Re: [PATCH] ALSA: usb-audio: fix race condition to UAF in snd_usbmidi_free

Hillf Danton
2025-09-27T09:19:50Z

On Sat, 27 Sep 2025 10:01:37 +0200 Takashi Iwai wrote:
>On Sat, 27 Sep 2025 06:41:06 +0200 Jeongjun Park wrote:
>>
>> The previous commit 0718a78f6a9f ("ALSA: usb-audio: Kill timer properly at
>> removal") patched a UAF issue caused by the error timer.
>>
>> However, because the error timer kill added in this patch occurs after the
>> endpoint delete, a race condition to UAF still occurs, albeit rarely.
>>
>> Therefore, to prevent this, the error timer must be killed before freeing
>> the heap memory.
>>
>> Cc: <stable@vger.kernel.org>
>> Fixes: 0718a78f6a9f ("ALSA: usb-audio: Kill timer properly at removal")
>> Signed-off-by: Jeongjun Park <aha310510@gmail.com>
>
> I suppose it's a fix for the recent syzbot reports?
> https://lore.kernel.org/68d17f44.050a0220.13cd81.05b7.GAE@google.com
> https://lore.kernel.org/68d38327.a70a0220.1b52b.02be.GAE@google.com

#syz test upstream master

--- x/sound/usb/midi.c
+++ y/sound/usb/midi.c
@@ -1522,6 +1522,8 @@ static void snd_usbmidi_free(struct snd_
{
int i;

+ timer_shutdown_sync(&umidi->error_timer);
+
for (i = 0; i < MIDI_MAX_ENDPOINTS; ++i) {
struct snd_usb_midi_endpoint *ep = &umidi->endpoints[i];
if (ep->out)
@@ -1530,7 +1532,6 @@ static void snd_usbmidi_free(struct snd_
snd_usbmidi_in_endpoint_delete(ep->in);
}
mutex_destroy(&umidi->mutex);
- timer_shutdown_sync(&umidi->error_timer);
kfree(umidi);
}

--