overlayfs: check CAP_DAC_READ_SEARCH before issuing exportfs_decode_fh
overlayfs: check CAP_MKNOD before issuing vfs_whiteout
Assumption never checked, should fail if the mounter creds are not
sufficient.
overlayfs: override_creds=off option bypass creator_cred
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. This option default is set in the CONFIG
OVERLAY_FS_OVERRIDE_CREDS or in the module option override_creds.
The module boolean parameter and mount option override_creds is also
added as a presence check for this "feature" by checking existence of
/sys/module/overlay/parameters/overlay_creds. This will allow user
space to determine if the option can be supplied successfully to the
mount(2) operation.
Signed-off-by: Mark Salyzyn <[email protected]>
Cc: Miklos Szeredi <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Vivek Goyal <[email protected]>
Cc: Eric W. Biederman <[email protected]>
Cc: Amir Goldstein <[email protected]>
Cc: Randy Dunlap <[email protected]>
Cc: Stephen Smalley <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
v2:
- Forward port changed attr to stat, resulting in a build error.
- altered commit message.
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
v4:
- spelling and grammar errors in text
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