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

Takashi Iwai
2025-09-27T08:01:41Z

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

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.

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);
> }
>
> --