[PATCH] ALSA: aloop: Fix peer runtime UAF during format-change stop

[Cássio Gabriel]

loopback_check_format() may stop the capture side when playback starts
with parameters that no longer match a running capture stream. Commit
826af7fa62e3 ("ALSA: aloop: Fix racy access at PCM trigger") moved
the peer lookup under cable->lock, but the actual snd_pcm_stop() still
runs after dropping that lock.

A concurrent close can clear the capture entry from cable->streams[] and
detach or free its runtime while the playback trigger path still holds a
stale peer substream pointer.

Keep a per-cable count of in-flight peer stops before dropping
cable->lock, make free_cable() wait for those stops before detaching the
runtime, and take the peer stream lock around snd_pcm_stop(). This
preserves the existing behavior while making the peer runtime lifetime
explicit.

Reported-by: syzbot+8fa95c41eafbc9d2ff6f@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=8fa95c41eafbc9d2ff6f
Fixes: 597603d615d2 ("ALSA: introduce the snd-aloop module for the PCM loopback")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Cássio Gabriel <cassiogabrielcontato@xxxxxxxxx>
---
 sound/drivers/aloop.c | 56 ++++++++++++++++++++++++++++++++++++---------------
 1 file changed, 40 insertions(+), 16 deletions(-)

diff --git a/sound/drivers/aloop.c b/sound/drivers/aloop.c
index aa0d2fcb1a18..a997ee262740 100644
--- a/sound/drivers/aloop.c
+++ b/sound/drivers/aloop.c
@@ -99,6 +99,9 @@ struct loopback_ops {
 struct loopback_cable {
 	spinlock_t lock;
 	struct loopback_pcm *streams[2];
+	/* in-flight peer stops running outside cable->lock */
+	atomic_t stop_count;
+	wait_queue_head_t stop_wait;
 	struct snd_pcm_hardware hw;
 	/* flags */
 	unsigned int valid;
@@ -337,10 +340,10 @@ static bool is_access_interleaved(snd_pcm_access_t access)
 static int loopback_check_format(struct loopback_cable *cable, int stream)
 {
 	struct loopback_pcm *dpcm_play, *dpcm_capt;
+	struct loopback_pcm *stop_dpcm = NULL;
 	struct snd_pcm_runtime *runtime, *cruntime;
 	struct loopback_setup *setup;
 	struct snd_card *card;
-	bool stop_capture = false;
 	int check;
 
 	scoped_guard(spinlock_irqsave, &cable->lock) {
@@ -366,8 +369,11 @@ static int loopback_check_format(struct loopback_cable *cable, int stream)
 				return 0;
 			if (stream == SNDRV_PCM_STREAM_CAPTURE)
 				return -EIO;
-			else if (cruntime->state == SNDRV_PCM_STATE_RUNNING)
-				stop_capture = true;
+			else if (cruntime->state == SNDRV_PCM_STATE_RUNNING) {
+				/* close must not free the peer runtime below */
+				atomic_inc(&cable->stop_count);
+				stop_dpcm = dpcm_capt;
+			}
 		}
 
 		setup = get_setup(dpcm_play);
@@ -396,8 +402,18 @@ static int loopback_check_format(struct loopback_cable *cable, int stream)
 		}
 	}
 
-	if (stop_capture)
-		snd_pcm_stop(dpcm_capt->substream, SNDRV_PCM_STATE_DRAINING);
+	if (stop_dpcm) {
+		struct snd_pcm_substream *stop_substream = stop_dpcm->substream;
+		unsigned long flags;
+
+		snd_pcm_stream_lock_irqsave_nested(stop_substream, flags);
+		if (stop_substream->runtime && snd_pcm_running(stop_substream))
+			snd_pcm_stop(stop_substream, SNDRV_PCM_STATE_DRAINING);
+		snd_pcm_stream_unlock_irqrestore(stop_substream, flags);
+
+		if (atomic_dec_and_test(&cable->stop_count))
+			wake_up(&cable->stop_wait);
+	}
 
 	return 0;
 }
@@ -1049,23 +1065,29 @@ static void free_cable(struct snd_pcm_substream *substream)
 	struct loopback *loopback = substream->private_data;
 	int dev = get_cable_index(substream);
 	struct loopback_cable *cable;
+	struct loopback_pcm *dpcm;
+	bool other_alive;
 
 	cable = loopback->cables[substream->number][dev];
 	if (!cable)
 		return;
-	if (cable->streams[!substream->stream]) {
-		/* other stream is still alive */
-		guard(spinlock_irq)(&cable->lock);
-		cable->streams[substream->stream] = NULL;
-	} else {
-		struct loopback_pcm *dpcm = substream->runtime->private_data;
 
-		if (cable->ops && cable->ops->close_cable && dpcm)
-			cable->ops->close_cable(dpcm);
-		/* free the cable */
-		loopback->cables[substream->number][dev] = NULL;
-		kfree(cable);
+	scoped_guard(spinlock_irq, &cable->lock) {
+		cable->streams[substream->stream] = NULL;
+		other_alive = cable->streams[!substream->stream];
 	}
+
+	/* Pair with the stop_count increment in loopback_check_format(). */
+	wait_event(cable->stop_wait, !atomic_read(&cable->stop_count));
+	if (other_alive)
+		return;
+
+	dpcm = substream->runtime->private_data;
+	if (cable->ops && cable->ops->close_cable && dpcm)
+		cable->ops->close_cable(dpcm);
+	/* free the cable */
+	loopback->cables[substream->number][dev] = NULL;
+	kfree(cable);
 }
 
 static int loopback_jiffies_timer_open(struct loopback_pcm *dpcm)
@@ -1260,6 +1282,8 @@ static int loopback_open(struct snd_pcm_substream *substream)
 			goto unlock;
 		}
 		spin_lock_init(&cable->lock);
+		atomic_set(&cable->stop_count, 0);
+		init_waitqueue_head(&cable->stop_wait);
 		cable->hw = loopback_pcm_hardware;
 		if (loopback->timer_source)
 			cable->ops = &loopback_snd_timer_ops;

---
base-commit: 03ef04a70af3ddfa925d78015713fdb4a15e4e88
change-id: 20260422-alsa-aloop-peer-stop-uaf-004de2120b52

Best regards,
--  
Cássio Gabriel <cassiogabrielcontato@xxxxxxxxx>