[PATCH v2] xattr: Enable security.capability in user namespaces

[Serge E. Hallyn]

Quoting Eric W. Biederman (ebiederm at xmission.com):
> Stefan Berger <stefanb at linux.vnet.ibm.com> writes:
> 
> > On 07/18/2017 03:01 AM, James Morris wrote:
> >> On Thu, 13 Jul 2017, Stefan Berger wrote:
> >>
> >>> A file shared by 2 containers, one mapping root to uid=1000, the other mapping
> >>> root to uid=2000, will show these two xattrs on the host (init_user_ns) once
> >>> these containers set xattrs on that file.
> >> I may be missing something here, but what happens when say the uid=2000
> >> container and associated user is deleted from the system, then another is
> >> created with the same uid?
> >>
> >> Won't this mean that you have unexpected capabilities turning up in the
> >> new container?
> >>
> >
> > Yes, that's right. I don't know any solution for that. We would have to walk the
> > filesystems and find all 'stale' xattrs with such a uid. This is independent of
> > whether the uid is encoded on the name side, as in this patch, or on the value
> > side, as in Serge's original proposal. And uids of a mapped container root user
> > don't necessarily have to have an account on the host so that an account
> > deletion could trigger that.
> 
> This problem is actually independent of this piece of code entirely.
> Any lingering files owned by that uid have the same issue.

In particular, any setuid-root files in that container have the precisely
analogous issue.

-serge