Re: INFO: rcu detected stall in memcpy

[Dmitry Vyukov]

On Thu, Jan 4, 2018 at 6:03 PM, Takashi Iwai <tiwai@xxxxxxx> wrote:
> On Thu, 04 Jan 2018 15:17:23 +0100,
> Takashi Iwai wrote:
>>
>> On Thu, 04 Jan 2018 15:01:06 +0100,
>> Dmitry Vyukov wrote:
>> >
>> > On Thu, Jan 4, 2018 at 1:57 PM, Takashi Iwai <tiwai@xxxxxxx> wrote:
>> > > On Thu, 04 Jan 2018 13:08:45 +0100,
>> > > Dmitry Vyukov wrote:
>> > >>
>> > >> On Thu, Jan 4, 2018 at 1:03 PM, syzbot
>> > >> <syzbot+387f48da65cb522abfe8@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
>> > >> > Hello,
>> > >> >
>> > >> > syzkaller hit the following crash on
>> > >> > 30a7acd573899fd8b8ac39236eff6468b195ac7d
>> > >> > git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/master
>> > >> > compiler: gcc (GCC) 7.1.1 20170620
>> > >> > .config is attached
>> > >> > Raw console output is attached.
>> > >> > Unfortunately, I don't have any reproducer for this bug yet.
>> > >> >
>> > >> >
>> > >> > IMPORTANT: if you fix the bug, please add the following tag to the commit:
>> > >> > Reported-by: syzbot+387f48da65cb522abfe8@xxxxxxxxxxxxxxxxxxxxxxxxx
>> > >> > It will help syzbot understand when the bug is fixed. See footer for
>> > >> > details.
>> > >> > If you forward the report, please keep this part and the footer.
>> > >>
>> > >> This looks ALSA-related. +ALSA maintainers.
>> > >
>> > > Not sure exactly what triggers it.  It's the simple memcpy(), and I
>> > > don't know where RCU is involved in that code path.
>> > >
>> > > BTW, other two suspicious RCU usage reports are actually stopped at
>> > > the second WARN_ON() after the RCU message, and the second WARN_ON()
>> > > is independent from RCU; it's the known spurious WARN_ON() and was
>> > > already removed in the sound git tree.
>> >
>> >
>> > Hi Takashi,
>> >
>> > Another similar one just popped up:
>> >
>> > https://groups.google.com/forum/#!topic/syzkaller-bugs/X3d6-PIrJM0
>> >
>> > This looks like mulaw_decode enters an infinite loop, or at least
>> > doing very large amount of computations without a resched, e.g.
>> > (uint64_t)-1 number of iterations of something along these lines.
>>
>> OK, that makes sense.
>>
>> My rough guess is that it's the misconfigured aloop device by
>> concurrent setup.  The aloop device allows to restrict the parameters
>> of the other side of the connection, and something bad may happen
>> there if both sides are updated concurrently.
>>
>> We've seen segfault by memset() at loopback_preapre() in
>> sound/drivers/aloop.c by syzbot+3902b5220e8ca27889ca, too, which
>> indicates also the wrongly setup parameters that overflows the
>> allocated buffer.
>
> Below two patches may possibly plug the holes, but I'm not entirely
> sure whether that's the exact culprit.  Could you put them into syzbot
> to watch whether they have any influence?

Hi Takashi,

I've gave an answer to this here:
https://groups.google.com/d/msg/syzkaller-bugs/7ucgCkAJKSk/skZjgavRAQAJ

> In anyway, they are obvious bugs to be fixed, so I'm going to queue to
> my tree.

The options are:
1. You can ask syzbot to test the patch separately. This requires a
reproducer, but there is this bug which has a reproducer and seems to
have the same root cause:
https://groups.google.com/d/msg/syzkaller-bugs/KrPUlf-nm5g/Vk0xEq-HAAAJ
2. You can reproduce it with the reproducer from here:
https://groups.google.com/d/msg/syzkaller-bugs/KrPUlf-nm5g/Vk0xEq-HAAAJ
and then test the patch as extensively as needed.
3. If you have some confidence that the patch fixes the problem, then
mark the commit with the tag:
Reported-by: syzbot+387f48da65cb522abfe8@xxxxxxxxxxxxxxxxxxxxxxxxx
then syzbot will notify if this still happens after the commit reaches
tested trees.