batman-adv: Only put gw_node list reference when removed

BugLink: https://bugs.launchpad.net/bugs/1868629

commit c18bdd01 upstream.

The batadv_gw_node reference counter in batadv_gw_node_update can only be
reduced when the list entry was actually removed. Otherwise the reference
counter may reach zero when batadv_gw_node_update is called from two
different contexts for the same gw_node but only one context is actually
removing the entry from the list.

The release function for this gw_node is not called inside the list_lock
spinlock protected region because the function batadv_gw_node_update still
holds a gw_node reference for the object pointer on the stack. Thus the
actual release function (when required) will be called only at the end of
the function.

Fixes: bd3524c1 ("batman-adv: remove obsolete deleted attribute for gateway node")
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <a@unstable.cc>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>
Signed-off-by: Kelsey Skunberg <kelsey.skunberg@canonical.com>

net/batman-adv/gateway_client.c

@@ -527,11 +527,12 @@ void batadv_gw_node_update(struct batadv_priv *bat_priv,
 		 * gets dereferenced.
 		 */
 		spin_lock_bh(&bat_priv->gw.list_lock);
-		hlist_del_init_rcu(&gw_node->list);
+		if (!hlist_unhashed(&gw_node->list)) {
+			hlist_del_init_rcu(&gw_node->list);
+			batadv_gw_node_free_ref(gw_node);
+		}
 		spin_unlock_bh(&bat_priv->gw.list_lock);
 
-		batadv_gw_node_free_ref(gw_node);
-
 		curr_gw = batadv_gw_get_selected_gw_node(bat_priv);
 		if (gw_node == curr_gw)
 			batadv_gw_reselect(bat_priv);