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

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Date:	Fri, 24 Jun 2016 15:48:53 +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 Fri, Jun 24, 2016 at 3:33 PM, Takashi Iwai <tiwai@...e.de> wrote:
> On Tue, 21 Jun 2016 19:41:28 +0200,
> Dmitry Vyukov wrote:
>>
>> On Mon, Jun 6, 2016 at 6:39 PM, Takashi Iwai <tiwai@...e.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@...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?
>> >
>> > 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 :)

Short term, run it under strace. It should show file names, decode
most of flags and structs.