linux-kernel - Re: [PATCH] prctl: Allow local CAP_SYS_ADMIN changing exe

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Date:   Fri, 12 May 2017 17:58:24 +0300
From:   Cyrill Gorcunov <gorcunov@...il.com>
To:     Kirill Tkhai <ktkhai@...tuozzo.com>
Cc:     mhocko@...e.com, avagin@...nvz.org, skinsbursky@...tuozzo.com,
        oleg@...hat.com, linux-kernel@...r.kernel.org,
        ebiederm@...ssion.com, akpm@...ux-foundation.org,
        ptikhomirov@...tuozzo.com, serge@...lyn.com
Subject: Re: [PATCH] prctl: Allow local CAP_SYS_ADMIN changing exe_file

On Fri, May 12, 2017 at 05:33:36PM +0300, Kirill Tkhai wrote:
> During checkpointing and restore of userspace tasks
> we bumped into the situation, that it's not possible
> to restore the tasks, which user namespace does not
> have uid 0 or gid 0 mapped.
> 
> People create user namespace mappings like they want,
> and there is no a limitation on obligatory uid and gid
> "must be mapped". So, if there is no uid 0 or gid 0
> in the mapping, it's impossible to restore mm->exe_file
> of the processes belonging to this user namespace.
> 
> Also, there is no a workaround. It's impossible
> to create a temporary uid/gid mapping, because
> only one write to /proc/[pid]/uid_map and gid_map
> is allowed during a namespace lifetime.
> If there is an entry, then no more mapings can't be
> written. If there isn't an entry, we can't write
> there too, otherwise user task won't be able
> to do that in the future.
> 
> The patch changes the check, and looks for CAP_SYS_ADMIN
> instead of zero uid and gid. This allows to restore
> a task independently of its user namespace mappings.
> 
> Signed-off-by: Kirill Tkhai <ktkhai@...tuozzo.com>
> CC: Andrew Morton <akpm@...ux-foundation.org>
> CC: Serge Hallyn <serge@...lyn.com>
> CC: "Eric W. Biederman" <ebiederm@...ssion.com>
> CC: Oleg Nesterov <oleg@...hat.com>
> CC: Michal Hocko <mhocko@...e.com>
> CC: Andrei Vagin <avagin@...nvz.org>
> CC: Cyrill Gorcunov <gorcunov@...nvz.org>
> CC: Stanislav Kinsburskiy <skinsbursky@...tuozzo.com>
> CC: Pavel Tikhomirov <ptikhomirov@...tuozzo.com>
> ---
>  kernel/sys.c |    8 ++------
>  1 file changed, 2 insertions(+), 6 deletions(-)

Reviewed-by: Cyrill Gorcunov <gorcunov@...nvz.org>