Re: sound: divide by 0 in snd_hrtimer_callback (or hang)

[Dmitry Vyukov]

On Wed, Sep 7, 2016 at 5:00 PM, Takashi Iwai <tiwai@xxxxxxx> wrote:
> On Tue, 06 Sep 2016 18:10:30 +0200,
> Takashi Iwai wrote:
>>
>> On Tue, 06 Sep 2016 16:06:08 +0200,
>> Dmitry Vyukov wrote:
>> >
>> > > Do you really see the zero-division error with sticks=1 fix?  That's
>> > > unexpected.
>> >
>> > Yes, just reproduced it again.
>> > I am on 0693c28cfc8e25f18c25b65a8942c026f1854a3c of linux-next now, so
>> > I already have:
>> >     commit 6b760bb2c63a9e322c0e4a0b5daf335ad93d5a33
>> >     ALSA: timer: fix division by zero after SNDRV_TIMER_IOCTL_CONTINUE
>> >
>> > Compiled the program verbatim and run it using stress utility (http
>> > s://godoc.org/golang.org/x/tools/cmd/stress namely just runs the
>> > program in parallel loop) and instantly get the following crash.
>>
>> OK, I'll take a deeper look.
>
> The fix patch below seems working on my system.  Please give it a
> try.


Pulled this from linux-next.
This should fix both division and stalls, right?
I will let you know if I see any of these again.


> -- 8< --
> From: Takashi Iwai <tiwai@xxxxxxx>
> Subject: [PATCH] ALSA: timer: Fix zero-division by continue of uninitialized
>  instance
>
> When a user timer instance is continued without the explicit start
> beforehand, the system gets eventually zero-division error like:
>
>   divide error: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN
>   CPU: 1 PID: 27320 Comm: syz-executor Not tainted 4.8.0-rc3-next-20160825+ #8
>   Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
>    task: ffff88003c9b2280 task.stack: ffff880027280000
>    RIP: 0010:[<ffffffff858e1a6c>]  [<     inline     >] ktime_divns include/linux/ktime.h:195
>    RIP: 0010:[<ffffffff858e1a6c>]  [<ffffffff858e1a6c>] snd_hrtimer_callback+0x1bc/0x3c0 sound/core/hrtimer.c:62
>   Call Trace:
>    <IRQ>
>    [<     inline     >] __run_hrtimer kernel/time/hrtimer.c:1238
>    [<ffffffff81504335>] __hrtimer_run_queues+0x325/0xe70 kernel/time/hrtimer.c:1302
>    [<ffffffff81506ceb>] hrtimer_interrupt+0x18b/0x420 kernel/time/hrtimer.c:1336
>    [<ffffffff8126d8df>] local_apic_timer_interrupt+0x6f/0xe0 arch/x86/kernel/apic/apic.c:933
>    [<ffffffff86e13056>] smp_apic_timer_interrupt+0x76/0xa0 arch/x86/kernel/apic/apic.c:957
>    [<ffffffff86e1210c>] apic_timer_interrupt+0x8c/0xa0 arch/x86/entry/entry_64.S:487
>    <EOI>
>    .....
>
> Although a similar issue was spotted and a fix patch was merged in
> commit [6b760bb2c63a: ALSA: timer: fix division by zero after
> SNDRV_TIMER_IOCTL_CONTINUE], it seems covering only a part of
> iceberg.
>
> In this patch, we fix the issue a bit more drastically.  Basically the
> continue of an uninitialized timer is supposed to be a fresh start, so
> we do it for user timers.  For the direct snd_timer_continue() call,
> there is no way to pass the initial tick value, so we kick out for the
> uninitialized case.
>
> Reported-by: Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
> ---
>  sound/core/timer.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
>
> diff --git a/sound/core/timer.c b/sound/core/timer.c
> index 2706061fc1ea..fc144f43faa6 100644
> --- a/sound/core/timer.c
> +++ b/sound/core/timer.c
> @@ -35,6 +35,9 @@
>  #include <sound/initval.h>
>  #include <linux/kmod.h>
>
> +/* internal flags */
> +#define SNDRV_TIMER_IFLG_PAUSED                0x00010000
> +
>  #if IS_ENABLED(CONFIG_SND_HRTIMER)
>  #define DEFAULT_TIMER_LIMIT 4
>  #else
> @@ -539,6 +542,10 @@ static int snd_timer_stop1(struct snd_timer_instance *timeri, bool stop)
>                 }
>         }
>         timeri->flags &= ~(SNDRV_TIMER_IFLG_RUNNING | SNDRV_TIMER_IFLG_START);
> +       if (stop)
> +               timeri->flags &= ~SNDRV_TIMER_IFLG_PAUSED;
> +       else
> +               timeri->flags |= SNDRV_TIMER_IFLG_PAUSED;
>         snd_timer_notify1(timeri, stop ? SNDRV_TIMER_EVENT_STOP :
>                           SNDRV_TIMER_EVENT_CONTINUE);
>   unlock:
> @@ -600,6 +607,10 @@ int snd_timer_stop(struct snd_timer_instance *timeri)
>   */
>  int snd_timer_continue(struct snd_timer_instance *timeri)
>  {
> +       /* timer can continue only after pause */
> +       if (!(timeri->flags & SNDRV_TIMER_IFLG_PAUSED))
> +               return -EINVAL;
> +
>         if (timeri->flags & SNDRV_TIMER_IFLG_SLAVE)
>                 return snd_timer_start_slave(timeri, false);
>         else
> @@ -1831,6 +1842,9 @@ static int snd_timer_user_continue(struct file *file)
>         tu = file->private_data;
>         if (!tu->timeri)
>                 return -EBADFD;
> +       /* start timer instead of continue if it's not used before */
> +       if (!(tu->timeri->flags & SNDRV_TIMER_IFLG_PAUSED))
> +               return snd_timer_user_start(file);
>         tu->timeri->lost = 0;
>         return (err = snd_timer_continue(tu->timeri)) < 0 ? err : 0;
>  }
> --
> 2.10.0
>