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: 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-unionfs@vger.kernel.org
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: kernel-team@android.com
---
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.
Signed-off-by: Mark Salyzyn <salyzyn@google.com>
(cherry picked from https://lore.kernel.org/patchwork/patch/1009299)
Bug: 109821005
Bug: 112955896
Bug: 127298877
Change-Id: I1d99298ec5e71174734481be3497763c6b9d42e1
f5f87ecac1