linux-kernel - Re: [PATCH] ALSA: usb-audio: fix race condition to UAF in snd_usbmidi

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Message-ID: <87bjmwb9y6.wl-tiwai@suse.de>
Date: Sat, 27 Sep 2025 10:01:37 +0200
From: Takashi Iwai <tiwai@...e.de>
To: Jeongjun Park <aha310510@...il.com>
Cc: clemens@...isch.de,
	perex@...ex.cz,
	tiwai@...e.com,
	linux-sound@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Subject: Re: [PATCH] ALSA: usb-audio: fix race condition to UAF in snd_usbmidi_free

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@...r.kernel.org>
> Fixes: 0718a78f6a9f ("ALSA: usb-audio: Kill timer properly at removal")
> Signed-off-by: Jeongjun Park <aha310510@...il.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);
>  }
>  
> --