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

[Takashi Iwai <tiwai@suse.de>]

On Mon, 09 Mar 2026 11:03:46 +0100,
Takashi Iwai wrote:
> 
> 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.

... and that check alone doesn't cover all cases, I'm afraid.  The
scarlett2 driver code parses over multiple interfaces.

I guess rather a patch like below should cover better.


thanks,

Takashi

--- a/sound/usb/mixer_scarlett2.c
+++ b/sound/usb/mixer_scarlett2.c
@@ -8251,6 +8251,8 @@ static int scarlett2_find_fc_interface(struct usb_device *dev,
 
 		if (desc->bInterfaceClass != 255)
 			continue;
+		if (desc->bNumEndpoints < 1)
+			continue;
 
 		epd = get_endpoint(intf->altsetting, 0);
 		private->bInterfaceNumber = desc->bInterfaceNumber;