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Date: Thu, 7 Jan 2016 12:52:56 +0300
From: Cyrill Gorcunov <gorcunov@gmail.com>
To: Mateusz Guzik <mguzik@redhat.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Alexey Dobriyan <adobriyan@gmail.com>, Jarod Wilson <jarod@redhat.com>,
        Jan Stancek <jstancek@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Al Viro <viro@zeniv.linux.org.uk>
Subject: Re: [PATCH 0/2] fix up {arg,env}_{start,end} vs prctl
Message-ID: <20160107095256.GA4306@uranus>
References: <1452056549-10048-1-git-send-email-mguzik@redhat.com>
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On Wed, Jan 06, 2016 at 06:02:27AM +0100, Mateusz Guzik wrote:
> An unprivileged user can trigger an oops on a kernel with
> CONFIG_CHECKPOINT_RESTORE.
> 
> proc_pid_cmdline_read takes mmap_sem for reading and obtains args + env
> start/end values. These get sanity checked as follows:
>         BUG_ON(arg_start > arg_end);
>         BUG_ON(env_start > env_end);
> 
> These can be changed by prctl_set_mm. Turns out also takes the semaphore for
> reading, effectively rendering it useless. This results in:

Thanks a lot for catching it! You know I tried to escape taking sem
for writing as long as I could so another option might be simply
zap these BUG_ON and rather exit with -EINVAL. On the other hands
modification under read-lock of course is not correct in terms
of "general approach" but these members are special so I took
a risk. Anyway,

Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>

Thanks again.