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