mac80211: Disallow to set multicast BSSID

Okay, here is the first of the five patches. After applying all
of them you should be able to build/join huge city mesh networks
(e.g. with the OLSR protocol) with the most of the mac80211 wireless
drivers by setting a fixed BSSID in the ad hoc mode. (If you found no
other bug/problem.) This was not specified in the original standard,
but is a widely used de facto standard.

The first patch now completely disallow to set multicast MAC addresses
as BSSID. The behavior before was really strange.
Signed-off-by: Alina Friedrichsen <x-alina@gmx.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/mac80211/mlme.c

@@ -2548,11 +2548,16 @@ int ieee80211_sta_set_bssid(struct ieee80211_sub_if_data *sdata, u8 *bssid)
 {
 	struct ieee80211_if_sta *ifsta;
 	int res;
+	bool valid;
 
 	ifsta = &sdata->u.sta;
+	valid = is_valid_ether_addr(bssid);
 
 	if (memcmp(ifsta->bssid, bssid, ETH_ALEN) != 0) {
-		memcpy(ifsta->bssid, bssid, ETH_ALEN);
+		if(valid)
+			memcpy(ifsta->bssid, bssid, ETH_ALEN);
+		else
+			memset(ifsta->bssid, 0, ETH_ALEN);
 		res = 0;
 		/*
 		 * Hack! See also ieee80211_sta_set_ssid.
@@ -2566,7 +2571,7 @@ int ieee80211_sta_set_bssid(struct ieee80211_sub_if_data *sdata, u8 *bssid)
 		}
 	}
 
-	if (is_valid_ether_addr(bssid))
+	if (valid)
 		ifsta->flags |= IEEE80211_STA_BSSID_SET;
 	else
 		ifsta->flags &= ~IEEE80211_STA_BSSID_SET;