[SRU][N][PATCH 1/1] net: bridge: fix use-after-free due to MST port state bypass

[Tim Whisonant]

From: Nikolay Aleksandrov <razor at blackwall.org>

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: syzbot+dd280197f0f7ab3917be at syzkaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/69088ffa.050a0220.29fc44.003d.GAE@google.com/
Signed-off-by: Nikolay Aleksandrov <razor at blackwall.org>
Reviewed-by: Ido Schimmel <idosch at nvidia.com>
Link: https://patch.msgid.link/20251105111919.1499702-2-razor@blackwall.org
Signed-off-by: Jakub Kicinski <kuba at kernel.org>
(cherry picked from commit 8dca36978aa80bab9d4da130c211db75c9e00048)
CVE-2025-40297
Signed-off-by: Tim Whisonant <tim.whisonant at canonical.com>
---
 net/bridge/br_forward.c | 2 +-
 net/bridge/br_input.c   | 4 ++--
 net/bridge/br_private.h | 8 +++++---
 3 files changed, 8 insertions(+), 6 deletions(-)

diff --git a/net/bridge/br_forward.c b/net/bridge/br_forward.c
index e19b583ff2c6d..1935203218046 100644
--- a/net/bridge/br_forward.c
+++ b/net/bridge/br_forward.c
@@ -25,7 +25,7 @@ static inline int should_deliver(const struct net_bridge_port *p,
 
 	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);
 }
diff --git a/net/bridge/br_input.c b/net/bridge/br_input.c
index ceaa5a89b947f..2eb2bb6643885 100644
--- a/net/bridge/br_input.c
+++ b/net/bridge/br_input.c
@@ -93,7 +93,7 @@ int br_handle_frame_finish(struct net *net, struct sock *sk, struct sk_buff *skb
 
 	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)
@@ -411,7 +411,7 @@ static rx_handler_result_t br_handle_frame(struct sk_buff **pskb)
 		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) {
diff --git a/net/bridge/br_private.h b/net/bridge/br_private.h
index 6a1bce8959afa..4088d5dfe9ea6 100644
--- a/net/bridge/br_private.h
+++ b/net/bridge/br_private.h
@@ -1902,10 +1902,12 @@ static inline bool br_vlan_state_allowed(u8 state, bool learn_allow)
 /* 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,
@@ -1920,7 +1922,7 @@ int br_mst_fill_info(struct sk_buff *skb,
 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;
 }
-- 
2.43.0