linux-kernel - Re: [PATCH] sanitize task->comm to avoid leaking escape codes

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Date:	Tue, 29 Jun 2010 08:09:52 -0700
From:	Kees Cook <kees.cook@...onical.com>
To:	Alexey Dobriyan <adobriyan@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Neil Horman <nhorman@...driver.com>,
	Roland McGrath <roland@...hat.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] sanitize task->comm to avoid leaking escape codes

On Tue, Jun 29, 2010 at 11:45:14AM +0300, Alexey Dobriyan wrote:
> On Tue, Jun 29, 2010 at 12:03 AM, Kees Cook <kees.cook@...onical.com> wrote:
> > On Mon, Jun 28, 2010 at 01:00:28PM -0700, Andrew Morton wrote:
> >> Surely it would be better to fix the tools which display this info
> >> rather than making the kernel tell fibs.
> >
> > The strncpy in get_task_comm() is totally wrong -- it's testing the length
> > of task->comm.
> 
> It also fills not just any buffer but buffer which is TASK_COMM_LEN byte wide.
> 
> > Why should get_task_comm not take a destination buffer length argument?
> 
> If you pass too small, you needlessly truncate output.

If you pass too small a buffer, get_task_comm will happily write all over
the caller's stack past the end of the buffer if the contents of task->comm
are large enough:

        strncpy(buf, tsk->comm, sizeof(tsk->comm));

The "n" argument to get_task_comm's use of strncpy is totally wrong --
it needs to be the size of the destination, not the size of the source.
Luckily, everyone using get_task_comm currently uses buffers that are
sizeof(task->comm).

> If you pass too large, you waste space.

Right.

-Kees

-- 
Kees Cook
Ubuntu Security Team
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