linux-kernel - Re: [PATCH v2] xattr: Enable security.capability in user namespaces

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Date:   Tue, 18 Jul 2017 18:13:50 -0500
From:   "Serge E. Hallyn" <serge@...lyn.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Stefan Berger <stefanb@...ux.vnet.ibm.com>,
        James Morris <jmorris@...ei.org>,
        Theodore Ts'o <tytso@....edu>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        containers@...ts.linux-foundation.org, lkp@...org,
        linux-kernel@...r.kernel.org, zohar@...ux.vnet.ibm.com,
        tycho@...ker.com, James.Bottomley@...senPartnership.com,
        vgoyal@...hat.com, christian.brauner@...lbox.org,
        amir73il@...il.com, linux-security-module@...r.kernel.org,
        casey@...aufler-ca.com
Subject: Re: [PATCH v2] xattr: Enable security.capability in user namespaces

Quoting Eric W. Biederman (ebiederm@...ssion.com):
> Stefan Berger <stefanb@...ux.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