net: bridge: fix use-after-free due to MST port state bypass

syzbot reported[1] a use-after-free when deleting an expired fdb. It is
due to a race condition between learning still happening and a port being
deleted, after all its fdbs have been flushed. The port's state has been
toggled to disabled so no learning should happen at that time, but if we
have MST enabled, it will bypass the port's state, that together with VLAN
filtering disabled can lead to fdb learning at a time when it shouldn't
happen while the port is being deleted. VLAN filtering must be disabled
because we flush the port VLANs when it's being deleted which will stop
learning. This fix adds a check for the port's vlan group which is
initialized to NULL when the port is getting deleted, that avoids the port
state bypass. When MST is enabled there would be a minimal new overhead
in the fast-path because the port's vlan group pointer is cache-hot.

[1] https://syzkaller.appspot.com/bug?extid=dd280197f0f7ab3917be

Fixes: ec7328b59176 ("net: bridge: mst: Multiple Spanning Tree (MST) mode")
Reported-by: [e-mail]
Closes: https://lore.kernel.org/netdev/[e-mail]/
Signed-off-by: Nikolay Aleksandrov <[e-mail]>
Reviewed-by: Ido Schimmel <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/bridge/br_forward.c

···
 
 	vg = nbp_vlan_group_rcu(p);
 	return ((p->flags & BR_HAIRPIN_MODE) || skb->dev != p->dev) &&
-		(br_mst_is_enabled(p->br) || p->state == BR_STATE_FORWARDING) &&
+		(br_mst_is_enabled(p) || p->state == BR_STATE_FORWARDING) &&
 		br_allowed_egress(vg, skb) && nbp_switchdev_allowed_egress(p, skb) &&
 		!br_skb_isolated(p, skb);
 }

net/bridge/br_input.c

···
 
 	br = p->br;
 
-	if (br_mst_is_enabled(br)) {
+	if (br_mst_is_enabled(p)) {
 		state = BR_STATE_FORWARDING;
 	} else {
 		if (p->state == BR_STATE_DISABLED) {
···
 		return RX_HANDLER_PASS;
 
 forward:
-	if (br_mst_is_enabled(p->br))
+	if (br_mst_is_enabled(p))
 		goto defer_stp_filtering;
 
 	switch (p->state) {

net/bridge/br_private.h

···
 /* br_mst.c */
 #ifdef CONFIG_BRIDGE_VLAN_FILTERING
 DECLARE_STATIC_KEY_FALSE(br_mst_used);
-static inline bool br_mst_is_enabled(struct net_bridge *br)
+static inline bool br_mst_is_enabled(const struct net_bridge_port *p)
 {
+	/* check the port's vlan group to avoid racing with port deletion */
 	return static_branch_unlikely(&br_mst_used) &&
-		br_opt_get(br, BROPT_MST_ENABLED);
+	       br_opt_get(p->br, BROPT_MST_ENABLED) &&
+	       rcu_access_pointer(p->vlgrp);
 }
 
 int br_mst_set_state(struct net_bridge_port *p, u16 msti, u8 state,
···
 int br_mst_process(struct net_bridge_port *p, const struct nlattr *mst_attr,
 		   struct netlink_ext_ack *extack);
 #else
-static inline bool br_mst_is_enabled(struct net_bridge *br)
+static inline bool br_mst_is_enabled(const struct net_bridge_port *p)
 {
 	return false;
 }