linux-kernel - Re: [PATCH 0/2] fix up {arg,env}

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Message-ID: <20160107095256.GA4306@uranus>
Date:	Thu, 7 Jan 2016 12:52:56 +0300
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Mateusz Guzik <mguzik@...hat.com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Alexey Dobriyan <adobriyan@...il.com>,
	Jarod Wilson <jarod@...hat.com>,
	Jan Stancek <jstancek@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH 0/2] fix up {arg,env}_{start,end} vs prctl

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@...nvz.org>

Thanks again.
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