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

[Takashi Iwai]

On Thu, 05 Mar 2026 20:35:07 +0100,
Mehul Rao wrote:
> 
> 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@xxxxxxxxxxxxxxx
> Signed-off-by: Mehul Rao <mehulrao@xxxxxxxxx>

Do you have the actual crash by fuzzer or such?  Or is it only
theoretical?

In anyway, I applied now as the fix looks fine and safe.


thanks,

Takashi