[PATCH] ALSA: pcm: prevent snd_pcm_action after substream detach

[Soham Kute <officialsohamkute@gmail.com>]

syzbot reported a slab use-after-free in snd_pcm_post_stop() caused by
snd_pcm_action() being invoked after snd_pcm_detach_substream() has
already freed the PCM runtime.

The previous approach attempted to guard against NULL runtime access in
the post-action callback, which only masked the symptom.  As pointed out
in review, this does not address the underlying lifetime issue.

Fix the root cause by preventing snd_pcm_action() from running once the
substream runtime has been detached, ensuring that no PCM actions are
executed after teardown.

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

Signed-off-by: Soham Kute <officialsohamkute@gmail.com>
---
 sound/core/pcm_native.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
index 932a9bf98..a29dbbc21 100644
--- a/sound/core/pcm_native.c
+++ b/sound/core/pcm_native.c
@@ -1391,6 +1391,9 @@ static int snd_pcm_action(const struct action_ops *ops,
 	struct snd_pcm_group *group;
 	int res;
 
+	if (WARN_ON_ONCE(!substream->runtime))
+		return 0;
+	
 	group = snd_pcm_stream_group_ref(substream);
 	if (group)
 		res = snd_pcm_action_group(ops, substream, state, true);
-- 
2.34.1