ALSA: usb-audio: Fix an OOB bug in parse_audio_mixer_unit

BugLink: https://bugs.launchpad.net/bugs/1845036

commit daac0715 upstream.

The `uac_mixer_unit_descriptor` shown as below is read from the
device side. In `parse_audio_mixer_unit`, `baSourceID` field is
accessed from index 0 to `bNrInPins` - 1, the current implementation
assumes that descriptor is always valid (the length  of descriptor
is no shorter than 5 + `bNrInPins`). If a descriptor read from
the device side is invalid, it may trigger out-of-bound memory
access.

```
struct uac_mixer_unit_descriptor {
	__u8 bLength;
	__u8 bDescriptorType;
	__u8 bDescriptorSubtype;
	__u8 bUnitID;
	__u8 bNrInPins;
	__u8 baSourceID[];
}
```

This patch fixes the bug by add a sanity check on the length of
the descriptor.
Reported-by: Hui Peng <benquike@gmail.com>
Reported-by: Mathias Payer <mathias.payer@nebelwelt.net>
Cc: <stable@vger.kernel.org>
Signed-off-by: Hui Peng <benquike@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

sound/usb/mixer.c

@@ -1647,6 +1647,7 @@ static int parse_audio_mixer_unit(struct mixer_build *state, int unitid,
 	int pin, ich, err;
 
 	if (desc->bLength < 11 || !(input_pins = desc->bNrInPins) ||
+	    desc->bLength < sizeof(*desc) + desc->bNrInPins ||
 	    !(num_outs = uac_mixer_unit_bNrChannels(desc))) {
 		usb_audio_err(state->chip,
 			      "invalid MIXER UNIT descriptor %d\n",