[PATCH 5.10 194/491] ALSA: pcm: fix use-after-free on linked stream runtime in snd_pcm_drain()

Greg Kroah-Hartman
2026-04-13T16:51:00Z

From: Mehul Rao <mehulrao@gmail.com>

[ Upstream commit 9b1dbd69ba6f8f8c69bc7b77c2ce3b9c6ed05ba6 ]

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: f2b3614cefb6 ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
sound/core/pcm_native.c | 19 ++++++++++++++++---
1 file changed, 16 insertions(+), 3 deletions(-)

--- a/sound/core/pcm_native.c
+++ b/sound/core/pcm_native.c
@@ -2129,6 +2129,10 @@ static int snd_pcm_drain(struct snd_pcm_
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;
@@ -2148,16 +2152,25 @@ static int snd_pcm_drain(struct snd_pcm_
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);