[PATCH v2] ALSA: sh: aica: reorder cleanup operations to avoid UAF bugs

[Duoming Zhou]

The dreamcastcard->timer could schedule the spu_dma_work and the
spu_dma_work could also arm the dreamcastcard->timer.

When the snd_pcm_substream is closing, the aica_channel will be
deallocated. But it could still be dereferenced in the worker
thread. The reason is that del_timer() will return directly
regardless of whether the timer handler is running or not and
the worker could be rescheduled in the timer handler. As a result,
the UAF bug will happen. The racy situation is shown below:

      (Thread 1)                 |      (Thread 2)
snd_aicapcm_pcm_close()          |
 ...                             |  run_spu_dma() //worker
                                 |    mod_timer()
  flush_work()                   |
  del_timer()                    |  aica_period_elapsed() //timer
  kfree(dreamcastcard->channel)  |    schedule_work()
                                 |  run_spu_dma() //worker
  ...                            |    dreamcastcard->channel-> //USE

In order to mitigate this bug and other possible corner cases,
call mod_timer() conditionally in run_spu_dma(), then implement
PCM sync_stop op to cancel both the timer and worker. The sync_stop
op will be called from PCM core appropriately when needed.

Fixes: 198de43d758c ("[ALSA] Add ALSA support for the SEGA Dreamcast PCM device")
Suggested-by: Takashi Iwai <tiwai@xxxxxxx>
Signed-off-by: Duoming Zhou <duoming@xxxxxxxxxx>
---
Changes in v2:
  - call mod_timer() conditionally and implement PCM sync_stop op.

 sound/sh/aica.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/sound/sh/aica.c b/sound/sh/aica.c
index 320ac792c7f..3182c634464 100644
--- a/sound/sh/aica.c
+++ b/sound/sh/aica.c
@@ -278,7 +278,8 @@ static void run_spu_dma(struct work_struct *work)
 		dreamcastcard->clicks++;
 		if (unlikely(dreamcastcard->clicks >= AICA_PERIOD_NUMBER))
 			dreamcastcard->clicks %= AICA_PERIOD_NUMBER;
-		mod_timer(&dreamcastcard->timer, jiffies + 1);
+		if (snd_pcm_running(dreamcastcard->substream))
+			mod_timer(&dreamcastcard->timer, jiffies + 1);
 	}
 }
 
@@ -290,6 +291,8 @@ static void aica_period_elapsed(struct timer_list *t)
 	/*timer function - so cannot sleep */
 	int play_period;
 	struct snd_pcm_runtime *runtime;
+	if (!snd_pcm_running(substream))
+		return;
 	runtime = substream->runtime;
 	dreamcastcard = substream->pcm->private_data;
 	/* Have we played out an additional period? */
@@ -350,12 +353,19 @@ static int snd_aicapcm_pcm_open(struct snd_pcm_substream
 	return 0;
 }
 
+static int snd_aicapcm_pcm_sync_stop(struct snd_pcm_substream *substream)
+{
+	struct snd_card_aica *dreamcastcard = substream->pcm->private_data;
+
+	del_timer_sync(&dreamcastcard->timer);
+	cancel_work_sync(&dreamcastcard->spu_dma_work);
+	return 0;
+}
+
 static int snd_aicapcm_pcm_close(struct snd_pcm_substream
 				 *substream)
 {
 	struct snd_card_aica *dreamcastcard = substream->pcm->private_data;
-	flush_work(&(dreamcastcard->spu_dma_work));
-	del_timer(&dreamcastcard->timer);
 	dreamcastcard->substream = NULL;
 	kfree(dreamcastcard->channel);
 	spu_disable();
@@ -401,6 +411,7 @@ static const struct snd_pcm_ops snd_aicapcm_playback_ops = {
 	.prepare = snd_aicapcm_pcm_prepare,
 	.trigger = snd_aicapcm_pcm_trigger,
 	.pointer = snd_aicapcm_pcm_pointer,
+	.sync_stop = snd_aicapcm_pcm_sync_stop,
 };
 
 /* TO DO: set up to handle more than one pcm instance */
-- 
2.17.1