linux-kernel - Re: [patch 5/5] futex: Simplify futex_lock_pi

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Date:	Fri, 13 Jun 2014 10:34:33 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Darren Hart <darren@...art.com>
cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Davidlohr Bueso <davidlohr@...com>,
	Kees Cook <kees@...flux.net>, wad@...omium.org
Subject: Re: [patch 5/5] futex: Simplify futex_lock_pi_atomic() and make it
 more robust

On Thu, 12 Jun 2014, Darren Hart wrote:

> On Wed, 2014-06-11 at 20:45 +0000, Thomas Gleixner wrote:
> > futex_lock_pi_atomic() is a maze of retry hoops and loops.
> > 
> > Reduce it to simple and understandable states:
> 
> Heh... well...
> 
> With this patch applied (1-4 will not reproduce without 5), if userspace
> wrongly sets the uval to 0, the pi_state can end up being NULL for a
> subsequent requeue_pi operation:
> 
> [   10.426159] requeue: 00000000006022e0 to 00000000006022e4
> [   10.427737]   this:ffff88013a637da8
> [   10.428749]   waking:ffff88013a637da8
> fut2 = 0
> [   10.429994]   comparing requeue_pi_key
> [   10.431034]   prepare waiter to take the rt_mutex
> [   10.432344]   pi_state:          (null)
> [   10.433414] BUG: unable to handle kernel NULL pointer dereference at
> 0000000000000038
> 
> This occurs in the requeue loop, in the requeue_pi block at:
> 
> 	atomic_inc(&pi_state->refcount);

Hmm. Took me some time to reproduce. Digging into it now.

Thanks,

	tglx
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