linux-kernel - Re: [PATCH] capabilities: Introduce CAP

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Date:   Fri, 22 May 2020 11:02:28 -0700
From:   Andrei Vagin <avagin@...il.com>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     Adrian Reber <areber@...hat.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        Pavel Emelyanov <ovzxemul@...il.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Nicolas Viennot <Nicolas.Viennot@...sigma.com>,
        Michał Cłapiński <mclapinski@...gle.com>,
        Kamil Yurtsever <kyurtsever@...gle.com>,
        Dirk Petersen <dipeit@...il.com>,
        Christine Flood <chf@...hat.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Radostin Stoyanov <rstoyanov1@...il.com>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        Serge Hallyn <serge@...lyn.com>,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        Sargun Dhillon <sargun@...gun.me>,
        Arnd Bergmann <arnd@...db.de>,
        Aaron Goidel <acgoide@...ho.nsa.gov>,
        linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, selinux@...r.kernel.org,
        Eric Paris <eparis@...isplace.org>,
        Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH] capabilities: Introduce CAP_RESTORE

On Fri, May 22, 2020 at 09:53:31AM +0200, Christian Brauner wrote:
> On Fri, May 22, 2020 at 07:53:50AM +0200, Adrian Reber wrote:
> > 
> > There are probably a few more things guarded by CAP_SYS_ADMIN required
> > to run checkpoint/restore as non-root, but by applying this patch I can
> > already checkpoint and restore processes as non-root. As there are
> > already multiple workarounds I would prefer to do it correctly in the
> > kernel to avoid that CRIU users are starting to invent more workarounds.
> 
> It sounds ok to me as long as this feature is guarded by any sensible
> capability. I don't want users to be able to randomly choose their pid
> without any capability required.
> 
> We've heard the plea for unprivileged checkpoint/restore through the
> grapevine and a few times about CAP_RESTORE at plumbers but it's one of
> those cases where nobody pushed for it so it's urgency was questionable.
> This is 5.9 material though and could you please add selftests?
> 
> It also seems you have future changes planned that would make certain
> things accessible via CAP_RESTORE that are currently guarded by other
> capabilities. Any specific things in mind? It might be worth knowing
> what we'd be getting ourselves into if you're planning on flipping
> switches in other places.

/proc/pid/map_files is one of the first candidate what we need to think
about. CRIU opens files from /proc/pid/map_files to dump file mappings,
shared memory mappings, memfd files.

Right now, it is impossible to open these files without CAP_SYS_ADMIN in
the root user-namespace (proc_map_files_get_link).