Code: void snd_pcm_period_elapsed(struct snd_pcm_substream *substream)
{
	struct snd_pcm_runtime *runtime;
	unsigned long flags;

	if (PCM_RUNTIME_CHECK(substream))
		return;
	runtime = substream->runtime;

	snd_pcm_stream_lock_irqsave(substream, flags);
	if (!snd_pcm_running(substream) ||
	    snd_pcm_update_hw_ptr0(substream, 1) < 0)
		goto _end;

#ifdef CONFIG_SND_PCM_TIMER
	if (substream->timer_running)
 		snd_timer_interrupt(substream->timer, 1);
 #endif
  _end:
	snd_pcm_stream_unlock_irqrestore(substream, flags);
 	kill_fasync(&runtime->fasync, SIGIO, POLL_IN);
 }

Vulnerability Type: DoS 
CWE ID: CWE-416
Summary: Race condition in the snd_pcm_period_elapsed function in sound/core/pcm_lib.c in the ALSA subsystem in the Linux kernel before 4.7 allows local users to cause a denial of service (use-after-free) or possibly have unspecified other impact via a crafted SNDRV_PCM_TRIGGER_START command.
Commit Message: ALSA: pcm : Call kill_fasync() in stream lock

Currently kill_fasync() is called outside the stream lock in
snd_pcm_period_elapsed().  This is potentially racy, since the stream
may get released even during the irq handler is running.  Although
snd_pcm_release_substream() calls snd_pcm_drop(), this doesn't
guarantee that the irq handler finishes, thus the kill_fasync() call
outside the stream spin lock may be invoked after the substream is
detached, as recently reported by KASAN.

As a quick workaround, move kill_fasync() call inside the stream
lock.  The fasync is rarely used interface, so this shouldn't have a
big impact from the performance POV.

Ideally, we should implement some sync mechanism for the proper finish
of stream and irq handler.  But this oneliner should suffice for most
cases, so far.

Reported-by: Baozeng Ding <sploving1@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>