[PATCH 3.2 097/152] ALSA: timer: Fix zero-division by continue of uninitialized instance

[Ben Hutchings <ben@decadent.org.uk>]

3.2.84-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Takashi Iwai <tiwai@suse.de>

commit 9f8a7658bcafb2a7853f7a2eae8a94e87e6e695b 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 [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: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
[bwh: Backported to 3.2:
 - Adjust context
 - In _snd_timer_stop(), check the value of 'event' instead of 'stop']
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
 sound/core/timer.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

--- a/sound/core/timer.c
+++ b/sound/core/timer.c
@@ -34,6 +34,9 @@
 #include <sound/initval.h>
 #include <linux/kmod.h>
 
+/* internal flags */
+#define SNDRV_TIMER_IFLG_PAUSED		0x00010000
+
 #if defined(CONFIG_SND_HRTIMER) || defined(CONFIG_SND_HRTIMER_MODULE)
 #define DEFAULT_TIMER_LIMIT 4
 #elif defined(CONFIG_SND_RTCTIMER) || defined(CONFIG_SND_RTCTIMER_MODULE)
@@ -552,6 +555,10 @@ static int _snd_timer_stop(struct snd_ti
 		}
 	}
 	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)
@@ -594,6 +601,10 @@ int snd_timer_continue(struct snd_timer_
 
 	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;
@@ -1798,6 +1809,9 @@ static int snd_timer_user_continue(struc
 	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;
 }