[PATCH] ALSA: pcm: fix use-after-free in snd_pcm_post_stop

[Soham Kute <officialsohamkute@gmail.com>]

syzbot reported a slab-use-after-free in snd_pcm_post_stop() when the
PCM runtime may already be freed during teardown.

snd_pcm_post_stop() can be called after snd_pcm_detach_substream()
releases the runtime, leading to a use-after-free when accessing
runtime state and wait queues.

Add a defensive check to avoid dereferencing a freed runtime pointer.

Reported-by: syzbot+16b2b67ae905feb8a289@syzkaller.appspotmail.com

Signed-off-by: Soham Kute <officialsohamkute@gmail.com>
---
 sound/core/pcm_native.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
index 932a9bf98..7b9e2aea5 100644
--- a/sound/core/pcm_native.c
+++ b/sound/core/pcm_native.c
@@ -1542,6 +1542,10 @@ static void snd_pcm_post_stop(struct snd_pcm_substream *substream,
 			      snd_pcm_state_t state)
 {
 	struct snd_pcm_runtime *runtime = substream->runtime;
+
+	if (!runtime)
+		return;
+		
 	if (runtime->state != state) {
 		snd_pcm_trigger_tstamp(substream);
 		__snd_pcm_set_state(runtime, state);
-- 
2.34.1