[PATCH v2] ALSA: aloop: Fix peer runtime UAF during format-change stop

["Cássio Gabriel" <cassiogabrielcontato@gmail.com>]

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, and make free_cable() wait for those stops before
detaching the runtime. This preserves the existing behavior while
making the peer runtime lifetime explicit.

Reported-by: syzbot+8fa95c41eafbc9d2ff6f@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=8fa95c41eafbc9d2ff6f
Fixes: 597603d615d2 ("ALSA: introduce the snd-aloop module for the PCM loopback")
Cc: stable@vger.kernel.org
Suggested-by: Takashi Iwai <tiwai@suse.com>
Signed-off-by: Cássio Gabriel <cassiogabrielcontato@gmail.com>
---
Changes in v2:
- simplify the peer-stop path in loopback_check_format()
- drop the extra stop_dpcm indirection and runtime/running recheck
- keep only the stop_count/stop_wait lifetime serialization
- Link to v1: https://patch.msgid.link/20260423-alsa-aloop-peer-stop-uaf-v1-1-25d8a9745f6c@gmail.com
---
 sound/drivers/aloop.c | 43 ++++++++++++++++++++++++++++++-------------
 1 file changed, 30 insertions(+), 13 deletions(-)

diff --git a/sound/drivers/aloop.c b/sound/drivers/aloop.c
index aa0d2fcb1a18..a37a1695f51c 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;
@@ -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)
+			else if (cruntime->state == SNDRV_PCM_STATE_RUNNING) {
+				/* close must not free the peer runtime below */
+				atomic_inc(&cable->stop_count);
 				stop_capture = true;
+			}
 		}
 
 		setup = get_setup(dpcm_play);
@@ -396,8 +402,11 @@ static int loopback_check_format(struct loopback_cable *cable, int stream)
 		}
 	}
 
-	if (stop_capture)
+	if (stop_capture) {
 		snd_pcm_stop(dpcm_capt->substream, SNDRV_PCM_STATE_DRAINING);
+		if (atomic_dec_and_test(&cable->stop_count))
+			wake_up(&cable->stop_wait);
+	}
 
 	return 0;
 }
@@ -1049,23 +1058,29 @@ 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 +1275,8 @@ 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: 876c495d412ef67bd4d0bdc4b74b0bd3d9f4e890
change-id: 20260422-alsa-aloop-peer-stop-uaf-004de2120b52

Best regards,
--  
Cássio Gabriel <cassiogabrielcontato@gmail.com>