linux-kernel - Re: sound: use-after-free in snd_timer

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Date:	Wed, 20 Apr 2016 12:31:00 +0200
From:	Takashi Iwai <tiwai@...e.de>
To:	Dmitry Vyukov <dvyukov@...gle.com>
Cc:	alsa-devel@...a-project.org, Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jaroslav Kysela <perex@...ex.cz>,
	LKML <linux-kernel@...r.kernel.org>,
	Alexander Potapenko <glider@...gle.com>,
	Kostya Serebryany <kcc@...gle.com>,
	syzkaller <syzkaller@...glegroups.com>,
	Sasha Levin <sasha.levin@...cle.com>
Subject: Re: sound: use-after-free in snd_timer_interrupt

On Wed, 20 Apr 2016 10:08:55 +0200,
Takashi Iwai wrote:
> 
> On Wed, 20 Apr 2016 09:56:04 +0200,
> Dmitry Vyukov wrote:
> > 
> > On Sun, Apr 3, 2016 at 8:33 AM, Takashi Iwai <tiwai@...e.de> wrote:
> > >> >> It is not easily reproducible. I've hit several times while running
> > >> >> fuzzer for a week. Here is one of the logs for the record:
> > >> >> https://gist.githubusercontent.com/dvyukov/c84798ee55721563ecb537c4d51dc9f5/raw/f00b865a85877656f13b41917f7321730f140d35/gistfile1.txt
> > >> >
> > >> > There are a few more fixes in sound/core/timer.c since 4.5, and they
> > >> > possibly already cover this.
> > >> >
> > >> > Please let me know if this is still seen on the upcoming 4.6-rc2.
> > >>
> > >> Hi Takashi,
> > >>
> > >> I've updated fuzzer to 05cf8077e54b20dddb756eaa26f3aeb5c38dd3cf (Apr
> > >> 1) yesterday. Let's see if it still happens.
> > >>
> > >> Out of curiosity, how was the bug found?
> > >
> > > Well, I'm not entirely sure whether they really cover.  It's just a
> > > hope, as these are patches to close some possible races :)
> > >
> > > 9984d1b5835ca29fc7025186a891ee7398d21cc7
> > >     ALSA: timer: Protect the whole snd_timer_close() with open race
> > > f65e0d299807d8a11812845c972493c3f9a18e10
> > >     ALSA: timer: Call notifier in the same spinlock
> > > 4a07083ed613644c96c34a7dd2853dc5d7c70902
> > >     ALSA: timer: Use mod_timer() for rearming the system timer
> > 
> > 
> > Hi Takashi,
> > 
> > I've hit it again on 806fdcce017dc98c4dbf8ed001750a0d7d2bb0af (Apr
> > 14), all 3 commits are already in my tree.
> > 
> > [  343.222218] ------------[ cut here ]------------
> > [  343.222218] WARNING: CPU: 3 PID: 7040 at kernel/time/hrtimer.c:837
> > hrtimer_forward+0x26a/0x3e0
> 
> This is a different warning.  The previous was use-after-free, and
> this is a warning about re-arming the queued hrtimer.
> Maybe there is a slightly remaining race about hrtimer_start() and the
> interrupt handler in snd-hrtimer.

Could you check whether two patches below help anything?
This should harden against the race between hrtimer callback and
another start/stop calls.


Takashi


Download attachment "0001-ALSA-timer-Allow-backend-disabling-start-stop-from-h.patch" of type "application/octet-stream" (4254 bytes)

Download attachment "0002-ALSA-hrtimer-Use-manual-start-stop-in-callback.patch" of type "application/octet-stream" (3932 bytes)