Commit 0431fcc2 authored by Takashi Iwai's avatar Takashi Iwai Committed by Ben Hutchings

ALSA: timer: Fix zero-division by continue of uninitialized instance

commit 9f8a7658 upstream.

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 [6b760bb2: 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: default avatarDmitry Vyukov <dvyukov@google.com>
Signed-off-by: default avatarTakashi Iwai <tiwai@suse.de>
[bwh: Backported to 3.16:
 - Adjust context
 - In _snd_timer_stop(), check the value of 'event' instead of 'stop']
Signed-off-by: default avatarBen Hutchings <ben@decadent.org.uk>
parent 151b56d3
......@@ -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
#elif IS_ENABLED(CONFIG_SND_RTCTIMER)
......@@ -569,6 +572,10 @@ static int _snd_timer_stop(struct snd_timer_instance *timeri, int event)
}
}
timeri->flags &= ~(SNDRV_TIMER_IFLG_RUNNING | SNDRV_TIMER_IFLG_START);
if (event == SNDRV_TIMER_EVENT_STOP)
timeri->flags &= ~SNDRV_TIMER_IFLG_PAUSED;
else
timeri->flags |= SNDRV_TIMER_IFLG_PAUSED;
spin_unlock_irqrestore(&timer->lock, flags);
__end:
if (event != SNDRV_TIMER_EVENT_RESOLUTION)
......@@ -611,6 +618,10 @@ int snd_timer_continue(struct snd_timer_instance *timeri)
if (timeri == NULL)
return result;
/* 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);
timer = timeri->timer;
......@@ -1844,6 +1855,9 @@ 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;
}
......
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