Re: [syzbot] [sound?] [usb?] general protection fault in snd_usbmidi_do_output

Takashi Iwai
2025-09-27T11:52:56Z

On Sat, 27 Sep 2025 12:36:07 +0200,
Jeongjun Park wrote:
>
> syzbot <syzbot+f02665daa2abeef4a947@syzkaller.appspotmail.com> wrote:
> >
> > Hello,
> >
> > syzbot has tested the proposed patch but the reproducer is still triggering an issue:
> > KASAN: slab-use-after-free Write in snd_usbmidi_in_urb_complete
> >
> > ==================================================================
> > BUG: KASAN: slab-use-after-free in snd_usbmidi_in_urb_complete+0x389/0x3c0 sound/usb/midi.c:251
> > Write of size 1 at addr ffff888074717943 by task kworker/1:3/5866
> >
>
> Wow, the UAF bug still occurs?
>
> But... this UAF seems to be a problem with how midi handles urb rather
> than a problem with my patch.
>
> Is there something wrong with the way snd_usbmidi_in_urb_complete() is
> implemented?

This can be rather a missing kill-and-cleanup in the code path.
So the patch like below.

Could you check whether this works for you instead of your fix, too?
timer_shutdown_sync() is already called in snd_usbmidi_disconnect(),
and the call in snd_usbmidi_free() should be superfluous after this
change.

thanks,

Takashi

--- a/sound/usb/midi.c
+++ b/sound/usb/midi.c
@@ -1522,6 +1522,9 @@ static void snd_usbmidi_free(struct snd_usb_midi *umidi)
{
int i;

+ if (!umidi->disconnected)
+ snd_usbmidi_disconnect(&umidi->list);
+
for (i = 0; i < MIDI_MAX_ENDPOINTS; ++i) {
struct snd_usb_midi_endpoint *ep = &umidi->endpoints[i];
if (ep->out)