *path, unsigned int flags, __u64 mask) > > { > > int ret; > > + unsigned int obj_type; > > > > pr_debug("%s: dfd=%d filename=%p flags=%x\n", __func__, > > dfd, filename, flags); > > @@ -567,8 +568,30 @@ static int fanotify_find_path(int dfd, const char __user *filename, > > > > /* you can only watch an inode if you have read permissions on it */ > > ret = inode_permission(path->dentry->d_inode, MAY_READ); > > + if (ret) { > > + path_put(path); > > + goto out; > > + } > > + > > + switch (flags & FANOTIFY_MARK_TYPE_BITS) { > > + case FAN_MARK_MOUNT: > > + obj_type = FSNOTIFY_OBJ_TYPE_VFSMOUNT; > > + break; > > + case FAN_MARK_FILESYSTEM: > > + obj_type = FSNOTIFY_OBJ_TYPE_SB; > > + break; > > + case FAN_MARK_INODE: > > + obj_type = FSNOTIFY_OBJ_TYPE_INODE; > > + break; > > + default: > > + ret = -EINVAL; > > + goto out; > > + } Sorry, I just can't stand this extra switch statement here. Please initialize obj_type at the very first switch statement in do_fanotify_mark() and pass it to fanotify_find_path(). Preferably also make it a helper that returns either valid obj_type or <0 for error. > > + > > + ret = security_path_notify(path, mask, obj_type); > > if (ret) > > path_put(path); It is probably best to mask out FANOTIFY_EVENT_FLAGS when calling the hook. Although FAN_EVENT_ON_CHILD and FAN_ONDIR do map to corresponding FS_ constants, the security hooks from dnotify and inotify do not pass these flags, so the security module cannot use them as reliable information, so it will have to assume that they have been requested anyway. Alternatively, make sure that dnotify/inotify security hooks always set these two flags, by fixing up and using the dnotify/inotify arg_to_mask conversion helpers before calling the security hook. > > + > > out: > > return ret; > > } > > @@ -1014,7 +1037,7 @@ static int do_fanotify_mark(int fanotify_fd, unsigned int flags, __u64 mask, > > goto fput_and_out; > > } > > > > - ret = fanotify_find_path(dfd, pathname, &path, flags); > > + ret = fanotify_find_path(dfd, pathname, &path, flags, mask); > > if (ret) > > goto fput_and_out; > > > > diff --git a/fs/notify/inotify/inotify_user.c b/fs/notify/inotify/inotify_user.c > > index 7b53598c8804..e0d593c2d437 100644 > > --- a/fs/notify/inotify/inotify_user.c > > +++ b/fs/notify/inotify/inotify_user.c > > @@ -39,6 +39,7 @@ > > #include <linux/poll.h> > > #include <linux/wait.h> > > #include <linux/memcontrol.h> > > +#include <linux/security.h> > > > > #include "inotify.h" > > #include "../fdinfo.h" > > @@ -342,7 +343,8 @@ static const struct file_operations inotify_fops = { > > /* > > * find_inode - resolve a user-given path to a specific inode > > */ > > -static int inotify_find_inode(const char __user *dirname, struct path *path, unsigned flags) > > +static int inotify_find_inode(const char __user *dirname, struct path *path, > > + unsigned int flags, __u64 mask) > > { > > int error; > > > > @@ -351,8 +353,15 @@ static int inotify_find_inode(const char __user *dirname, struct path *path, uns > > return error; > > /* you can only watch an inode if you have read permissions on it */ > > error = inode_permission(path->dentry->d_inode, MAY_READ); > > + if (error) { > > + path_put(path); > > + return error; > > + } > > + error = security_path_notify(path, mask, > > + FSNOTIFY_OBJ_TYPE_INODE); > > if (error) > > path_put(path); > > + > > return error; > > } > > > > @@ -744,7 +753,7 @@ SYSCALL_DEFINE3(inotify_add_watch, int, fd, const char __user *, pathname, > > if (mask & IN_ONLYDIR) > > flags |= LOOKUP_DIRECTORY; > > > > - ret = inotify_find_inode(pathname, &path, flags); > > + ret = inotify_find_inode(pathname, &path, flags, mask); Please use (mask & IN_ALL_EVENTS) for converting to common FS_ flags or use the inotify_arg_to_mask() conversion helper, which contains more details irrelevant for the security hook. Otherwise mask may contain flags like IN_MASK_CREATE, which mean different things on different backends and the security module cannot tell the difference. Also note that at this point, before inotify_arg_to_mask(), the mask does not yet contain FS_EVENT_ON_CHILD, which could be interesting for the security hook (fanotify users can opt-in with FAN_EVENT_ON_CHILD). Not a big deal though as security hook can assume the worse (that events on child are requested). Thanks, Amir.
On Fri, Aug 9, 2019 at 5:06 AM Amir Goldstein <amir73il@gmail.com> wrote: > On Thu, Aug 8, 2019 at 9:33 PM Paul Moore <paul@paul-moore.com> wrote: > > On Wed, Jul 31, 2019 at 11:35 AM Aaron Goidel <acgoide@tycho.nsa.gov> wrote: > > > As of now, setting watches on filesystem objects has, at most, applied a > > > check for read access to the inode, and in the case of fanotify, requires > > > CAP_SYS_ADMIN. No specific security hook or permission check has been > > > provided to control the setting of watches. Using any of inotify, dnotify, > > > or fanotify, it is possible to observe, not only write-like operations, but > > > even read access to a file. Modeling the watch as being merely a read from > > > the file is insufficient for the needs of SELinux. This is due to the fact > > > that read access should not necessarily imply access to information about > > > when another process reads from a file. Furthermore, fanotify watches grant > > > more power to an application in the form of permission events. While > > > notification events are solely, unidirectional (i.e. they only pass > > > information to the receiving application), permission events are blocking. > > > Permission events make a request to the receiving application which will > > > then reply with a decision as to whether or not that action may be > > > completed. This causes the issue of the watching application having the > > > ability to exercise control over the triggering process. Without drawing a > > > distinction within the permission check, the ability to read would imply > > > the greater ability to control an application. Additionally, mount and > > > superblock watches apply to all files within the same mount or superblock. > > > Read access to one file should not necessarily imply the ability to watch > > > all files accessed within a given mount or superblock. > > > > > > In order to solve these issues, a new LSM hook is implemented and has been > > > placed within the system calls for marking filesystem objects with inotify, > > > fanotify, and dnotify watches. These calls to the hook are placed at the > > > point at which the target path has been resolved and are provided with the > > > path struct, the mask of requested notification events, and the type of > > > object on which the mark is being set (inode, superblock, or mount). The > > > mask and obj_type have already been translated into common FS_* values > > > shared by the entirety of the fs notification infrastructure. The path > > > struct is passed rather than just the inode so that the mount is available, > > > particularly for mount watches. This also allows for use of the hook by > > > pathname-based security modules. However, since the hook is intended for > > > use even by inode based security modules, it is not placed under the > > > CONFIG_SECURITY_PATH conditional. Otherwise, the inode-based security > > > modules would need to enable all of the path hooks, even though they do not > > > use any of them. > > > > > > This only provides a hook at the point of setting a watch, and presumes > > > that permission to set a particular watch implies the ability to receive > > > all notification about that object which match the mask. This is all that > > > is required for SELinux. If other security modules require additional hooks > > > or infrastructure to control delivery of notification, these can be added > > > by them. It does not make sense for us to propose hooks for which we have > > > no implementation. The understanding that all notifications received by the > > > requesting application are all strictly of a type for which the application > > > has been granted permission shows that this implementation is sufficient in > > > its coverage. > > > > > > Security modules wishing to provide complete control over fanotify must > > > also implement a security_file_open hook that validates that the access > > > requested by the watching application is authorized. Fanotify has the issue > > > that it returns a file descriptor with the file mode specified during > > > fanotify_init() to the watching process on event. This is already covered > > > by the LSM security_file_open hook if the security module implements > > > checking of the requested file mode there. Otherwise, a watching process > > > can obtain escalated access to a file for which it has not been authorized. > > > > > > The selinux_path_notify hook implementation works by adding five new file > > > permissions: watch, watch_mount, watch_sb, watch_reads, and watch_with_perm > > > (descriptions about which will follow), and one new filesystem permission: > > > watch (which is applied to superblock checks). The hook then decides which > > > subset of these permissions must be held by the requesting application > > > based on the contents of the provided mask and the obj_type. The > > > selinux_file_open hook already checks the requested file mode and therefore > > > ensures that a watching process cannot escalate its access through > > > fanotify. > > > > > > The watch, watch_mount, and watch_sb permissions are the baseline > > > permissions for setting a watch on an object and each are a requirement for > > > any watch to be set on a file, mount, or superblock respectively. It should > > > be noted that having either of the other two permissions (watch_reads and > > > watch_with_perm) does not imply the watch, watch_mount, or watch_sb > > > permission. Superblock watches further require the filesystem watch > > > permission to the superblock. As there is no labeled object in view for > > > mounts, there is no specific check for mount watches beyond watch_mount to > > > the inode. Such a check could be added in the future, if a suitable labeled > > > object existed representing the mount. > > > > > > The watch_reads permission is required to receive notifications from > > > read-exclusive events on filesystem objects. These events include accessing > > > a file for the purpose of reading and closing a file which has been opened > > > read-only. This distinction has been drawn in order to provide a direct > > > indication in the policy for this otherwise not obvious capability. Read > > > access to a file should not necessarily imply the ability to observe read > > > events on a file. > > > > > > Finally, watch_with_perm only applies to fanotify masks since it is the > > > only way to set a mask which allows for the blocking, permission event. > > > This permission is needed for any watch which is of this type. Though > > > fanotify requires CAP_SYS_ADMIN, this is insufficient as it gives implicit > > > trust to root, which we do not do, and does not support least privilege. > > > > > > Signed-off-by: Aaron Goidel <acgoide@tycho.nsa.gov> > > > --- > > > fs/notify/dnotify/dnotify.c | 15 +++++++-- > > > fs/notify/fanotify/fanotify_user.c | 27 +++++++++++++++-- > > > fs/notify/inotify/inotify_user.c | 13 ++++++-- > > > include/linux/lsm_hooks.h | 9 +++++- > > > include/linux/security.h | 10 ++++-- > > > security/security.c | 6 ++++ > > > security/selinux/hooks.c | 47 +++++++++++++++++++++++++++++ > > > security/selinux/include/classmap.h | 5 +-- > > > 8 files changed, 120 insertions(+), 12 deletions(-) > > > > Other than Casey's comments, and ACK, I'm not seeing much commentary > > on this patch so FS and LSM folks consider this your last chance - if > > I don't hear any objections by the end of this week I'll plan on > > merging this into selinux/next next week. > > Please consider it is summer time so people may be on vacation like I was... This is one of the reasons why I was speaking to the mailing list and not a particular individual :) > First a suggestion, take it or leave it. > The name of the hook _notify() seems misleading to me. > naming the hook security_path_watch() seems much more > appropriate and matching the name of the constants FILE__WATCH > used by selinux. I guess I'm not too bothered by either name, Aaron? FWIW, if I was writing this hook, I would probably name it security_fsnotify_path(...).
On 8/9/19 8:55 AM, Paul Moore wrote: > On Fri, Aug 9, 2019 at 5:06 AM Amir Goldstein <amir73il@gmail.com> wrote: >> On Thu, Aug 8, 2019 at 9:33 PM Paul Moore <paul@paul-moore.com> wrote: >>> On Wed, Jul 31, 2019 at 11:35 AM Aaron Goidel <acgoide@tycho.nsa.gov> wrote: >>>> As of now, setting watches on filesystem objects has, at most, applied a >>>> check for read access to the inode, and in the case of fanotify, requires >>>> CAP_SYS_ADMIN. No specific security hook or permission check has been >>>> provided to control the setting of watches. Using any of inotify, dnotify, >>>> or fanotify, it is possible to observe, not only write-like operations, but >>>> even read access to a file. Modeling the watch as being merely a read from >>>> the file is insufficient for the needs of SELinux. This is due to the fact >>>> that read access should not necessarily imply access to information about >>>> when another process reads from a file. Furthermore, fanotify watches grant >>>> more power to an application in the form of permission events. While >>>> notification events are solely, unidirectional (i.e. they only pass >>>> information to the receiving application), permission events are blocking. >>>> Permission events make a request to the receiving application which will >>>> then reply with a decision as to whether or not that action may be >>>> completed. This causes the issue of the watching application having the >>>> ability to exercise control over the triggering process. Without drawing a >>>> distinction within the permission check, the ability to read would imply >>>> the greater ability to control an application. Additionally, mount and >>>> superblock watches apply to all files within the same mount or superblock. >>>> Read access to one file should not necessarily imply the ability to watch >>>> all files accessed within a given mount or superblock. >>>> >>>> In order to solve these issues, a new LSM hook is implemented and has been >>>> placed within the system calls for marking filesystem objects with inotify, >>>> fanotify, and dnotify watches. These calls to the hook are placed at the >>>> point at which the target path has been resolved and are provided with the >>>> path struct, the mask of requested notification events, and the type of >>>> object on which the mark is being set (inode, superblock, or mount). The >>>> mask and obj_type have already been translated into common FS_* values >>>> shared by the entirety of the fs notification infrastructure. The path >>>> struct is passed rather than just the inode so that the mount is available, >>>> particularly for mount watches. This also allows for use of the hook by >>>> pathname-based security modules. However, since the hook is intended for >>>> use even by inode based security modules, it is not placed under the >>>> CONFIG_SECURITY_PATH conditional. Otherwise, the inode-based security >>>> modules would need to enable all of the path hooks, even though they do not >>>> use any of them. >>>> >>>> This only provides a hook at the point of setting a watch, and presumes >>>> that permission to set a particular watch implies the ability to receive >>>> all notification about that object which match the mask. This is all that >>>> is required for SELinux. If other security modules require additional hooks >>>> or infrastructure to control delivery of notification, these can be added >>>> by them. It does not make sense for us to propose hooks for which we have >>>> no implementation. The understanding that all notifications received by the >>>> requesting application are all strictly of a type for which the application >>>> has been granted permission shows that this implementation is sufficient in >>>> its coverage. >>>> >>>> Security modules wishing to provide complete control over fanotify must >>>> also implement a security_file_open hook that validates that the access >>>> requested by the watching application is authorized. Fanotify has the issue >>>> that it returns a file descriptor with the file mode specified during >>>> fanotify_init() to the watching process on event. This is already covered >>>> by the LSM security_file_open hook if the security module implements >>>> checking of the requested file mode there. Otherwise, a watching process >>>> can obtain escalated access to a file for which it has not been authorized. >>>> >>>> The selinux_path_notify hook implementation works by adding five new file >>>> permissions: watch, watch_mount, watch_sb, watch_reads, and watch_with_perm >>>> (descriptions about which will follow), and one new filesystem permission: >>>> watch (which is applied to superblock checks). The hook then decides which >>>> subset of these permissions must be held by the requesting application >>>> based on the contents of the provided mask and the obj_type. The >>>> selinux_file_open hook already checks the requested file mode and therefore >>>> ensures that a watching process cannot escalate its access through >>>> fanotify. >>>> >>>> The watch, watch_mount, and watch_sb permissions are the baseline >>>> permissions for setting a watch on an object and each are a requirement for >>>> any watch to be set on a file, mount, or superblock respectively. It should >>>> be noted that having either of the other two permissions (watch_reads and >>>> watch_with_perm) does not imply the watch, watch_mount, or watch_sb >>>> permission. Superblock watches further require the filesystem watch >>>> permission to the superblock. As there is no labeled object in view for >>>> mounts, there is no specific check for mount watches beyond watch_mount to >>>> the inode. Such a check could be added in the future, if a suitable labeled >>>> object existed representing the mount. >>>> >>>> The watch_reads permission is required to receive notifications from >>>> read-exclusive events on filesystem objects. These events include accessing >>>> a file for the purpose of reading and closing a file which has been opened >>>> read-only. This distinction has been drawn in order to provide a direct >>>> indication in the policy for this otherwise not obvious capability. Read >>>> access to a file should not necessarily imply the ability to observe read >>>> events on a file. >>>> >>>> Finally, watch_with_perm only applies to fanotify masks since it is the >>>> only way to set a mask which allows for the blocking, permission event. >>>> This permission is needed for any watch which is of this type. Though >>>> fanotify requires CAP_SYS_ADMIN, this is insufficient as it gives implicit >>>> trust to root, which we do not do, and does not support least privilege. >>>> >>>> Signed-off-by: Aaron Goidel <acgoide@tycho.nsa.gov> >>>> --- >>>> fs/notify/dnotify/dnotify.c | 15 +++++++-- >>>> fs/notify/fanotify/fanotify_user.c | 27 +++++++++++++++-- >>>> fs/notify/inotify/inotify_user.c | 13 ++++++-- >>>> include/linux/lsm_hooks.h | 9 +++++- >>>> include/linux/security.h | 10 ++++-- >>>> security/security.c | 6 ++++ >>>> security/selinux/hooks.c | 47 +++++++++++++++++++++++++++++ >>>> security/selinux/include/classmap.h | 5 +-- >>>> 8 files changed, 120 insertions(+), 12 deletions(-) >>> >>> Other than Casey's comments, and ACK, I'm not seeing much commentary >>> on this patch so FS and LSM folks consider this your last chance - if >>> I don't hear any objections by the end of this week I'll plan on >>> merging this into selinux/next next week. >> >> Please consider it is summer time so people may be on vacation like I was... > > This is one of the reasons why I was speaking to the mailing list and > not a particular individual :) > >> First a suggestion, take it or leave it. >> The name of the hook _notify() seems misleading to me. >> naming the hook security_path_watch() seems much more >> appropriate and matching the name of the constants FILE__WATCH >> used by selinux. > > I guess I'm not too bothered by either name, Aaron? FWIW, if I was > writing this hook, I would probably name it > security_fsnotify_path(...). > While I'm not necessarily attached to the name, I feel as though "misleading" is too strong a word here. Notify seems to be an appropriate enough term to me as every call to the hook, and thus all the logic to which the hook adds security, lives in the notify/ subtree.
On 8/9/19 5:06 AM, Amir Goldstein wrote: > On Thu, Aug 8, 2019 at 9:33 PM Paul Moore <paul@paul-moore.com> wrote: >> >> On Wed, Jul 31, 2019 at 11:35 AM Aaron Goidel <acgoide@tycho.nsa.gov> wrote: >>> As of now, setting watches on filesystem objects has, at most, applied a >>> check for read access to the inode, and in the case of fanotify, requires >>> CAP_SYS_ADMIN. No specific security hook or permission check has been >>> provided to control the setting of watches. Using any of inotify, dnotify, >>> or fanotify, it is possible to observe, not only write-like operations, but >>> even read access to a file. Modeling the watch as being merely a read from >>> the file is insufficient for the needs of SELinux. This is due to the fact >>> that read access should not necessarily imply access to information about >>> when another process reads from a file. Furthermore, fanotify watches grant >>> more power to an application in the form of permission events. While >>> notification events are solely, unidirectional (i.e. they only pass >>> information to the receiving application), permission events are blocking. >>> Permission events make a request to the receiving application which will >>> then reply with a decision as to whether or not that action may be >>> completed. This causes the issue of the watching application having the >>> ability to exercise control over the triggering process. Without drawing a >>> distinction within the permission check, the ability to read would imply >>> the greater ability to control an application. Additionally, mount and >>> superblock watches apply to all files within the same mount or superblock. >>> Read access to one file should not necessarily imply the ability to watch >>> all files accessed within a given mount or superblock. >>> >>> In order to solve these issues, a new LSM hook is implemented and has been >>> placed within the system calls for marking filesystem objects with inotify, >>> fanotify, and dnotify watches. These calls to the hook are placed at the >>> point at which the target path has been resolved and are provided with the >>> path struct, the mask of requested notification events, and the type of >>> object on which the mark is being set (inode, superblock, or mount). The >>> mask and obj_type have already been translated into common FS_* values >>> shared by the entirety of the fs notification infrastructure. The path >>> struct is passed rather than just the inode so that the mount is available, >>> particularly for mount watches. This also allows for use of the hook by >>> pathname-based security modules. However, since the hook is intended for >>> use even by inode based security modules, it is not placed under the >>> CONFIG_SECURITY_PATH conditional. Otherwise, the inode-based security >>> modules would need to enable all of the path hooks, even though they do not >>> use any of them. >>> >>> This only provides a hook at the point of setting a watch, and presumes >>> that permission to set a particular watch implies the ability to receive >>> all notification about that object which match the mask. This is all that >>> is required for SELinux. If other security modules require additional hooks >>> or infrastructure to control delivery of notification, these can be added >>> by them. It does not make sense for us to propose hooks for which we have >>> no implementation. The understanding that all notifications received by the >>> requesting application are all strictly of a type for which the application >>> has been granted permission shows that this implementation is sufficient in >>> its coverage. >>> >>> Security modules wishing to provide complete control over fanotify must >>> also implement a security_file_open hook that validates that the access >>> requested by the watching application is authorized. Fanotify has the issue >>> that it returns a file descriptor with the file mode specified during >>> fanotify_init() to the watching process on event. This is already covered >>> by the LSM security_file_open hook if the security module implements >>> checking of the requested file mode there. Otherwise, a watching process >>> can obtain escalated access to a file for which it has not been authorized. >>> >>> The selinux_path_notify hook implementation works by adding five new file >>> permissions: watch, watch_mount, watch_sb, watch_reads, and watch_with_perm >>> (descriptions about which will follow), and one new filesystem permission: >>> watch (which is applied to superblock checks). The hook then decides which >>> subset of these permissions must be held by the requesting application >>> based on the contents of the provided mask and the obj_type. The >>> selinux_file_open hook already checks the requested file mode and therefore >>> ensures that a watching process cannot escalate its access through >>> fanotify. >>> >>> The watch, watch_mount, and watch_sb permissions are the baseline >>> permissions for setting a watch on an object and each are a requirement for >>> any watch to be set on a file, mount, or superblock respectively. It should >>> be noted that having either of the other two permissions (watch_reads and >>> watch_with_perm) does not imply the watch, watch_mount, or watch_sb >>> permission. Superblock watches further require the filesystem watch >>> permission to the superblock. As there is no labeled object in view for >>> mounts, there is no specific check for mount watches beyond watch_mount to >>> the inode. Such a check could be added in the future, if a suitable labeled >>> object existed representing the mount. >>> >>> The watch_reads permission is required to receive notifications from >>> read-exclusive events on filesystem objects. These events include accessing >>> a file for the purpose of reading and closing a file which has been opened >>> read-only. This distinction has been drawn in order to provide a direct >>> indication in the policy for this otherwise not obvious capability. Read >>> access to a file should not necessarily imply the ability to observe read >>> events on a file. >>> >>> Finally, watch_with_perm only applies to fanotify masks since it is the >>> only way to set a mask which allows for the blocking, permission event. >>> This permission is needed for any watch which is of this type. Though >>> fanotify requires CAP_SYS_ADMIN, this is insufficient as it gives implicit >>> trust to root, which we do not do, and does not support least privilege. >>> >>> Signed-off-by: Aaron Goidel <acgoide@tycho.nsa.gov> >>> --- >>> fs/notify/dnotify/dnotify.c | 15 +++++++-- >>> fs/notify/fanotify/fanotify_user.c | 27 +++++++++++++++-- >>> fs/notify/inotify/inotify_user.c | 13 ++++++-- >>> include/linux/lsm_hooks.h | 9 +++++- >>> include/linux/security.h | 10 ++++-- >>> security/security.c | 6 ++++ >>> security/selinux/hooks.c | 47 +++++++++++++++++++++++++++++ >>> security/selinux/include/classmap.h | 5 +-- >>> 8 files changed, 120 insertions(+), 12 deletions(-) >> >> Other than Casey's comments, and ACK, I'm not seeing much commentary >> on this patch so FS and LSM folks consider this your last chance - if >> I don't hear any objections by the end of this week I'll plan on >> merging this into selinux/next next week. > > Please consider it is summer time so people may be on vacation like I was... > > First a suggestion, take it or leave it. > The name of the hook _notify() seems misleading to me. > naming the hook security_path_watch() seems much more > appropriate and matching the name of the constants FILE__WATCH > used by selinux. > > Few more comments below.. > >> >>> diff --git a/fs/notify/dnotify/dnotify.c b/fs/notify/dnotify/dnotify.c >>> index 250369d6901d..87a7f61bc91c 100644 >>> --- a/fs/notify/dnotify/dnotify.c >>> +++ b/fs/notify/dnotify/dnotify.c >>> @@ -22,6 +22,7 @@ >>> #include <linux/sched/signal.h> >>> #include <linux/dnotify.h> >>> #include <linux/init.h> >>> +#include <linux/security.h> >>> #include <linux/spinlock.h> >>> #include <linux/slab.h> >>> #include <linux/fdtable.h> >>> @@ -288,6 +289,17 @@ int fcntl_dirnotify(int fd, struct file *filp, unsigned long arg) >>> goto out_err; >>> } >>> >>> + /* >>> + * convert the userspace DN_* "arg" to the internal FS_* >>> + * defined in fsnotify >>> + */ >>> + mask = convert_arg(arg); >>> + >>> + error = security_path_notify(&filp->f_path, mask, >>> + FSNOTIFY_OBJ_TYPE_INODE); >>> + if (error) >>> + goto out_err; >>> + >>> /* expect most fcntl to add new rather than augment old */ >>> dn = kmem_cache_alloc(dnotify_struct_cache, GFP_KERNEL); >>> if (!dn) { >>> @@ -302,9 +314,6 @@ int fcntl_dirnotify(int fd, struct file *filp, unsigned long arg) >>> goto out_err; >>> } >>> >>> - /* convert the userspace DN_* "arg" to the internal FS_* defines in fsnotify */ >>> - mask = convert_arg(arg); >>> - >>> /* set up the new_fsn_mark and new_dn_mark */ >>> new_fsn_mark = &new_dn_mark->fsn_mark; >>> fsnotify_init_mark(new_fsn_mark, dnotify_group); >>> diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c >>> index a90bb19dcfa2..b83f27021f54 100644 >>> --- a/fs/notify/fanotify/fanotify_user.c >>> +++ b/fs/notify/fanotify/fanotify_user.c >>> @@ -528,9 +528,10 @@ static const struct file_operations fanotify_fops = { >>> }; >>> >>> static int fanotify_find_path(int dfd, const char __user *filename, >>> - struct path *path, unsigned int flags) >>> + struct path *path, unsigned int flags, __u64 mask) >>> { >>> int ret; >>> + unsigned int obj_type; >>> >>> pr_debug("%s: dfd=%d filename=%p flags=%x\n", __func__, >>> dfd, filename, flags); >>> @@ -567,8 +568,30 @@ static int fanotify_find_path(int dfd, const char __user *filename, >>> >>> /* you can only watch an inode if you have read permissions on it */ >>> ret = inode_permission(path->dentry->d_inode, MAY_READ); >>> + if (ret) { >>> + path_put(path); >>> + goto out; >>> + } >>> + >>> + switch (flags & FANOTIFY_MARK_TYPE_BITS) { >>> + case FAN_MARK_MOUNT: >>> + obj_type = FSNOTIFY_OBJ_TYPE_VFSMOUNT; >>> + break; >>> + case FAN_MARK_FILESYSTEM: >>> + obj_type = FSNOTIFY_OBJ_TYPE_SB; >>> + break; >>> + case FAN_MARK_INODE: >>> + obj_type = FSNOTIFY_OBJ_TYPE_INODE; >>> + break; >>> + default: >>> + ret = -EINVAL; >>> + goto out; >>> + } > > Sorry, I just can't stand this extra switch statement here. > Please initialize obj_type at the very first switch statement in > do_fanotify_mark() and pass it to fanotify_find_path(). > Preferably also make it a helper that returns either > valid obj_type or <0 for error. > > I have no problem moving the initialization of obj_type up one level to do_fanotify_mark(). I don't think that a helper is necessary at this juncture as this logic seems to only exist in one place. Should this change, then there would be merit to having a separate function. >>> + >>> + ret = security_path_notify(path, mask, obj_type); >>> if (ret) >>> path_put(path); > > It is probably best to mask out FANOTIFY_EVENT_FLAGS > when calling the hook. Although FAN_EVENT_ON_CHILD > and FAN_ONDIR do map to corresponding FS_ constants, > the security hooks from dnotify and inotify do not pass these > flags, so the security module cannot use them as reliable > information, so it will have to assume that they have been > requested anyway. > > Alternatively, make sure that dnotify/inotify security hooks > always set these two flags, by fixing up and using the > dnotify/inotify arg_to_mask conversion helpers before calling > the security hook. > I think that at this point either approach you mentioned makes just as much sense. If it's all the same to you, Amir, I'll just change the caller in fanotify to include (mask) & ~(FANOTIFY_EVENT_FLAGS) >>> + >>> out: >>> return ret; >>> } >>> @@ -1014,7 +1037,7 @@ static int do_fanotify_mark(int fanotify_fd, unsigned int flags, __u64 mask, >>> goto fput_and_out; >>> } >>> >>> - ret = fanotify_find_path(dfd, pathname, &path, flags); >>> + ret = fanotify_find_path(dfd, pathname, &path, flags, mask); >>> if (ret) >>> goto fput_and_out; >>> >>> diff --git a/fs/notify/inotify/inotify_user.c b/fs/notify/inotify/inotify_user.c >>> index 7b53598c8804..e0d593c2d437 100644 >>> --- a/fs/notify/inotify/inotify_user.c >>> +++ b/fs/notify/inotify/inotify_user.c >>> @@ -39,6 +39,7 @@ >>> #include <linux/poll.h> >>> #include <linux/wait.h> >>> #include <linux/memcontrol.h> >>> +#include <linux/security.h> >>> >>> #include "inotify.h" >>> #include "../fdinfo.h" >>> @@ -342,7 +343,8 @@ static const struct file_operations inotify_fops = { >>> /* >>> * find_inode - resolve a user-given path to a specific inode >>> */ >>> -static int inotify_find_inode(const char __user *dirname, struct path *path, unsigned flags) >>> +static int inotify_find_inode(const char __user *dirname, struct path *path, >>> + unsigned int flags, __u64 mask) >>> { >>> int error; >>> >>> @@ -351,8 +353,15 @@ static int inotify_find_inode(const char __user *dirname, struct path *path, uns >>> return error; >>> /* you can only watch an inode if you have read permissions on it */ >>> error = inode_permission(path->dentry->d_inode, MAY_READ); >>> + if (error) { >>> + path_put(path); >>> + return error; >>> + } >>> + error = security_path_notify(path, mask, >>> + FSNOTIFY_OBJ_TYPE_INODE); >>> if (error) >>> path_put(path); >>> + >>> return error; >>> } >>> >>> @@ -744,7 +753,7 @@ SYSCALL_DEFINE3(inotify_add_watch, int, fd, const char __user *, pathname, >>> if (mask & IN_ONLYDIR) >>> flags |= LOOKUP_DIRECTORY; >>> >>> - ret = inotify_find_inode(pathname, &path, flags); >>> + ret = inotify_find_inode(pathname, &path, flags, mask); > > Please use (mask & IN_ALL_EVENTS) for converting to common FS_ flags > or use the inotify_arg_to_mask() conversion helper, which contains more > details irrelevant for the security hook. > Otherwise mask may contain flags like IN_MASK_CREATE, which mean > different things on different backends and the security module cannot tell > the difference. > > Also note that at this point, before inotify_arg_to_mask(), the mask does > not yet contain FS_EVENT_ON_CHILD, which could be interesting for > the security hook (fanotify users can opt-in with FAN_EVENT_ON_CHILD). > Not a big deal though as security hook can assume the worse > (that events on child are requested). > I'll use (mask & IN_ALL_EVENTS). > Thanks, > Amir. > I can implement the changes in the ways I mentioned above. I don't see a need for anything more in the cases you brought up since none of them change the logic of the hook itself or would make a substantive difference to the operation of the hook given its current implementation.
... > >> First a suggestion, take it or leave it. > >> The name of the hook _notify() seems misleading to me. > >> naming the hook security_path_watch() seems much more > >> appropriate and matching the name of the constants FILE__WATCH > >> used by selinux. > > > > I guess I'm not too bothered by either name, Aaron? FWIW, if I was > > writing this hook, I would probably name it > > security_fsnotify_path(...). > > Or even just security_fsnotify() > > While I'm not necessarily attached to the name, I feel as though > "misleading" is too strong a word here. Agree. It is not misleading, but I should note that you yourself named the security class "watch", so why the inconsistency? > Notify seems to be an > appropriate enough term to me as every call to the hook, and thus all > the logic to which the hook adds security, lives in the notify/ subtree. > Well, if nobody cares about the name, it's fine by me. I wanted to point your attention to this proposal by David Howells: https://lore.kernel.org/linux-fsdevel/155991706847.15579.4702772917586301113.stgit@warthog.procyon.org.uk/ His proposal adds new types of watches, for keyring changes, mount changes, etc and he proposed security hooks for setting new watches named "watch_XXX" and for posting notifications called "post_notification". The latter was later rejected by Stephen Smalley: https://lore.kernel.org/linux-fsdevel/cd657aab-e11c-c0b1-2e36-dd796ca75b75@tycho.nsa.gov/ https://lore.kernel.org/linux-fsdevel/541e5cb3-142b-fe87-dff6-260b46d34f2d@tycho.nsa.gov/ Just to have a perspective why the hook name "notify_path" may end up being a bit ambiguous down the road. Thanks, Amir.
> >>> + switch (flags & FANOTIFY_MARK_TYPE_BITS) { > >>> + case FAN_MARK_MOUNT: > >>> + obj_type = FSNOTIFY_OBJ_TYPE_VFSMOUNT; > >>> + break; > >>> + case FAN_MARK_FILESYSTEM: > >>> + obj_type = FSNOTIFY_OBJ_TYPE_SB; > >>> + break; > >>> + case FAN_MARK_INODE: > >>> + obj_type = FSNOTIFY_OBJ_TYPE_INODE; > >>> + break; > >>> + default: > >>> + ret = -EINVAL; > >>> + goto out; > >>> + } > > > > Sorry, I just can't stand this extra switch statement here. > > Please initialize obj_type at the very first switch statement in > > do_fanotify_mark() and pass it to fanotify_find_path(). > > Preferably also make it a helper that returns either > > valid obj_type or <0 for error. > > > > > I have no problem moving the initialization of obj_type up one level to > do_fanotify_mark(). I don't think that a helper is necessary at this > juncture as this logic seems to only exist in one place. Should this > change, then there would be merit to having a separate function. Ok. > >>> + > >>> + ret = security_path_notify(path, mask, obj_type); > >>> if (ret) > >>> path_put(path); > > > > It is probably best to mask out FANOTIFY_EVENT_FLAGS > > when calling the hook. Although FAN_EVENT_ON_CHILD > > and FAN_ONDIR do map to corresponding FS_ constants, > > the security hooks from dnotify and inotify do not pass these > > flags, so the security module cannot use them as reliable > > information, so it will have to assume that they have been > > requested anyway. > > > > Alternatively, make sure that dnotify/inotify security hooks > > always set these two flags, by fixing up and using the > > dnotify/inotify arg_to_mask conversion helpers before calling > > the security hook. > > > I think that at this point either approach you mentioned makes just as > much sense. If it's all the same to you, Amir, I'll just change the > caller in fanotify to include (mask) & ~(FANOTIFY_EVENT_FLAGS) On second look, let's go with (mask & ALL_FSNOTIFY_EVENTS) It seems simpler and more appropriate way to convert to FS_ flags. [...] > >>> > >>> - ret = inotify_find_inode(pathname, &path, flags); > >>> + ret = inotify_find_inode(pathname, &path, flags, mask); > > > > Please use (mask & IN_ALL_EVENTS) for converting to common FS_ flags > > or use the inotify_arg_to_mask() conversion helper, which contains more > > details irrelevant for the security hook. > > Otherwise mask may contain flags like IN_MASK_CREATE, which mean > > different things on different backends and the security module cannot tell > > the difference. > > > > Also note that at this point, before inotify_arg_to_mask(), the mask does > > not yet contain FS_EVENT_ON_CHILD, which could be interesting for > > the security hook (fanotify users can opt-in with FAN_EVENT_ON_CHILD). > > Not a big deal though as security hook can assume the worse > > (that events on child are requested). > > > I'll use (mask & IN_ALL_EVENTS). OK. > > I can implement the changes in the ways I mentioned above. I don't see a > need for anything more in the cases you brought up since none of them > change the logic of the hook itself or would make a substantive > difference to the operation of the hook given its current implementation. > Agree. If more flags are needed for LSMs they could be added later. Thanks, Amir.
> > > Other than Casey's comments, and ACK, I'm not seeing much commentary > > > on this patch so FS and LSM folks consider this your last chance - if > > > I don't hear any objections by the end of this week I'll plan on > > > merging this into selinux/next next week. > > > > Please consider it is summer time so people may be on vacation like I was... > > This is one of the reasons why I was speaking to the mailing list and > not a particular individual :) > Jan is fsnotify maintainer, so I think you should wait for an explicit ACK from Jan or just merge the hook definition and ask Jan to merge to fsnotify security hooks. Thanks, Amir.
On August 10, 2019 6:05:27 AM Amir Goldstein <amir73il@gmail.com> wrote: >>>> Other than Casey's comments, and ACK, I'm not seeing much commentary >>>> on this patch so FS and LSM folks consider this your last chance - if >>>> I don't hear any objections by the end of this week I'll plan on >>>> merging this into selinux/next next week. >>> >>> Please consider it is summer time so people may be on vacation like I was... >> >> This is one of the reasons why I was speaking to the mailing list and >> not a particular individual :) > > Jan is fsnotify maintainer, so I think you should wait for an explicit ACK > from Jan or just merge the hook definition and ask Jan to merge to > fsnotify security hooks. Aaron posted his first patch a month ago in the beginning of July and I don't recall seeing any comments from Jan on any of the patch revisions. I would feel much better with an ACK/Reviewed-by from Jan, or you - which is why I sent that email - but I'm not going to wait forever and I'd like to get this into -next soon so we can get some testing. -- paul moore www.paul-moore.com
On Sat 10-08-19 11:01:16, Paul Moore wrote: > On August 10, 2019 6:05:27 AM Amir Goldstein <amir73il@gmail.com> wrote: > > >>>> Other than Casey's comments, and ACK, I'm not seeing much commentary > >>>> on this patch so FS and LSM folks consider this your last chance - if > >>>> I don't hear any objections by the end of this week I'll plan on > >>>> merging this into selinux/next next week. > >>> > >>> Please consider it is summer time so people may be on vacation like I was... > >> > >> This is one of the reasons why I was speaking to the mailing list and > >> not a particular individual :) > > > > Jan is fsnotify maintainer, so I think you should wait for an explicit ACK > > from Jan or just merge the hook definition and ask Jan to merge to > > fsnotify security hooks. > > Aaron posted his first patch a month ago in the beginning of July and I > don't recall seeing any comments from Jan on any of the patch revisions. > I would feel much better with an ACK/Reviewed-by from Jan, or you - which > is why I sent that email - but I'm not going to wait forever and I'd like > to get this into -next soon so we can get some testing. Yeah, sorry for the delays. I'm aware of the patch but I was also on vacation and pretty busy at work so Amir always beat me in commenting on the patch and I didn't have much to add. Once Aaron fixes the latest comments from Amir, I'll give the patch the final look and give my ack. Honza
On 8/12/19 9:41 AM, Jan Kara wrote: > On Sat 10-08-19 11:01:16, Paul Moore wrote: >> On August 10, 2019 6:05:27 AM Amir Goldstein <amir73il@gmail.com> wrote: >> >>>>>> Other than Casey's comments, and ACK, I'm not seeing much commentary >>>>>> on this patch so FS and LSM folks consider this your last chance - if >>>>>> I don't hear any objections by the end of this week I'll plan on >>>>>> merging this into selinux/next next week. >>>>> >>>>> Please consider it is summer time so people may be on vacation like I was... >>>> >>>> This is one of the reasons why I was speaking to the mailing list and >>>> not a particular individual :) >>> >>> Jan is fsnotify maintainer, so I think you should wait for an explicit ACK >>> from Jan or just merge the hook definition and ask Jan to merge to >>> fsnotify security hooks. >> >> Aaron posted his first patch a month ago in the beginning of July and I >> don't recall seeing any comments from Jan on any of the patch revisions. >> I would feel much better with an ACK/Reviewed-by from Jan, or you - which >> is why I sent that email - but I'm not going to wait forever and I'd like >> to get this into -next soon so we can get some testing. > > Yeah, sorry for the delays. I'm aware of the patch but I was also on > vacation and pretty busy at work so Amir always beat me in commenting on > the patch and I didn't have much to add. Once Aaron fixes the latest > comments from Amir, I'll give the patch the final look and give my ack. > > Honza > I already re-spun the patch with the changes Amir and I agreed to. There was an email with the PATCH v2. It may have flown under the radar a bit, so just wanted to point that out.
On Mon, Aug 12, 2019 at 9:41 AM Jan Kara <jack@suse.cz> wrote: > On Sat 10-08-19 11:01:16, Paul Moore wrote: > > On August 10, 2019 6:05:27 AM Amir Goldstein <amir73il@gmail.com> wrote: > > > > >>>> Other than Casey's comments, and ACK, I'm not seeing much commentary > > >>>> on this patch so FS and LSM folks consider this your last chance - if > > >>>> I don't hear any objections by the end of this week I'll plan on > > >>>> merging this into selinux/next next week. > > >>> > > >>> Please consider it is summer time so people may be on vacation like I was... > > >> > > >> This is one of the reasons why I was speaking to the mailing list and > > >> not a particular individual :) > > > > > > Jan is fsnotify maintainer, so I think you should wait for an explicit ACK > > > from Jan or just merge the hook definition and ask Jan to merge to > > > fsnotify security hooks. > > > > Aaron posted his first patch a month ago in the beginning of July and I > > don't recall seeing any comments from Jan on any of the patch revisions. > > I would feel much better with an ACK/Reviewed-by from Jan, or you - which > > is why I sent that email - but I'm not going to wait forever and I'd like > > to get this into -next soon so we can get some testing. > > Yeah, sorry for the delays. I'm aware of the patch but I was also on > vacation and pretty busy at work so Amir always beat me in commenting on > the patch and I didn't have much to add. Once Aaron fixes the latest > comments from Amir, I'll give the patch the final look and give my ack. That is prefect, thanks.
diff --git a/fs/notify/dnotify/dnotify.c b/fs/notify/dnotify/dnotify.c index 250369d6901d..87a7f61bc91c 100644 --- a/fs/notify/dnotify/dnotify.c +++ b/fs/notify/dnotify/dnotify.c @@ -22,6 +22,7 @@ #include <linux/sched/signal.h> #include <linux/dnotify.h> #include <linux/init.h> +#include <linux/security.h> #include <linux/spinlock.h> #include <linux/slab.h> #include <linux/fdtable.h> @@ -288,6 +289,17 @@ int fcntl_dirnotify(int fd, struct file *filp, unsigned long arg) goto out_err; } + /* + * convert the userspace DN_* "arg" to the internal FS_* + * defined in fsnotify + */ + mask = convert_arg(arg); + + error = security_path_notify(&filp->f_path, mask, + FSNOTIFY_OBJ_TYPE_INODE); + if (error) + goto out_err; + /* expect most fcntl to add new rather than augment old */ dn = kmem_cache_alloc(dnotify_struct_cache, GFP_KERNEL); if (!dn) { @@ -302,9 +314,6 @@ int fcntl_dirnotify(int fd, struct file *filp, unsigned long arg) goto out_err; } - /* convert the userspace DN_* "arg" to the internal FS_* defines in fsnotify */ - mask = convert_arg(arg); - /* set up the new_fsn_mark and new_dn_mark */ new_fsn_mark = &new_dn_mark->fsn_mark; fsnotify_init_mark(new_fsn_mark, dnotify_group); diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c index a90bb19dcfa2..b83f27021f54 100644 --- a/fs/notify/fanotify/fanotify_user.c +++ b/fs/notify/fanotify/fanotify_user.c @@ -528,9 +528,10 @@ static const struct file_operations fanotify_fops = { }; static int fanotify_find_path(int dfd, const char __user *filename, - struct path *path, unsigned int flags) + struct path *path, unsigned int flags, __u64 mask) { int ret; + unsigned int obj_type; pr_debug("%s: dfd=%d filename=%p flags=%x\n", __func__, dfd, filename, flags); @@ -567,8 +568,30 @@ static int fanotify_find_path(int dfd, const char __user *filename, /* you can only watch an inode if you have read permissions on it */ ret = inode_permission(path->dentry->d_inode, MAY_READ); + if (ret) { + path_put(path); + goto out; + } + + switch (flags & FANOTIFY_MARK_TYPE_BITS) { + case FAN_MARK_MOUNT: + obj_type = FSNOTIFY_OBJ_TYPE_VFSMOUNT; + break; + case FAN_MARK_FILESYSTEM: + obj_type = FSNOTIFY_OBJ_TYPE_SB; + break; + case FAN_MARK_INODE: + obj_type = FSNOTIFY_OBJ_TYPE_INODE; + break; + default: + ret = -EINVAL; + goto out; + } + + ret = security_path_notify(path, mask, obj_type); if (ret) path_put(path); + out: return ret; } @@ -1014,7 +1037,7 @@ static int do_fanotify_mark(int fanotify_fd, unsigned int flags, __u64 mask, goto fput_and_out; } - ret = fanotify_find_path(dfd, pathname, &path, flags); + ret = fanotify_find_path(dfd, pathname, &path, flags, mask); if (ret) goto fput_and_out; diff --git a/fs/notify/inotify/inotify_user.c b/fs/notify/inotify/inotify_user.c index 7b53598c8804..e0d593c2d437 100644 --- a/fs/notify/inotify/inotify_user.c +++ b/fs/notify/inotify/inotify_user.c @@ -39,6 +39,7 @@ #include <linux/poll.h> #include <linux/wait.h> #include <linux/memcontrol.h> +#include <linux/security.h> #include "inotify.h" #include "../fdinfo.h" @@ -342,7 +343,8 @@ static const struct file_operations inotify_fops = { /* * find_inode - resolve a user-given path to a specific inode */ -static int inotify_find_inode(const char __user *dirname, struct path *path, unsigned flags) +static int inotify_find_inode(const char __user *dirname, struct path *path, + unsigned int flags, __u64 mask) { int error; @@ -351,8 +353,15 @@ static int inotify_find_inode(const char __user *dirname, struct path *path, uns return error; /* you can only watch an inode if you have read permissions on it */ error = inode_permission(path->dentry->d_inode, MAY_READ); + if (error) { + path_put(path); + return error; + } + error = security_path_notify(path, mask, + FSNOTIFY_OBJ_TYPE_INODE); if (error) path_put(path); + return error; } @@ -744,7 +753,7 @@ SYSCALL_DEFINE3(inotify_add_watch, int, fd, const char __user *, pathname, if (mask & IN_ONLYDIR) flags |= LOOKUP_DIRECTORY; - ret = inotify_find_inode(pathname, &path, flags); + ret = inotify_find_inode(pathname, &path, flags, mask); if (ret) goto fput_and_out; diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h index 47f58cfb6a19..ead98af9c602 100644 --- a/include/linux/lsm_hooks.h +++ b/include/linux/lsm_hooks.h @@ -339,6 +339,9 @@ * Check for permission to change root directory. * @path contains the path structure. * Return 0 if permission is granted. + * @path_notify: + * Check permissions before setting a watch on events as defined by @mask, + * on an object at @path, whose type is defined by @obj_type. * @inode_readlink: * Check the permission to read the symbolic link. * @dentry contains the dentry structure for the file link. @@ -1535,7 +1538,9 @@ union security_list_options { int (*path_chown)(const struct path *path, kuid_t uid, kgid_t gid); int (*path_chroot)(const struct path *path); #endif - + /* Needed for inode based security check */ + int (*path_notify)(const struct path *path, u64 mask, + unsigned int obj_type); int (*inode_alloc_security)(struct inode *inode); void (*inode_free_security)(struct inode *inode); int (*inode_init_security)(struct inode *inode, struct inode *dir, @@ -1860,6 +1865,8 @@ struct security_hook_heads { struct hlist_head path_chown; struct hlist_head path_chroot; #endif + /* Needed for inode based modules as well */ + struct hlist_head path_notify; struct hlist_head inode_alloc_security; struct hlist_head inode_free_security; struct hlist_head inode_init_security; diff --git a/include/linux/security.h b/include/linux/security.h index 659071c2e57c..7d9c1da1f659 100644 --- a/include/linux/security.h +++ b/include/linux/security.h @@ -259,7 +259,8 @@ int security_dentry_create_files_as(struct dentry *dentry, int mode, struct qstr *name, const struct cred *old, struct cred *new); - +int security_path_notify(const struct path *path, u64 mask, + unsigned int obj_type); int security_inode_alloc(struct inode *inode); void security_inode_free(struct inode *inode); int security_inode_init_security(struct inode *inode, struct inode *dir, @@ -387,7 +388,6 @@ int security_ismaclabel(const char *name); int security_secid_to_secctx(u32 secid, char **secdata, u32 *seclen); int security_secctx_to_secid(const char *secdata, u32 seclen, u32 *secid); void security_release_secctx(char *secdata, u32 seclen); - void security_inode_invalidate_secctx(struct inode *inode); int security_inode_notifysecctx(struct inode *inode, void *ctx, u32 ctxlen); int security_inode_setsecctx(struct dentry *dentry, void *ctx, u32 ctxlen); @@ -621,6 +621,12 @@ static inline int security_move_mount(const struct path *from_path, return 0; } +static inline int security_path_notify(const struct path *path, u64 mask, + unsigned int obj_type) +{ + return 0; +} + static inline int security_inode_alloc(struct inode *inode) { return 0; diff --git a/security/security.c b/security/security.c index 613a5c00e602..30687e1366b7 100644 --- a/security/security.c +++ b/security/security.c @@ -871,6 +871,12 @@ int security_move_mount(const struct path *from_path, const struct path *to_path return call_int_hook(move_mount, 0, from_path, to_path); } +int security_path_notify(const struct path *path, u64 mask, + unsigned int obj_type) +{ + return call_int_hook(path_notify, 0, path, mask, obj_type); +} + int security_inode_alloc(struct inode *inode) { int rc = lsm_inode_alloc(inode); diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c index c61787b15f27..a1aaf79421df 100644 --- a/security/selinux/hooks.c +++ b/security/selinux/hooks.c @@ -92,6 +92,8 @@ #include <linux/kernfs.h> #include <linux/stringhash.h> /* for hashlen_string() */ #include <uapi/linux/mount.h> +#include <linux/fsnotify.h> +#include <linux/fanotify.h> #include "avc.h" #include "objsec.h" @@ -3261,6 +3263,50 @@ static int selinux_inode_removexattr(struct dentry *dentry, const char *name) return -EACCES; } +static int selinux_path_notify(const struct path *path, u64 mask, + unsigned int obj_type) +{ + int ret; + u32 perm; + + struct common_audit_data ad; + + ad.type = LSM_AUDIT_DATA_PATH; + ad.u.path = *path; + + /* + * Set permission needed based on the type of mark being set. + * Performs an additional check for sb watches. + */ + switch (obj_type) { + case FSNOTIFY_OBJ_TYPE_VFSMOUNT: + perm = FILE__WATCH_MOUNT; + break; + case FSNOTIFY_OBJ_TYPE_SB: + perm = FILE__WATCH_SB; + ret = superblock_has_perm(current_cred(), path->dentry->d_sb, + FILESYSTEM__WATCH, &ad); + if (ret) + return ret; + break; + case FSNOTIFY_OBJ_TYPE_INODE: + perm = FILE__WATCH; + break; + default: + return -EINVAL; + } + + // check if the mask is requesting ability to set a blocking watch + if (mask & (ALL_FSNOTIFY_PERM_EVENTS)) + perm |= FILE__WATCH_WITH_PERM; // if so, check that permission + + // is the mask asking to watch file reads? + if (mask & (FS_ACCESS | FS_ACCESS_PERM | FS_CLOSE_NOWRITE)) + perm |= FILE__WATCH_READS; // check that permission as well + + return path_has_perm(current_cred(), path, perm); +} + /* * Copy the inode security context value to the user. * @@ -6797,6 +6843,7 @@ static struct security_hook_list selinux_hooks[] __lsm_ro_after_init = { LSM_HOOK_INIT(inode_getsecid, selinux_inode_getsecid), LSM_HOOK_INIT(inode_copy_up, selinux_inode_copy_up), LSM_HOOK_INIT(inode_copy_up_xattr, selinux_inode_copy_up_xattr), + LSM_HOOK_INIT(path_notify, selinux_path_notify), LSM_HOOK_INIT(kernfs_init_security, selinux_kernfs_init_security), diff --git a/security/selinux/include/classmap.h b/security/selinux/include/classmap.h index 201f7e588a29..32e9b03be3dd 100644 --- a/security/selinux/include/classmap.h +++ b/security/selinux/include/classmap.h @@ -7,7 +7,8 @@ #define COMMON_FILE_PERMS COMMON_FILE_SOCK_PERMS, "unlink", "link", \ "rename", "execute", "quotaon", "mounton", "audit_access", \ - "open", "execmod" + "open", "execmod", "watch", "watch_mount", "watch_sb", \ + "watch_with_perm", "watch_reads" #define COMMON_SOCK_PERMS COMMON_FILE_SOCK_PERMS, "bind", "connect", \ "listen", "accept", "getopt", "setopt", "shutdown", "recvfrom", \ @@ -60,7 +61,7 @@ struct security_class_mapping secclass_map[] = { { "filesystem", { "mount", "remount", "unmount", "getattr", "relabelfrom", "relabelto", "associate", "quotamod", - "quotaget", NULL } }, + "quotaget", "watch", NULL } }, { "file", { COMMON_FILE_PERMS, "execute_no_trans", "entrypoint", NULL } },
As of now, setting watches on filesystem objects has, at most, applied a check for read access to the inode, and in the case of fanotify, requires CAP_SYS_ADMIN. No specific security hook or permission check has been provided to control the setting of watches. Using any of inotify, dnotify, or fanotify, it is possible to observe, not only write-like operations, but even read access to a file. Modeling the watch as being merely a read from the file is insufficient for the needs of SELinux. This is due to the fact that read access should not necessarily imply access to information about when another process reads from a file. Furthermore, fanotify watches grant more power to an application in the form of permission events. While notification events are solely, unidirectional (i.e. they only pass information to the receiving application), permission events are blocking. Permission events make a request to the receiving application which will then reply with a decision as to whether or not that action may be completed. This causes the issue of the watching application having the ability to exercise control over the triggering process. Without drawing a distinction within the permission check, the ability to read would imply the greater ability to control an application. Additionally, mount and superblock watches apply to all files within the same mount or superblock. Read access to one file should not necessarily imply the ability to watch all files accessed within a given mount or superblock. In order to solve these issues, a new LSM hook is implemented and has been placed within the system calls for marking filesystem objects with inotify, fanotify, and dnotify watches. These calls to the hook are placed at the point at which the target path has been resolved and are provided with the path struct, the mask of requested notification events, and the type of object on which the mark is being set (inode, superblock, or mount). The mask and obj_type have already been translated into common FS_* values shared by the entirety of the fs notification infrastructure. The path struct is passed rather than just the inode so that the mount is available, particularly for mount watches. This also allows for use of the hook by pathname-based security modules. However, since the hook is intended for use even by inode based security modules, it is not placed under the CONFIG_SECURITY_PATH conditional. Otherwise, the inode-based security modules would need to enable all of the path hooks, even though they do not use any of them. This only provides a hook at the point of setting a watch, and presumes that permission to set a particular watch implies the ability to receive all notification about that object which match the mask. This is all that is required for SELinux. If other security modules require additional hooks or infrastructure to control delivery of notification, these can be added by them. It does not make sense for us to propose hooks for which we have no implementation. The understanding that all notifications received by the requesting application are all strictly of a type for which the application has been granted permission shows that this implementation is sufficient in its coverage. Security modules wishing to provide complete control over fanotify must also implement a security_file_open hook that validates that the access requested by the watching application is authorized. Fanotify has the issue that it returns a file descriptor with the file mode specified during fanotify_init() to the watching process on event. This is already covered by the LSM security_file_open hook if the security module implements checking of the requested file mode there. Otherwise, a watching process can obtain escalated access to a file for which it has not been authorized. The selinux_path_notify hook implementation works by adding five new file permissions: watch, watch_mount, watch_sb, watch_reads, and watch_with_perm (descriptions about which will follow), and one new filesystem permission: watch (which is applied to superblock checks). The hook then decides which subset of these permissions must be held by the requesting application based on the contents of the provided mask and the obj_type. The selinux_file_open hook already checks the requested file mode and therefore ensures that a watching process cannot escalate its access through fanotify. The watch, watch_mount, and watch_sb permissions are the baseline permissions for setting a watch on an object and each are a requirement for any watch to be set on a file, mount, or superblock respectively. It should be noted that having either of the other two permissions (watch_reads and watch_with_perm) does not imply the watch, watch_mount, or watch_sb permission. Superblock watches further require the filesystem watch permission to the superblock. As there is no labeled object in view for mounts, there is no specific check for mount watches beyond watch_mount to the inode. Such a check could be added in the future, if a suitable labeled object existed representing the mount. The watch_reads permission is required to receive notifications from read-exclusive events on filesystem objects. These events include accessing a file for the purpose of reading and closing a file which has been opened read-only. This distinction has been drawn in order to provide a direct indication in the policy for this otherwise not obvious capability. Read access to a file should not necessarily imply the ability to observe read events on a file. Finally, watch_with_perm only applies to fanotify masks since it is the only way to set a mask which allows for the blocking, permission event. This permission is needed for any watch which is of this type. Though fanotify requires CAP_SYS_ADMIN, this is insufficient as it gives implicit trust to root, which we do not do, and does not support least privilege. Signed-off-by: Aaron Goidel <acgoide@tycho.nsa.gov> --- fs/notify/dnotify/dnotify.c | 15 +++++++-- fs/notify/fanotify/fanotify_user.c | 27 +++++++++++++++-- fs/notify/inotify/inotify_user.c | 13 ++++++-- include/linux/lsm_hooks.h | 9 +++++- include/linux/security.h | 10 ++++-- security/security.c | 6 ++++ security/selinux/hooks.c | 47 +++++++++++++++++++++++++++++ security/selinux/include/classmap.h | 5 +-- 8 files changed, 120 insertions(+), 12 deletions(-)