cfg80211: validate SSID/MBSSID element ordering assumption

The code copying the data assumes that the SSID element is
before the MBSSID element, but since the data is untrusted
from the AP, this cannot be guaranteed.

Validate that this is indeed the case and ignore the MBSSID
otherwise, to avoid having to deal with both cases for the
copy of data that should be between them.

Cc: stable@vger.kernel.org
Fixes: 0b8fb823 ("cfg80211: Parsing of Multiple BSSID information in scanning")
Link: https://lore.kernel.org/r/1569009255-I1673911f5eae02964e21bdc11b2bf58e5e207e59@changeidSigned-off-by: Johannes Berg <johannes.berg@intel.com>

net/wireless/scan.c

@@ -1723,7 +1723,12 @@ cfg80211_update_notlisted_nontrans(struct wiphy *wiphy,
 		return;
 	new_ie_len -= trans_ssid[1];
 	mbssid = cfg80211_find_ie(WLAN_EID_MULTIPLE_BSSID, ie, ielen);
-	if (!mbssid)
+	/*
+	 * It's not valid to have the MBSSID element before SSID
+	 * ignore if that happens - the code below assumes it is
+	 * after (while copying things inbetween).
+	 */
+	if (!mbssid || mbssid < trans_ssid)
 		return;
 	new_ie_len -= mbssid[1];
 	rcu_read_lock();