net: dsa: fix NULL pointer dereference in dsa_port_reset_vlan_filtering

The "ds" iterator variable used in dsa_port_reset_vlan_filtering() ->
dsa_switch_for_each_port() overwrites the "dp" received as argument,
which is later used to call dsa_port_vlan_filtering() proper.

As a result, switches which do enter that code path (the ones with
vlan_filtering_is_global=true) will dereference an invalid dp in
dsa_port_reset_vlan_filtering() after leaving a VLAN-aware bridge.

Use a dedicated "other_dp" iterator variable to avoid this from
happening.

Fixes: d0004a0 ("net: dsa: remove the "dsa_to_port in a loop" antipattern from the core")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: vladimiroltean

net/dsa/port.c

@@ -248,6 +248,7 @@ static void dsa_port_reset_vlan_filtering(struct dsa_port *dp,
 	struct netlink_ext_ack extack = {0};
 	bool change_vlan_filtering = false;
 	struct dsa_switch *ds = dp->ds;
+	struct dsa_port *other_dp;
 	bool vlan_filtering;
 	int err;
 
@@ -270,8 +271,8 @@ static void dsa_port_reset_vlan_filtering(struct dsa_port *dp,
 	 * VLAN-aware bridge.
 	 */
 	if (change_vlan_filtering && ds->vlan_filtering_is_global) {
-		dsa_switch_for_each_port(dp, ds) {
-			struct net_device *br = dsa_port_bridge_dev_get(dp);
+		dsa_switch_for_each_port(other_dp, ds) {
+			struct net_device *br = dsa_port_bridge_dev_get(other_dp);
 
 			if (br && br_vlan_enabled(br)) {
 				change_vlan_filtering = false;