ALSA: pcm: fix use-after-free on linked stream runtime in snd_pcm_drain()

In the drain loop, the local variable 'runtime' is reassigned to a
linked stream's runtime (runtime = s->runtime at line 2157).  After
releasing the stream lock at line 2169, the code accesses
runtime->no_period_wakeup, runtime->rate, and runtime->buffer_size
(lines 2170-2178) — all referencing the linked stream's runtime without
any lock or refcount protecting its lifetime.

A concurrent close() on the linked stream's fd triggers
snd_pcm_release_substream() → snd_pcm_drop() → pcm_release_private()
→ snd_pcm_unlink() → snd_pcm_detach_substream() → kfree(runtime).
No synchronization prevents kfree(runtime) from completing while the
drain path dereferences the stale pointer.

Fix by caching the needed runtime fields (no_period_wakeup, rate,
buffer_size) into local variables while still holding the stream lock,
and using the cached values after the lock is released.

Fixes: f2b3614 ("ALSA: PCM - Don't check DMA time-out too shortly")
Cc: stable@vger.kernel.org
Signed-off-by: Mehul Rao <mehulrao@gmail.com>
Link: https://patch.msgid.link/20260305193508.311096-1-mehulrao@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>

Author: mehulrao

sound/core/pcm_native.c

@@ -2144,6 +2144,10 @@ static int snd_pcm_drain(struct snd_pcm_substream *substream,
 	for (;;) {
 		long tout;
 		struct snd_pcm_runtime *to_check;
+		unsigned int drain_rate;
+		snd_pcm_uframes_t drain_bufsz;
+		bool drain_no_period_wakeup;
+
 		if (signal_pending(current)) {
 			result = -ERESTARTSYS;
 			break;
@@ -2163,16 +2167,25 @@ static int snd_pcm_drain(struct snd_pcm_substream *substream,
 		snd_pcm_group_unref(group, substream);
 		if (!to_check)
 			break; /* all drained */
+		/*
+		 * Cache the runtime fields needed after unlock.
+		 * A concurrent close() on the linked stream may free
+		 * its runtime via snd_pcm_detach_substream() once we
+		 * release the stream lock below.
+		 */
+		drain_no_period_wakeup = to_check->no_period_wakeup;
+		drain_rate = to_check->rate;
+		drain_bufsz = to_check->buffer_size;
 		init_waitqueue_entry(&wait, current);
 		set_current_state(TASK_INTERRUPTIBLE);
 		add_wait_queue(&to_check->sleep, &wait);
 		snd_pcm_stream_unlock_irq(substream);
-		if (runtime->no_period_wakeup)
+		if (drain_no_period_wakeup)
 			tout = MAX_SCHEDULE_TIMEOUT;
 		else {
 			tout = 100;
-			if (runtime->rate) {
-				long t = runtime->buffer_size * 1100 / runtime->rate;
+			if (drain_rate) {
+				long t = drain_bufsz * 1100 / drain_rate;
 				tout = max(t, tout);
 			}
 			tout = msecs_to_jiffies(tout);