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 <redacted>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Eric W. Biederman <redacted>
Cc: Amir Goldstein <amir73il@gmail.com>
Cc: Randy Dunlap <redacted>
Cc: Stephen Smalley <redacted>
Cc: linux-unionfs@vger.kernel.org
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org

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
---
 Documentation/filesystems/overlayfs.txt | 29 +++++++++++++++++++++++++
 fs/overlayfs/Kconfig                    | 22 +++++++++++++++++++
 fs/overlayfs/copy_up.c                  |  2 +-
 fs/overlayfs/dir.c                      |  9 ++++----
 fs/overlayfs/inode.c                    | 16 +++++++-------
 fs/overlayfs/namei.c                    |  6 ++---
 fs/overlayfs/overlayfs.h                |  1 +
 fs/overlayfs/ovl_entry.h                |  1 +
 fs/overlayfs/readdir.c                  |  4 ++--
 fs/overlayfs/super.c                    | 23 ++++++++++++++++++++
 fs/overlayfs/util.c                     | 12 ++++++++--
 11 files changed, 105 insertions(+), 20 deletions(-)

diff --git a/Documentation/filesystems/overlayfs.txt b/Documentation/filesystems/overlayfs.txt
index 72615a2c0752..953e52971eb0 100644
--- a/Documentation/filesystems/overlayfs.txt
+++ b/Documentation/filesystems/overlayfs.txt

@@ -106,6 +106,35 @@ Only the lists of names from directories are merged.  Other content
 such as metadata and extended attributes are reported for the upper
 directory only.  These attributes of the lower directory are hidden.

+credentials
+-----------
+
+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, but is MAC denied to the
+generally higher DAC privileged mounter, to prevent an attack vector
+executing with the increased privileges of the mounter.  One option is
+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.
+Fundamentally The mounter has privileges, its ability to execute,
+for example, files and grant them these higher privileges is to be
+blocked except to lower privileged and appropriate callers.  This
+option turned off permits this kind of security policy.
+
+With override_creds turned off, several unintended side effects will
+occur.  The caller with a lower privilege will not be able to delete
+files or directories, create nodes, or search some directories.  The
+uneven security model where upperdir and workdir are opened at
+privilege, but accessed without, should only be used with strict
+understanding of the side effects and of the security policies.
+
 whiteouts and opaque directories
 --------------------------------

diff --git a/fs/overlayfs/Kconfig b/fs/overlayfs/Kconfig
index 9384164253ac..b55bb0d48415 100644
--- a/fs/overlayfs/Kconfig
+++ b/fs/overlayfs/Kconfig

@@ -103,3 +103,25 @@ config OVERLAY_FS_XINO_AUTO
          For more information, see Documentation/filesystems/overlayfs.txt

          If unsure, say N.
+
+config OVERLAY_FS_OVERRIDE_CREDS
+       bool "Overlay filesystem override credentials"
+       depends on OVERLAY_FS
+       default y
+       help
+         If set, 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.  The mount option
+         "override_creds=off" drops the mounter's credential check, so that
+         all subsequent operations, after mount, on the filesystem will only
+         be the caller's credentials.  This option sets the default for the
+         module option override_creds, and thus the default for all mounts
+         that do not specify this option.
+