mac80211: discard multicast and 4-addr A-MSDUs

In mac80211, multicast A-MSDUs are accepted in many cases that
they shouldn't be accepted in:
 * drop A-MSDUs with a multicast A1 (RA), as required by the
   spec in 9.11 (802.11-2012 version)
 * drop A-MSDUs with a 4-addr header, since the fourth address
   can't actually be useful for them; unless 4-address frame
   format is actually requested, even though the fourth address
   is still not useful in this case, but ignored

Accepting the first case, in particular, is very problematic
since it allows anyone else with possession of a GTK to send
unicast frames encapsulated in a multicast A-MSDU, even when
the AP has client isolation enabled.

Cc: stable@vger.kernel.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/rx.c

@@ -2308,16 +2308,22 @@ ieee80211_rx_h_amsdu(struct ieee80211_rx_data *rx)
 	if (!(status->rx_flags & IEEE80211_RX_AMSDU))
 		return RX_CONTINUE;
 
-	if (ieee80211_has_a4(hdr->frame_control) &&
-	    rx->sdata->vif.type == NL80211_IFTYPE_AP_VLAN &&
-	    !rx->sdata->u.vlan.sta)
-		return RX_DROP_UNUSABLE;
+	if (unlikely(ieee80211_has_a4(hdr->frame_control))) {
+		switch (rx->sdata->vif.type) {
+		case NL80211_IFTYPE_AP_VLAN:
+			if (!rx->sdata->u.vlan.sta)
+				return RX_DROP_UNUSABLE;
+			break;
+		case NL80211_IFTYPE_STATION:
+			if (!rx->sdata->u.mgd.use_4addr)
+				return RX_DROP_UNUSABLE;
+			break;
+		default:
+			return RX_DROP_UNUSABLE;
+		}
+	}
 
-	if (is_multicast_ether_addr(hdr->addr1) &&
-	    ((rx->sdata->vif.type == NL80211_IFTYPE_AP_VLAN &&
-	      rx->sdata->u.vlan.sta) ||
-	     (rx->sdata->vif.type == NL80211_IFTYPE_STATION &&
-	      rx->sdata->u.mgd.use_4addr)))
+	if (is_multicast_ether_addr(hdr->addr1))
 		return RX_DROP_UNUSABLE;
 
 	skb->dev = dev;