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Date: Fri, 24 Jun 2016 15:33:38 +0200
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From: Takashi Iwai <tiwai@suse.de>
To: Dmitry Vyukov <dvyukov@google.com>
Cc: Jaroslav Kysela <perex@perex.cz>, alsa-devel@alsa-project.org,
        LKML <linux-kernel@vger.kernel.org>,
        Sasha Levin <sasha.levin@oracle.com>,
        Alexander Potapenko <glider@google.com>,
        Kostya Serebryany <kcc@google.com>,
        syzkaller <syzkaller@googlegroups.com>
Subject: Re: sound: use-after-free in hrtimer_cancel
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On Tue, 21 Jun 2016 19:41:28 +0200,
Dmitry Vyukov wrote:
> 
> On Mon, Jun 6, 2016 at 6:39 PM, Takashi Iwai <tiwai@suse.de> wrote:
> > On Mon, 06 Jun 2016 18:29:25 +0200,
> > Dmitry Vyukov wrote:
> >>
> >> On Mon, Jun 6, 2016 at 4:11 PM, Takashi Iwai <tiwai@suse.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@google.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?
> >
> > If it's about snd-dummy driver, hrtimer is created at open, and
> > started/stopped at PCM trigger, and removed at close.
> >
> > Is there any good way to decode which syscalls are executed in the
> > test code?
> 
> What do you mean?
> Here are the syscalls in the program:
> 
>     r[2] = syscall(SYS_open, "/dev/audio", 0xa40ul, 0, 0, 0);
>     // r[2] is in the descriptor passed to SYS_poll
>     r[15] = syscall(SYS_poll, 0x2001dde8ul, 0x4ul, 0x8ul, 0, 0, 0);
>     r[18] = syscall(SYS_readv, r[2], 0x20dc13c0ul, 0x1ul, 0, 0, 0);
>     r[19] = syscall(SYS_read, r[2], 0x20dbefe0ul, 0x20ul, 0, 0, 0);

I meant some nice way to decode these magic numbers to be more
understandable :)


Takashi