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

Jeongjun Park
2025-09-27T08:48:05Z

Hi,

Takashi Iwai <tiwai@suse.de> 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
>

Oh, I didn't know it was already reported on syzbot.

> I had the very same fix in mind, as posted in
> https://lore.kernel.org/87plbhn16a.wl-tiwai@suse.de
> so I'll happily apply if that's the case (and it was verified to
> work). I'm just back from vacation and trying to catch up things.
>

Although it's difficult to disclose right now, I have already completed
writing a PoC that triggers a UAF due to the error timer in a slightly
different way than the backtrace reported to syzbot, and I have confirmed
that no bugs occur when testing this patch through this PoC.

>
> thanks,
>
> Takashi
>
> > ---
> > sound/usb/midi.c | 3 ++-
> > 1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/sound/usb/midi.c b/sound/usb/midi.c
> > index acb3bf92857c..8d15f1caa92b 100644
> > --- a/sound/usb/midi.c
> > +++ b/sound/usb/midi.c
> > @@ -1522,6 +1522,8 @@ static void snd_usbmidi_free(struct snd_usb_midi *umidi)
> > {
> > 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_usb_midi *umidi)
> > snd_usbmidi_in_endpoint_delete(ep->in);
> > }
> > mutex_destroy(&umidi->mutex);
> > - timer_shutdown_sync(&umidi->error_timer);
> > kfree(umidi);
> > }
> >
> > --

Regards,
Jeongjun Park