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

Takashi Iwai
2025-09-27T09:39:05Z

On Sat, 27 Sep 2025 10:48:02 +0200,
Jeongjun Park wrote:
>
> 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.

OK, so this sounds like a coincidence, but it's very likely the same
issue, so I'm going to put mark those syzbot reports.

thanks,

Takashi