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

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Date:	Mon, 6 Jun 2016 18:29:25 +0200
From:	Dmitry Vyukov <dvyukov@...gle.com>
To:	Takashi Iwai <tiwai@...e.de>
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 Mon, Jun 6, 2016 at 4:11 PM, Takashi Iwai <tiwai@...e.de> wrote:
> 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?

I use a VM with 4 cores and use 20 parallel test processes.

> 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?


I think poll with timeout is related. It is poll who sets hrtimer, right?


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

# cat /proc/asound/cards
 0 [Dummy          ]: Dummy - Dummy
                      Dummy 1
 1 [Loopback       ]: Loopback - Loopback
                      Loopback 1
 2 [VirMIDI        ]: VirMIDI - VirMIDI
                      Virtual MIDI Card 1
 3 [pcsp           ]: PC-Speaker - pcsp
                      Internal PC-Speaker at port 0x61
 4 [Intel          ]: HDA-Intel - HDA Intel
                      HDA Intel at 0xfebf0000 irq 24



> 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);
>  }
>