wifi: cfg80211: fix buffer overflow in elem comparison

For vendor elements, the code here assumes that 5 octets
are present without checking. Since the element itself is
already checked to fit, we only need to check the length.
Reported-and-tested-by: Sönke Huster <shuster@seemoo.tu-darmstadt.de>
Fixes: 0b8fb823 ("cfg80211: Parsing of Multiple BSSID information in scanning")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/wireless/scan.c

@@ -330,7 +330,8 @@ static size_t cfg80211_gen_new_ie(const u8 *ie, size_t ielen,
 			 * determine if they are the same ie.
 			 */
 			if (tmp_old[0] == WLAN_EID_VENDOR_SPECIFIC) {
-				if (!memcmp(tmp_old + 2, tmp + 2, 5)) {
+				if (tmp_old[1] >= 5 && tmp[1] >= 5 &&
+				    !memcmp(tmp_old + 2, tmp + 2, 5)) {
 					/* same vendor ie, copy from
 					 * subelement
 					 */