Re: [PATCH] capabilities: require CAP_SETFCAP to map uid 0 (v3.3)

[Serge E. Hallyn]

On Mon, Apr 19, 2021 at 06:09:11PM +0200, Christian Brauner wrote:
> On Mon, Apr 19, 2021 at 07:25:14AM -0500, Serge Hallyn wrote:
> > cap_setfcap is required to create file capabilities.
> > 
> > Since 8db6c34f1dbc ("Introduce v3 namespaced file capabilities"), a
> > process running as uid 0 but without cap_setfcap is able to work around
> > this as follows: unshare a new user namespace which maps parent uid 0
> > into the child namespace.  While this task will not have new
> > capabilities against the parent namespace, there is a loophole due to
> > the way namespaced file capabilities are represented as xattrs.  File
> > capabilities valid in userns 1 are distinguished from file capabilities
> > valid in userns 2 by the kuid which underlies uid 0.  Therefore the
> > restricted root process can unshare a new self-mapping namespace, add a
> > namespaced file capability onto a file, then use that file capability in
> > the parent namespace.
> > 
> > To prevent that, do not allow mapping parent uid 0 if the process which
> > opened the uid_map file does not have CAP_SETFCAP, which is the capability
> > for setting file capabilities.
> > 
> > As a further wrinkle:  a task can unshare its user namespace, then
> > open its uid_map file itself, and map (only) its own uid.  In this
> > case we do not have the credential from before unshare,  which was
> > potentially more restricted.  So, when creating a user namespace, we
> > record whether the creator had CAP_SETFCAP.  Then we can use that
> > during map_write().
> > 
> > With this patch:
> > 
> > 1. Unprivileged user can still unshare -Ur
> > 
> > ubuntu@caps:~$ unshare -Ur
> > root@caps:~# logout
> > 
> > 2. Root user can still unshare -Ur
> > 
> > ubuntu@caps:~$ sudo bash
> > root@caps:/home/ubuntu# unshare -Ur
> > root@caps:/home/ubuntu# logout
> > 
> > 3. Root user without CAP_SETFCAP cannot unshare -Ur:
> > 
> > root@caps:/home/ubuntu# /sbin/capsh --drop=cap_setfcap --
> > root@caps:/home/ubuntu# /sbin/setcap cap_setfcap=p /sbin/setcap
> > unable to set CAP_SETFCAP effective capability: Operation not permitted
> > root@caps:/home/ubuntu# unshare -Ur
> > unshare: write failed /proc/self/uid_map: Operation not permitted
> > 
> > Note: an alternative solution would be to allow uid 0 mappings by
> > processes without CAP_SETFCAP, but to prevent such a namespace from
> > writing any file capabilities.  This approach can be seen here:
> >     https://git.kernel.org/pub/scm/linux/kernel/git/sergeh/linux.git/log/?h=2021-04-15/setfcap-nsfscaps-v4
> > 
> 
> Ah, can you link to the previous fix and its revert, please? I think
> that was mentioned in the formerly private thread as well but we forgot:
> 
> commit 95ebabde382c371572297915b104e55403674e73
> Author: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
> Date:   Thu Dec 17 09:42:00 2020 -0600
> 
>     capabilities: Don't allow writing ambiguous v3 file capabilities
> 
> commit 3b0c2d3eaa83da259d7726192cf55a137769012f
> Author: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
> Date:   Fri Mar 12 15:07:09 2021 -0600
> 
>     Revert 95ebabde382c ("capabilities: Don't allow writing ambiguous v3 file capabilities")

Sure.

Is there a tag for that kind of thing or do I just mention it at the end
of the description?