Commit 8dca3697 authored by Nikolay Aleksandrov's avatar Nikolay Aleksandrov Committed by Jakub Kicinski
Browse files

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: ec7328b5 ("net: bridge: mst: Multiple Spanning Tree (MST) mode")
Reported-by: default avatar <syzbot+dd280197f0f7ab3917be@syzkaller.appspotmail.com>
Closes: https://lore.kernel.org/netdev/69088ffa.050a0220.29fc44.003d.GAE@google.com/


Signed-off-by: default avatarNikolay Aleksandrov <razor@blackwall.org>
Reviewed-by: default avatarIdo Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20251105111919.1499702-2-razor@blackwall.org


Signed-off-by: default avatarJakub Kicinski <kuba@kernel.org>
parent 0216721c

net/bridge/br_forward.c

+1 −1
Original line number Diff line number Diff line
@@ -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);

}

net/bridge/br_input.c

+2 −2
Original line number Diff line number Diff line
@@ -94,7 +94,7 @@ 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) {
@@ -429,7 +429,7 @@ 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) {

net/bridge/br_private.h

+5 −3
Original line number Diff line number Diff line
@@ -1935,10 +1935,12 @@ 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,
@@ -1953,7 +1955,7 @@ 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;

}