@@ -195,7 +195,7 @@ handle it in two different ways:
1. return EXDEV error: this error is returned by rename(2) when trying to
move a file or directory across filesystem boundaries. Hence
- applications are usually prepared to hande this error (mv(1) for example
+ applications are usually prepared to handle this error (mv(1) for example
recursively copies the directory tree). This is the default behavior.
2. If the "redirect_dir" feature is enabled, then the directory will be
@@ -324,6 +324,30 @@ and
The resulting access permissions should be the same. The difference is in
the time of copy (on-demand vs. up-front).
+### Non overlapping credentials
+
+As noted above, all access to the upper, lower and work directories is the
+recorded mounter's MAC and DAC credentials. The incoming accesses are
+checked against the caller's credentials.
+
+In the case where caller MAC or DAC credentials do not overlap the mounter, a
+use case available in older versions of the driver, the override_creds mount
+flag can be turned off. For when the use pattern has caller with legitimate
+credentials where the mounter does not. For example init may have been the
+mounter, but the caller would have execute or read MAC permissions where
+init would not. override_creds off means all access, incoming, upper, lower
+or working, will be tested against the caller.
+
+Several unintended side effects will occur though. The caller without certain
+key capabilities or lower privilege will not always be able to delete files or
+directories, create nodes, or search some restricted directories. The ability
+to search and read a directory entry is spotty as a result of the cache
+mechanism not re-testing the credentials because of the assumption, a
+privileged caller can fill cache, then a lower privilege can read the directory
+cache. The uneven security model where cache, upperdir and workdir are opened
+at privilege, but accessed without creating a form of privilege escalation,
+should only be used with strict understanding of the side effects and of the
+security policies.
Multiple lower layers
---------------------
@@ -962,7 +962,7 @@ static int ovl_copy_up_flags(struct dentry *dentry, int flags)
dput(parent);
dput(next);
}
- revert_creds(old_cred);
+ ovl_revert_creds(dentry->d_sb, old_cred);
return err;
}
@@ -565,7 +565,7 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
struct ovl_cattr *attr, bool origin)
{
int err;
- const struct cred *old_cred;
+ const struct cred *old_cred, *hold_cred = NULL;
struct cred *override_cred;
struct dentry *parent = dentry->d_parent;
@@ -592,14 +592,15 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
override_cred->fsgid = inode->i_gid;
if (!attr->hardlink) {
err = security_dentry_create_files_as(dentry,
- attr->mode, &dentry->d_name, old_cred,
+ attr->mode, &dentry->d_name,
+ old_cred ? old_cred : current_cred(),
override_cred);
if (err) {
put_cred(override_cred);
goto out_revert_creds;
}
}
- put_cred(override_creds(override_cred));
+ hold_cred = override_creds(override_cred);
put_cred(override_cred);
if (!ovl_dentry_is_whiteout(dentry))
@@ -608,7 +609,9 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
err = ovl_create_over_whiteout(dentry, inode, attr);
}
out_revert_creds:
- revert_creds(old_cred);
+ ovl_revert_creds(dentry->d_sb, old_cred ?: hold_cred);
+ if (old_cred && hold_cred)
+ put_cred(hold_cred);
return err;
}
@@ -684,7 +687,7 @@ static int ovl_set_link_redirect(struct dentry *dentry)
old_cred = ovl_override_creds(dentry->d_sb);
err = ovl_set_redirect(dentry, false);
- revert_creds(old_cred);
+ ovl_revert_creds(dentry->d_sb, old_cred);
return err;
}
@@ -903,7 +906,7 @@ static int ovl_do_remove(struct dentry *dentry, bool is_dir)
err = ovl_remove_upper(dentry, is_dir, &list);
else
err = ovl_remove_and_whiteout(dentry, &list);
- revert_creds(old_cred);
+ ovl_revert_creds(dentry->d_sb, old_cred);
if (!err) {
if (is_dir)
clear_nlink(dentry->d_inode);
@@ -1273,7 +1276,7 @@ static int ovl_rename(struct inode *olddir, struct dentry *old,
out_unlock:
unlock_rename(new_upperdir, old_upperdir);
out_revert_creds:
- revert_creds(old_cred);
+ ovl_revert_creds(old->d_sb, old_cred);
if (update_nlink)
ovl_nlink_end(new);
out_drop_write:
@@ -59,7 +59,7 @@ static struct file *ovl_open_realfile(const struct file *file,
realfile = open_with_fake_path(&file->f_path, flags, realinode,
current_cred());
}
- revert_creds(old_cred);
+ ovl_revert_creds(inode->i_sb, old_cred);
pr_debug("open(%p[%pD2/%c], 0%o) -> (%p, 0%o)\n",
file, file, ovl_whatisit(inode, realinode), file->f_flags,
@@ -209,7 +209,7 @@ static loff_t ovl_llseek(struct file *file, loff_t offset, int whence)
old_cred = ovl_override_creds(inode->i_sb);
ret = vfs_llseek(real.file, offset, whence);
- revert_creds(old_cred);
+ ovl_revert_creds(inode->i_sb, old_cred);
file->f_pos = real.file->f_pos;
ovl_inode_unlock(inode);
@@ -323,7 +323,7 @@ static ssize_t ovl_read_iter(struct kiocb *iocb, struct iov_iter *iter)
ovl_aio_cleanup_handler(aio_req);
}
out:
- revert_creds(old_cred);
+ ovl_revert_creds(file_inode(file)->i_sb, old_cred);
ovl_file_accessed(file);
fdput(real);
@@ -388,7 +388,7 @@ static ssize_t ovl_write_iter(struct kiocb *iocb, struct iov_iter *iter)
ovl_aio_cleanup_handler(aio_req);
}
out:
- revert_creds(old_cred);
+ ovl_revert_creds(file_inode(file)->i_sb, old_cred);
fdput(real);
out_unlock:
@@ -411,7 +411,7 @@ static ssize_t ovl_splice_read(struct file *in, loff_t *ppos,
old_cred = ovl_override_creds(file_inode(in)->i_sb);
ret = generic_file_splice_read(real.file, ppos, pipe, len, flags);
- revert_creds(old_cred);
+ ovl_revert_creds(file_inode(in)->i_sb, old_cred);
ovl_file_accessed(in);
fdput(real);
@@ -432,7 +432,7 @@ ovl_splice_write(struct pipe_inode_info *pipe, struct file *out,
old_cred = ovl_override_creds(file_inode(out)->i_sb);
ret = iter_file_splice_write(pipe, real.file, ppos, len, flags);
- revert_creds(old_cred);
+ ovl_revert_creds(file_inode(out)->i_sb, old_cred);
ovl_file_accessed(out);
fdput(real);
@@ -456,7 +456,7 @@ static int ovl_fsync(struct file *file, loff_t start, loff_t end, int datasync)
if (file_inode(real.file) == ovl_inode_upper(file_inode(file))) {
old_cred = ovl_override_creds(file_inode(file)->i_sb);
ret = vfs_fsync_range(real.file, start, end, datasync);
- revert_creds(old_cred);
+ ovl_revert_creds(file_inode(file)->i_sb, old_cred);
}
fdput(real);
@@ -480,7 +480,7 @@ static int ovl_mmap(struct file *file, struct vm_area_struct *vma)
old_cred = ovl_override_creds(file_inode(file)->i_sb);
ret = call_mmap(vma->vm_file, vma);
- revert_creds(old_cred);
+ ovl_revert_creds(file_inode(file)->i_sb, old_cred);
if (ret) {
/* Drop reference count from new vm_file value */
@@ -508,7 +508,7 @@ static long ovl_fallocate(struct file *file, int mode, loff_t offset, loff_t len
old_cred = ovl_override_creds(file_inode(file)->i_sb);
ret = vfs_fallocate(real.file, mode, offset, len);
- revert_creds(old_cred);
+ ovl_revert_creds(file_inode(file)->i_sb, old_cred);
/* Update size */
ovl_copyattr(ovl_inode_real(inode), inode);
@@ -530,7 +530,7 @@ static int ovl_fadvise(struct file *file, loff_t offset, loff_t len, int advice)
old_cred = ovl_override_creds(file_inode(file)->i_sb);
ret = vfs_fadvise(real.file, offset, len, advice);
- revert_creds(old_cred);
+ ovl_revert_creds(file_inode(file)->i_sb, old_cred);
fdput(real);
@@ -552,7 +552,7 @@ static long ovl_real_ioctl(struct file *file, unsigned int cmd,
ret = security_file_ioctl(real.file, cmd, arg);
if (!ret)
ret = vfs_ioctl(real.file, cmd, arg);
- revert_creds(old_cred);
+ ovl_revert_creds(file_inode(file)->i_sb, old_cred);
fdput(real);
@@ -741,7 +741,7 @@ static loff_t ovl_copyfile(struct file *file_in, loff_t pos_in,
flags);
break;
}
- revert_creds(old_cred);
+ ovl_revert_creds(file_inode(file_out)->i_sb, old_cred);
/* Update size */
ovl_copyattr(ovl_inode_real(inode_out), inode_out);
@@ -80,7 +80,7 @@ int ovl_setattr(struct dentry *dentry, struct iattr *attr)
inode_lock(upperdentry->d_inode);
old_cred = ovl_override_creds(dentry->d_sb);
err = notify_change(upperdentry, attr, NULL);
- revert_creds(old_cred);
+ ovl_revert_creds(dentry->d_sb, old_cred);
if (!err)
ovl_copyattr(upperdentry->d_inode, dentry->d_inode);
inode_unlock(upperdentry->d_inode);
@@ -272,7 +272,7 @@ int ovl_getattr(const struct path *path, struct kstat *stat,
stat->nlink = dentry->d_inode->i_nlink;
out:
- revert_creds(old_cred);
+ ovl_revert_creds(dentry->d_sb, old_cred);
return err;
}
@@ -306,7 +306,7 @@ int ovl_permission(struct inode *inode, int mask)
mask |= MAY_READ;
}
err = inode_permission(realinode, mask);
- revert_creds(old_cred);
+ ovl_revert_creds(inode->i_sb, old_cred);
return err;
}
@@ -323,7 +323,7 @@ static const char *ovl_get_link(struct dentry *dentry,
old_cred = ovl_override_creds(dentry->d_sb);
p = vfs_get_link(ovl_dentry_real(dentry), done);
- revert_creds(old_cred);
+ ovl_revert_creds(dentry->d_sb, old_cred);
return p;
}
@@ -366,7 +366,7 @@ int ovl_xattr_set(struct dentry *dentry, struct inode *inode, const char *name,
WARN_ON(flags != XATTR_REPLACE);
err = vfs_removexattr(realdentry, name);
}
- revert_creds(old_cred);
+ ovl_revert_creds(dentry->d_sb, old_cred);
/* copy c/mtime */
ovl_copyattr(d_inode(realdentry), inode);
@@ -388,7 +388,7 @@ int ovl_xattr_get(struct dentry *dentry, struct inode *inode, const char *name,
old_cred = ovl_override_creds(dentry->d_sb);
res = __vfs_getxattr(realdentry, d_inode(realdentry), name,
value, size, flags);
- revert_creds(old_cred);
+ ovl_revert_creds(dentry->d_sb, old_cred);
return res;
}
@@ -416,7 +416,7 @@ ssize_t ovl_listxattr(struct dentry *dentry, char *list, size_t size)
old_cred = ovl_override_creds(dentry->d_sb);
res = vfs_listxattr(realdentry, list, size);
- revert_creds(old_cred);
+ ovl_revert_creds(dentry->d_sb, old_cred);
if (res <= 0 || size == 0)
return res;
@@ -451,7 +451,7 @@ struct posix_acl *ovl_get_acl(struct inode *inode, int type)
old_cred = ovl_override_creds(inode->i_sb);
acl = get_acl(realinode, type);
- revert_creds(old_cred);
+ ovl_revert_creds(inode->i_sb, old_cred);
return acl;
}
@@ -485,7 +485,7 @@ static int ovl_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
old_cred = ovl_override_creds(inode->i_sb);
err = realinode->i_op->fiemap(realinode, fieinfo, start, len);
- revert_creds(old_cred);
+ ovl_revert_creds(inode->i_sb, old_cred);
return err;
}
@@ -1097,7 +1097,7 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
ovl_dentry_update_reval(dentry, upperdentry,
DCACHE_OP_REVALIDATE | DCACHE_OP_WEAK_REVALIDATE);
- revert_creds(old_cred);
+ ovl_revert_creds(dentry->d_sb, old_cred);
if (origin_path) {
dput(origin_path->dentry);
kfree(origin_path);
@@ -1124,7 +1124,7 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
kfree(upperredirect);
out:
kfree(d.redirect);
- revert_creds(old_cred);
+ ovl_revert_creds(dentry->d_sb, old_cred);
return ERR_PTR(err);
}
@@ -1176,7 +1176,7 @@ bool ovl_lower_positive(struct dentry *dentry)
dput(this);
}
}
- revert_creds(old_cred);
+ ovl_revert_creds(dentry->d_sb, old_cred);
return positive;
}
@@ -253,6 +253,7 @@ int ovl_want_write(struct dentry *dentry);
void ovl_drop_write(struct dentry *dentry);
struct dentry *ovl_workdir(struct dentry *dentry);
const struct cred *ovl_override_creds(struct super_block *sb);
+void ovl_revert_creds(struct super_block *sb, const struct cred *oldcred);
int ovl_can_decode_fh(struct super_block *sb);
struct dentry *ovl_indexdir(struct super_block *sb);
bool ovl_index_all(struct super_block *sb);
@@ -18,6 +18,7 @@ struct ovl_config {
int xino;
bool metacopy;
bool ovl_volatile;
+ bool override_creds;
};
struct ovl_sb {
@@ -286,7 +286,7 @@ static int ovl_check_whiteouts(struct dentry *dir, struct ovl_readdir_data *rdd)
}
inode_unlock(dir->d_inode);
}
- revert_creds(old_cred);
+ ovl_revert_creds(rdd->dentry->d_sb, old_cred);
return err;
}
@@ -796,7 +796,7 @@ static int ovl_iterate(struct file *file, struct dir_context *ctx)
}
err = 0;
out:
- revert_creds(old_cred);
+ ovl_revert_creds(dentry->d_sb, old_cred);
return err;
}
@@ -848,7 +848,7 @@ static struct file *ovl_dir_open_realfile(const struct file *file,
old_cred = ovl_override_creds(file_inode(file)->i_sb);
res = ovl_path_open(realpath, O_RDONLY | (file->f_flags & O_LARGEFILE));
- revert_creds(old_cred);
+ ovl_revert_creds(file_inode(file)->i_sb, old_cred);
return res;
}
@@ -986,7 +986,7 @@ int ovl_check_empty_dir(struct dentry *dentry, struct list_head *list)
old_cred = ovl_override_creds(dentry->d_sb);
err = ovl_dir_read_merged(dentry, list, &root);
- revert_creds(old_cred);
+ ovl_revert_creds(dentry->d_sb, old_cred);
if (err)
return err;
@@ -53,6 +53,11 @@ module_param_named(xino_auto, ovl_xino_auto_def, bool, 0644);
MODULE_PARM_DESC(xino_auto,
"Auto enable xino feature");
+static bool __read_mostly ovl_override_creds_def = true;
+module_param_named(override_creds, ovl_override_creds_def, bool, 0644);
+MODULE_PARM_DESC(ovl_override_creds_def,
+ "Use mounter's credentials for accesses");
+
static void ovl_entry_stack_free(struct ovl_entry *oe)
{
unsigned int i;
@@ -366,6 +371,9 @@ static int ovl_show_options(struct seq_file *m, struct dentry *dentry)
ofs->config.metacopy ? "on" : "off");
if (ofs->config.ovl_volatile)
seq_puts(m, ",volatile");
+ if (ofs->config.override_creds != ovl_override_creds_def)
+ seq_show_option(m, "override_creds",
+ ofs->config.override_creds ? "on" : "off");
return 0;
}
@@ -418,6 +426,8 @@ enum {
OPT_METACOPY_ON,
OPT_METACOPY_OFF,
OPT_VOLATILE,
+ OPT_OVERRIDE_CREDS_ON,
+ OPT_OVERRIDE_CREDS_OFF,
OPT_ERR,
};
@@ -437,6 +447,8 @@ static const match_table_t ovl_tokens = {
{OPT_METACOPY_ON, "metacopy=on"},
{OPT_METACOPY_OFF, "metacopy=off"},
{OPT_VOLATILE, "volatile"},
+ {OPT_OVERRIDE_CREDS_ON, "override_creds=on"},
+ {OPT_OVERRIDE_CREDS_OFF, "override_creds=off"},
{OPT_ERR, NULL}
};
@@ -496,6 +508,7 @@ static int ovl_parse_opt(char *opt, struct ovl_config *config)
config->redirect_mode = kstrdup(ovl_redirect_mode_def(), GFP_KERNEL);
if (!config->redirect_mode)
return -ENOMEM;
+ config->override_creds = ovl_override_creds_def;
while ((p = ovl_next_opt(&opt)) != NULL) {
int token;
@@ -585,6 +598,14 @@ static int ovl_parse_opt(char *opt, struct ovl_config *config)
config->ovl_volatile = true;
break;
+ case OPT_OVERRIDE_CREDS_ON:
+ config->override_creds = true;
+ break;
+
+ case OPT_OVERRIDE_CREDS_OFF:
+ config->override_creds = false;
+ break;
+
default:
pr_err("unrecognized mount option \"%s\" or missing value\n",
p);
@@ -2007,7 +2028,6 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
kfree(splitlower);
sb->s_root = root_dentry;
-
return 0;
out_free_oe:
@@ -37,9 +37,18 @@ const struct cred *ovl_override_creds(struct super_block *sb)
{
struct ovl_fs *ofs = sb->s_fs_info;
+ if (!ofs->config.override_creds)
+ return NULL;
return override_creds(ofs->creator_cred);
}
+void ovl_revert_creds(struct super_block *sb, const struct cred *old_cred)
+{
+ if (old_cred)
+ revert_creds(old_cred);
+}
+
+
/*
* Check if underlying fs supports file handles and try to determine encoding
* type, in order to deduce maximum inode number used by fs.
@@ -823,7 +832,7 @@ int ovl_nlink_start(struct dentry *dentry)
* value relative to the upper inode nlink in an upper inode xattr.
*/
err = ovl_set_nlink_upper(dentry);
- revert_creds(old_cred);
+ ovl_revert_creds(dentry->d_sb, old_cred);
out:
if (err)
@@ -841,7 +850,7 @@ void ovl_nlink_end(struct dentry *dentry)
old_cred = ovl_override_creds(dentry->d_sb);
ovl_cleanup_index(dentry);
- revert_creds(old_cred);
+ ovl_revert_creds(dentry->d_sb, old_cred);
}
ovl_inode_unlock(inode);
By default, all access to the upper, lower and work directories is the recorded mounter's MAC and DAC credentials. The incoming accesses are checked against the caller's credentials. If the principles of least privilege are applied, the mounter's credentials might not overlap the credentials of the caller's when accessing the overlayfs filesystem. For example, a file that a lower DAC privileged caller can execute, is MAC denied to the generally higher DAC privileged mounter, to prevent an attack vector. We add the option to turn off override_creds in the mount options; all subsequent operations after mount on the filesystem will be only the caller's credentials. The module boolean parameter and mount option override_creds is also added as a presence check for this "feature", existence of /sys/module/overlay/parameters/override_creds. It was not always this way. Circa 4.6 there was no recorded mounter's credentials, instead privileged access to upper or work directories were temporarily increased to perform the operations. The MAC (selinux) policies were caller's in all cases. override_creds=off partially returns us to this older access model minus the insecure temporary credential increases. This is to permit use in a system with non-overlapping security models for each executable including the agent that mounts the overlayfs filesystem. In Android this is the case since init, which performs the mount operations, has a minimal MAC set of privileges to reduce any attack surface, and services that use the content have a different set of MAC privileges (eg: read, for vendor labelled configuration, execute for vendor libraries and modules). The caveats are not a problem in the Android usage model, however they should be fixed for completeness and for general use in time. Signed-off-by: Mark Salyzyn <salyzyn@android.com> Cc: linux-fsdevel@vger.kernel.org Cc: linux-unionfs@vger.kernel.org Cc: Miklos Szeredi <miklos@szeredi.hu> Cc: Jonathan Corbet <corbet@lwn.net> Cc: Vivek Goyal <vgoyal@redhat.com> Cc: Eric W. Biederman <ebiederm@xmission.com> Cc: Amir Goldstein <amir73il@gmail.com> Cc: Randy Dunlap <rdunlap@infradead.org> Cc: Stephen Smalley <sds@tycho.nsa.gov> Cc: linux-security-module@vger.kernel.org Cc: linux-doc@vger.kernel.org Cc: linux-kernel@vger.kernel.org Cc: kernel-team@android.com Cc: selinux@vger.kernel.org v18 - rebase v17 - move credential section for override_creds=off as a level 3 heading subsection of the permissions section. v16 - Rebase, cover a few more new ovl_revert_creds callpoints. v15 - Rebase v14: - fix an issue in ovl_create_or_link which leaks credentials. v12 + v13 - Rebase v11: - add sb argument to ovl_revert_creds to match future work in progress in other commiter's hands. v10: - Rebase (and expand because of increased revert_cred usage) v9: - Add to the caveats v8: - drop pr_warn message after straw poll to remove it. - added a use case in the commit message v7: - change name of internal parameter to ovl_override_creds_def - report override_creds only if different than default v6: - Drop CONFIG_OVERLAY_FS_OVERRIDE_CREDS. - Do better with the documentation. - pr_warn message adjusted to report consequences. v5: - beefed up the caveats in the Documentation - Is dependent on "overlayfs: check CAP_DAC_READ_SEARCH before issuing exportfs_decode_fh" "overlayfs: check CAP_MKNOD before issuing vfs_whiteout" - Added prwarn when override_creds=off v4: - spelling and grammar errors in text v3: - Change name from caller_credentials / creator_credentials to the boolean override_creds. - Changed from creator to mounter credentials. - Updated and fortified the documentation. - Added CONFIG_OVERLAY_FS_OVERRIDE_CREDS v2: - Forward port changed attr to stat, resulting in a build error. - altered commit message. --- Documentation/filesystems/overlayfs.rst | 26 ++++++++++++++++++++++++- fs/overlayfs/copy_up.c | 2 +- fs/overlayfs/dir.c | 17 +++++++++------- fs/overlayfs/file.c | 24 +++++++++++------------ fs/overlayfs/inode.c | 18 ++++++++--------- fs/overlayfs/namei.c | 6 +++--- fs/overlayfs/overlayfs.h | 1 + fs/overlayfs/ovl_entry.h | 1 + fs/overlayfs/readdir.c | 8 ++++---- fs/overlayfs/super.c | 22 ++++++++++++++++++++- fs/overlayfs/util.c | 13 +++++++++++-- 11 files changed, 98 insertions(+), 40 deletions(-)