mac80211: fix virtual monitor interface iteration

During channel context assignment, the interface should
be found by interface iteration, so we need to assign the
pointer before the channel context.

Reported-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Tested-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/iface.c

@@ -418,20 +418,24 @@ int ieee80211_add_virtual_monitor(struct ieee80211_local *local)
 		return ret;
 	}
 
+	mutex_lock(&local->iflist_mtx);
+	rcu_assign_pointer(local->monitor_sdata, sdata);
+	mutex_unlock(&local->iflist_mtx);
+
 	mutex_lock(&local->mtx);
 	ret = ieee80211_vif_use_channel(sdata, &local->monitor_chandef,
 					IEEE80211_CHANCTX_EXCLUSIVE);
 	mutex_unlock(&local->mtx);
 	if (ret) {
+		mutex_lock(&local->iflist_mtx);
+		rcu_assign_pointer(local->monitor_sdata, NULL);
+		mutex_unlock(&local->iflist_mtx);
+		synchronize_net();
 		drv_remove_interface(local, sdata);
 		kfree(sdata);
 		return ret;
 	}
 
-	mutex_lock(&local->iflist_mtx);
-	rcu_assign_pointer(local->monitor_sdata, sdata);
-	mutex_unlock(&local->iflist_mtx);
-
 	return 0;
 }