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

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Date:	Mon, 06 Jun 2016 16:11:05 +0200
From:	Takashi Iwai <tiwai@...e.de>
To:	Dmitry Vyukov <dvyukov@...gle.com>
Cc:	Jaroslav Kysela <perex@...ex.cz>, alsa-devel@...a-project.org,
	LKML <linux-kernel@...r.kernel.org>,
	Sasha Levin <sasha.levin@...cle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Kostya Serebryany <kcc@...gle.com>,
	syzkaller <syzkaller@...glegroups.com>
Subject: Re: sound: use-after-free in hrtimer_cancel

On Sat, 04 Jun 2016 20:27:50 +0200,
Dmitry Vyukov wrote:
> 
> On Sat, Jun 4, 2016 at 8:00 PM, Dmitry Vyukov <dvyukov@...gle.com> wrote:
> > Hello,
> >
> > The following program triggers use-after-free:
> 
> Forget to mention that you need to run it in a tight parallel loop. It
> takes around 5 minutes to reproduce for me.

Hmm, this again is a bug that is difficult to trigger...  At least, I
couldn't reproduce locally.  How many processes are you running with
stress program?

It seems that there is nothing more than opening /dev/audio and does
some mmap in the job.  Is there any other relevant thing there?

Also, this assumes that the first sound card is Dummy driver, right?
Check /proc/asound/cards.

If it's about snd-dummy driver, one blind shot would be a patch like
below.  But even if it would fix, it doesn't explain why it's
triggered in that way...

thanks,

Takashi

---
diff --git a/sound/drivers/dummy.c b/sound/drivers/dummy.c
index c0f8f613f1f1..172dacd925f5 100644
--- a/sound/drivers/dummy.c
+++ b/sound/drivers/dummy.c
@@ -420,6 +420,7 @@ static int dummy_hrtimer_stop(struct snd_pcm_substream *substream)

 static inline void dummy_hrtimer_sync(struct dummy_hrtimer_pcm *dpcm)
 {
+	hrtimer_cancel(&dpcm->timer);
 	tasklet_kill(&dpcm->tasklet);
 }