ALSA: usb-audio: Fix NULL pointer dereference in snd_usb_mixer_controls_badd

In snd_usb_create_streams(), for UAC version 3 devices, the Interface
Association Descriptor (IAD) is retrieved via usb_ifnum_to_if(). If this
call fails, a fallback routine attempts to obtain the IAD from the next
interface and sets a BADD profile. However, snd_usb_mixer_controls_badd()
assumes that the IAD retrieved from usb_ifnum_to_if() is always valid,
without performing a NULL check. This can lead to a NULL pointer
dereference when usb_ifnum_to_if() fails to find the interface descriptor.

This patch adds a NULL pointer check after calling usb_ifnum_to_if() in
snd_usb_mixer_controls_badd() to prevent the dereference.

This issue was discovered by syzkaller, which triggered the bug by sending
a crafted USB device descriptor.

Fixes: 17156f2 ("ALSA: usb: add UAC3 BADD profiles support")
Signed-off-by: Haein Lee <lhi0729@kaist.ac.kr>
Link: https://patch.msgid.link/vwhzmoba9j2f.vwhzmob9u9e2.g6@dooray.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>

Author: haaeein

sound/usb/mixer.c

@@ -3086,6 +3086,8 @@ static int snd_usb_mixer_controls_badd(struct usb_mixer_interface *mixer,
 	int i;
 
 	assoc = usb_ifnum_to_if(dev, ctrlif)->intf_assoc;
+	if (!assoc)
+		return -EINVAL;
 
 	/* Detect BADD capture/playback channels from AS EP descriptors */
 	for (i = 0; i < assoc->bInterfaceCount; i++) {