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

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Message-Id: <aae67245-4c9c-f79e-b821-40753e732f65@linux.vnet.ibm.com>
Date:   Tue, 18 Jul 2017 08:12:13 -0400
From:   Stefan Berger <stefanb@...ux.vnet.ibm.com>
To:     James Morris <jmorris@...ei.org>
Cc:     "Theodore Ts'o" <tytso@....edu>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "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

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.

     Stefan