ALSA: usb-audio: Fix NULL dereference in create_fixed_stream_quirk()

commit 0f886ca1 upstream.

create_fixed_stream_quirk() may cause a NULL-pointer dereference by
accessing the non-existing endpoint when a USB device with a malformed
USB descriptor is used.

This patch avoids it simply by adding a sanity check of bNumEndpoints
before the accesses.

Bugzilla: https://bugzilla.suse.com/show_bug.cgi?id=971125Signed-off-by: Takashi Iwai <tiwai@suse.de>
[bwh: Backported to 3.2:
 - There's no altsd variable
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

sound/usb/quirks.c

@@ -165,6 +165,12 @@ static int create_fixed_stream_quirk(struct snd_usb_audio *chip,
 		return -EINVAL;
 	}
 	alts = &iface->altsetting[fp->altset_idx];
+	if (get_iface_desc(alts)->bNumEndpoints < 1) {
+		kfree(fp);
+		kfree(rate_table);
+		return -EINVAL;
+	}
+
 	fp->datainterval = snd_usb_parse_datainterval(chip, alts);
 	fp->maxpacksize = le16_to_cpu(get_endpoint(alts, 0)->wMaxPacketSize);
 	usb_set_interface(chip->dev, fp->iface, 0);