Re: [PATCH v2] ALSA: scarlett2: Add the number of endpoints checked was 0

[Takashi Iwai <tiwai@suse.de>]

On Mon, 09 Mar 2026 10:57:03 +0100,
Edward Adam Davis wrote:
> 
> The user constructed a corrupted USB device, causing the USB device
> enumeration phase to fail to resolve any endpoints. This resulted in
> a null pointer dereference reported in [1] when the USB sound card
> driver executed probe to initialize the mixer. 
> 
> To avoid the problem reported in [1], a check was added to ensure that
> the number of endpoints contained in the interface was 0 when creating
> mixer controls for the Focusrite Scarlett 2nd/3rd Gen USB sound card.
> 
> [1]
> KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
> RIP: 0010:usb_endpoint_num include/uapi/linux/usb/ch9.h:479 [inline]
> RIP: 0010:scarlett2_find_fc_interface sound/usb/mixer_scarlett2.c:8261 [inline]
> RIP: 0010:scarlett2_init_private sound/usb/mixer_scarlett2.c:8295 [inline]
> RIP: 0010:snd_scarlett2_controls_create sound/usb/mixer_scarlett2.c:8684 [inline]
> RIP: 0010:snd_scarlett2_init.cold+0xbad/0x6c79 sound/usb/mixer_scarlett2.c:9407
> Call Trace:
>  snd_usb_mixer_apply_create_quirk+0x1c21/0x2b80 sound/usb/mixer_quirks.c:4446
>  snd_usb_create_mixer+0x7a2/0x1910 sound/usb/mixer.c:3641
>  usb_audio_probe+0xf6d/0x3a90 sound/usb/card.c:1033
> 
> Reported-by: syzbot+ae893a8901067fde2741@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=ae893a8901067fde2741 
> Signed-off-by: Edward Adam Davis <eadavis@qq.com>
> ---
> v1 -> v2: move the check to scarlett2
> 
>  sound/usb/mixer_scarlett2.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/sound/usb/mixer_scarlett2.c b/sound/usb/mixer_scarlett2.c
> index ef3150581eab..4b300226f16c 100644
> --- a/sound/usb/mixer_scarlett2.c
> +++ b/sound/usb/mixer_scarlett2.c
> @@ -9393,6 +9393,15 @@ int snd_scarlett2_init(struct usb_mixer_interface *mixer)
>  		return 0;
>  	}
>  
> +	if (get_iface_desc(mixer->hostif)->bNumEndpoints == 0) {
> +		usb_audio_err(chip,
> +			"%s: There are no endpoints for %04x:%04x\n",
> +			__func__,
> +			USB_ID_VENDOR(chip->usb_id),
> +			USB_ID_PRODUCT(chip->usb_id));
> +		return 0;

This should be an error.  It's obviously a broken descriptor, and this
is the code specific to the certain configuration.


Takashi