On Tue, Aug 28, 2018 at 7:53 PM Mark Salyzyn <salyzyn@xxxxxxxxxxx> wrote:
>
> 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 <salyzyn@xxxxxxxxxxx>
> Cc: Miklos Szeredi <miklos@xxxxxxxxxx>
> Cc: Jonathan Corbet <corbet@xxxxxxx>
> Cc: Vivek Goyal <vgoyal@xxxxxxxxxx>
> Cc: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
> Cc: Amir Goldstein <amir73il@xxxxxxxxx>
> Cc: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
> Cc: Stephen Smalley <sds@xxxxxxxxxxxxx>
> Cc: linux-unionfs@xxxxxxxxxxxxxxx
> Cc: linux-doc@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
>
> 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.
> +
Was it me that suggested a config option??
because I can't thing of a reason to configure this as the default.
...
>
> sb->s_root = root_dentry;
> + if (!ofs->config.override_creds)
> + pr_warn("overlayfs: override_creds=off, uneven security model where mounter privileges do not overlap caller.\n");
>
This is not what the user needs to be warned about IMO.
User should be warned about consequences.
Thanks,
Amir.
