mac80211: make mgmt_tx accept a NULL channel

cfg80211 passes a NULL channel to mgmt_tx if the frame has
to be sent on the one currently in use by the device.
Make the implementation of mgmt_tx correctly handle this
case. Fail if offchan is required.
Signed-off-by: NAntonio Quartulli <antonio@open-mesh.com>
[fix RCU locking]
Signed-off-by: NJohannes Berg <johannes.berg@intel.com>

net/mac80211/cfg.c

@@ -2841,6 +2841,12 @@ static int ieee80211_mgmt_tx(struct wiphy *wiphy, struct wireless_dev *wdev,
 		return -EOPNOTSUPP;
 	}
 
+	/* configurations requiring offchan cannot work if no channel has been
+	 * specified
+	 */
+	if (need_offchan && !chan)
+		return -EINVAL;
+
 	mutex_lock(&local->mtx);
 
 	/* Check if the operating channel is the requested channel */
@@ -2850,10 +2856,15 @@ static int ieee80211_mgmt_tx(struct wiphy *wiphy, struct wireless_dev *wdev,
 		rcu_read_lock();
 		chanctx_conf = rcu_dereference(sdata->vif.chanctx_conf);
 
-		if (chanctx_conf)
-			need_offchan = chan != chanctx_conf->def.chan;
-		else
+		if (chanctx_conf) {
+			need_offchan = chan && (chan != chanctx_conf->def.chan);
+		} else if (!chan) {
+			ret = -EINVAL;
+			rcu_read_unlock();
+			goto out_unlock;
+		} else {
 			need_offchan = true;
+		}
 		rcu_read_unlock();
 	}