Re: INFO: rcu detected stall in memcpy

[Takashi Iwai]

On Sun, 07 Jan 2018 12:30:53 +0100,
Dmitry Vyukov wrote:
> 
> 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

OK, noted.

> > 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

Ah, I didn't know that each bot can test patches individually.

> 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.

Yes, I could test and find the culprit with the given reproducer.

> 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.

Some have been already tagged with reported-by.  Some are mostly
irrelevant but casually found during the debug session for the bugs
syzkaller spotted.


thanks,

Takashi