[v1] ALSA: usb-audio: fix race condition to UAF in snd_usbmidi_free

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

Posted by Jeongjun Park 4 days, 16 hours ago

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>
---
 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);
 }
 
--

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

Posted by Takashi Iwai 4 days, 13 hours ago

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);
>  }
>  
> --

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

Posted by Hillf Danton 4 days, 12 hours ago

On Sat, 27 Sep 2025 10:01:37 +0200 Takashi Iwai 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

#syz test upstream master

--- x/sound/usb/midi.c
+++ y/sound/usb/midi.c
@@ -1522,6 +1522,8 @@ static void snd_usbmidi_free(struct snd_
 {
 	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_
 			snd_usbmidi_in_endpoint_delete(ep->in);
 	}
 	mutex_destroy(&umidi->mutex);
-	timer_shutdown_sync(&umidi->error_timer);
 	kfree(umidi);
 }
 
--