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

[Hillf Danton]

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@xxxxxxxxxxxxxxx>
>> Fixes: 0718a78f6a9f ("ALSA: usb-audio: Kill timer properly at removal")
>> Signed-off-by: Jeongjun Park <aha310510@xxxxxxxxx>
>
> I suppose it's a fix for the recent syzbot reports?
>   https://lore.kernel.org/68d17f44.050a0220.13cd81.05b7.GAE@xxxxxxxxxx
>   https://lore.kernel.org/68d38327.a70a0220.1b52b.02be.GAE@xxxxxxxxxx

#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);
 }
 
--