Re: [PATCH v14 0/5] overlayfs override_creds=off & nested get xattr fix

[Amir Goldstein]

On Tue, Oct 22, 2019 at 11:45 PM Mark Salyzyn <salyzyn@xxxxxxxxxxx> wrote:
>
> Patch series:
>
> Mark Salyzyn (5):
>   Add flags option to get xattr method paired to __vfs_getxattr
>   overlayfs: check CAP_DAC_READ_SEARCH before issuing exportfs_decode_fh
>   overlayfs: handle XATTR_NOSECURITY flag for get xattr method
>   overlayfs: internal getxattr operations without sepolicy checking
>   overlayfs: override_creds=off option bypass creator_cred
>
> The first four patches address fundamental security issues that should
> be solved regardless of the override_creds=off feature.
>
> The fifth adds the feature depends on these other fixes.
>
> 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 for sepolicy, 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/overlay_creds
>
> 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
>
> ---
> v14:
> - Rejoin, rebase and a few adjustments.
>
> v13:
> - Pull out first patch and try to get it in alone feedback, some
>   Acks, and then <crickets> because people forgot why we were doing i.

Mark,

I do not see the first patch on fsdevel
and I am confused from all the suggested APIs
I recall Christoph's comment on v8 for not using xattr_gs_args
and just adding flags to existing get() method.
I agree to that comment.

I remember asking - don't remember the answer -
do you have any testing for this feature?
I have a WIP branch to run unionmount-testsuite not as root,
which is a start, but I didn't get to finish the work.
Let me know if you want to take up this work.

Thanks,
Amir.