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SubjectRe: BUG: KASAN: use-after-free in snd_complete_urb+0x109e/0x1740 [snd_usb_audio] (5.11-rc6)
On Sat, 06 Feb 2021 09:13:33 +0100,
Hillf Danton wrote:
>
> On Sat, 6 Feb 2021 Takashi Iwai wrote:
> > > Due to the reconnecting key word mentioned, no fix to
> > > d0f09d1e4a88 ("ALSA: usb-audio: Refactoring endpoint URB deactivation")
> > > will be added.
> > >
> > > What is added is to capture EP_FLAG_STOPPING and remove the one
> > > second wait limit if the reconnecting acts may make it easier to
> > > repro the uaf. The diff is only for idea show.
> >
> > If my understanding is right, this won't change. The problem is
> > rather the lack of this function call itself, i.e. the missing
> > synchronization for the stream stop.
>
> Thanks for taking a look at it.
> >
> > It worked casually in the past because the endpoint resource is
> > released at a later point that is after all streams are really closed.
> > Now it's released earlier and hitting the UAF.
>
> And add it if I dont misread you.
>
> Hillf
>
> --- a/sound/usb/endpoint.c
> +++ b/sound/usb/endpoint.c
> @@ -832,24 +832,14 @@ void snd_usb_endpoint_suspend(struct snd
> */
> static int wait_clear_urbs(struct snd_usb_endpoint *ep)
> {
> - unsigned long end_time = jiffies + msecs_to_jiffies(1000);
> - int alive;
> -
> - if (!test_bit(EP_FLAG_STOPPING, &ep->flags))
> - return 0;
> -
> + WARN_ON_ONCE(!test_bit(EP_FLAG_STOPPING, &ep->flags));

EP_FLAG_STOPPING is normally zero, hence putting a WARN_ON() shows
just a false-positive.

> do {
> - alive = bitmap_weight(&ep->active_mask, ep->nurbs);
> - if (!alive)
> + if (!bitmap_weight(&ep->active_mask, ep->nurbs))
> break;
>
> schedule_timeout_uninterruptible(1);
> - } while (time_before(jiffies, end_time));
> + } while (1);
>
> - if (alive)
> - usb_audio_err(ep->chip,
> - "timeout: still %d active urbs on EP #%x\n",
> - alive, ep->ep_num);

The original report didn't show the error, so it's not about the
timeout. You're likely scratching a wrong surface, I'm afraid.

> clear_bit(EP_FLAG_STOPPING, &ep->flags);
>
> ep->sync_sink = NULL;
> @@ -1433,7 +1423,7 @@ void snd_usb_endpoint_stop(struct snd_us
> WRITE_ONCE(ep->sync_source->sync_sink, NULL);
>
> if (!atomic_dec_return(&ep->running))
> - stop_and_unlink_urbs(ep, false, false);
> + stop_and_unlink_urbs(ep, false, true);

Please don't. This will lead to a sleep-in-atomic Oops.


Takashi

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