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

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Date:	Thu, 21 Apr 2016 10:14:10 +0200
From:	Dmitry Vyukov <dvyukov@...gle.com>
To:	Takashi Iwai <tiwai@...e.de>
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, Apr 20, 2016 at 12:31 PM, Takashi Iwai <tiwai@...e.de> wrote:
> 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.

I don't have a reliable way to reproduce it. I've tried to replay the
logs for hours, but no success. And I've hit it only three times:

-rw-r----- 1 346004 Apr 19 02:36 crash-qemu-23-1461026201572599961
-rw-r----- 1 393438 Mar 27 08:24 crash-qemu-8-1459059850150353721
-rw-r----- 1 393439 Mar 10 19:44 crash-qemu-16-1457635446972474955

I will merge the patches and restart the fuzzer. It will be difficult
to conclude whether it fixes the bug or not, but at least it will test
the patches.