CVE-2026-45913
Description
In the Linux kernel, the following vulnerability has been resolved:
net: bridge: mcast: always update mdb_n_entries for vlan contexts
syzbot triggered a warning[1] about the number of mdb entries in a context. It turned out that there are multiple ways to trigger that warning today (some got added during the years), the root cause of the problem is that the increase is done conditionally, and over the years these different conditions increased so there were new ways to trigger the warning, that is to do a decrease which wasn't paired with a previous increase.
For example one way to trigger it is with flush: $ ip l add br0 up type bridge vlan_filtering 1 mcast_snooping 1 $ ip l add dumdum up master br0 type dummy $ bridge mdb add dev br0 port dumdum grp 239.0.0.1 permanent vid 1 $ ip link set dev br0 down $ ip link set dev br0 type bridge mcast_vlan_snooping 1 ^^^^ this will enable snooping, but will not update mdb_n_entries because in __br_multicast_enable_port_ctx() we check !netif_running $ bridge mdb flush dev br0 ^^^ this will trigger the warning because it will delete the pg which we added above, which will try to decrease mdb_n_entries
Fix the problem by removing the conditional increase and always keep the count up-to-date while the vlan exists. In order to do that we have to first initialize it on port-vlan context creation, and then always increase or decrease the value regardless of mcast options. To keep the current behaviour we have to enforce the mdb limit only if the context is port's or if the port-vlan's mcast snooping is enabled.
[1] ------------[ cut here ]------------ n == 0 WARNING: net/bridge/br_multicast.c:718 at br_multicast_port_ngroups_dec_one net/bridge/br_multicast.c:718 [inline], CPU#0: syz.4.4607/22043 WARNING: net/bridge/br_multicast.c:718 at br_multicast_port_ngroups_dec net/bridge/br_multicast.c:771 [inline], CPU#0: syz.4.4607/22043 WARNING: net/bridge/br_multicast.c:718 at br_multicast_del_pg+0x1bbe/0x1e20 net/bridge/br_multicast.c:825, CPU#0: syz.4.4607/22043 Modules linked in: CPU: 0 UID: 0 PID: 22043 Comm: syz.4.4607 Not tainted syzkaller #0 PREEMPT(full) Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/24/2026 RIP: 0010:br_multicast_port_ngroups_dec_one net/bridge/br_multicast.c:718 [inline] RIP: 0010:br_multicast_port_ngroups_dec net/bridge/br_multicast.c:771 [inline] RIP: 0010:br_multicast_del_pg+0x1bbe/0x1e20 net/bridge/br_multicast.c:825 Code: 41 5f 5d e9 04 7a 48 f7 e8 3f 73 5c f7 90 0f 0b 90 e9 cf fd ff ff e8 31 73 5c f7 90 0f 0b 90 e9 16 fd ff ff e8 23 73 5c f7 90 <0f> 0b 90 e9 60 fd ff ff e8 15 73 5c f7 eb 05 e8 0e 73 5c f7 48 8b RSP: 0018:ffffc9000c207220 EFLAGS: 00010293 RAX: ffffffff8a68042d RBX: ffff88807c6f1800 RCX: ffff888066e90000 RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000 RBP: 0000000000000000 R08: ffff888066e90000 R09: 000000000000000c R10: 000000000000000c R11: 0000000000000000 R12: ffff8880303ef800 R13: dffffc0000000000 R14: ffff888050eb11c4 R15: 1ffff1100a1d6238 FS: 00007fa45921b6c0(0000) GS:ffff8881256f5000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007fa4591f9ff8 CR3: 0000000081df2000 CR4: 00000000003526f0 Call Trace:
br_mdb_flush_pgs net/bridge/br_mdb.c:1525 [inline] br_mdb_flush net/bridge/br_mdb.c:1544 [inline] br_mdb_del_bulk+0x5e2/0xb20 net/bridge/br_mdb.c:1561 rtnl_mdb_del+0x48a/0x640 net/core/rtnetlink.c:-1 rtnetlink_rcv_msg+0x77e/0xbe0 net/core/rtnetlink.c:6967 netlink_rcv_skb+0x232/0x4b0 net/netlink/af_netlink.c:2550 netlink_unicast_kernel net/netlink/af_netlink.c:1318 [inline] netlink_unicast+0x80f/0x9b0 net/netlink/af_netlink.c:1344 netlink_sendmsg+0x813/0xb40 net/netlink/af_netlink.c:1894 sock_sendmsg_nosec net/socket.c:727 [inline] __sock_sendmsg net/socket.c:742 [inline] ____sys_sendmsg+0xa68/0xad0 net/socket.c:2592 ___sys_sendmsg+0x2a5/0x360 net/socke ---truncated---
AI Insight
LLM-synthesized narrative grounded in this CVE's description and references.
In the Linux kernel's bridge multicast, failure to always update mdb_n_entries for VLAN contexts can trigger a kernel warning, leading to denial of service.
Vulnerability
The Linux kernel bridge multicast implementation (net/bridge/br_multicast.c) contains a bug where the mdb_n_entries counter for VLAN contexts is not always updated correctly. The increase was done conditionally, and over time, different code paths were added that decreased the counter without a corresponding paired increase, leading to a warning when the counter becomes negative. Affected versions include any kernel prior to the fix. The root cause is that mdb_n_entries is not always kept consistent while the VLAN exists, particularly when toggling mcast_vlan_snooping on a bridge that already has multicast entries.
Exploitation
An attacker with local access and CAP_NET_ADMIN capabilities can trigger the issue with a specific sequence of operations: create a bridge with vlan_filtering 1 and mcast_snooping 1, add a port and a permanent MDB entry (e.g., bridge mdb add dev br0 port dumdum grp 239.0.0.1 permanent vid 1), then bring the bridge down and enable mcast_vlan_snooping (which increments mdb_n_entries incorrectly), and finally flush all MDB entries. This sequence causes a decrement without a prior increase, hitting the WARN_ON in br_multicast_port_ngroups_dec_one.
Impact
Successful exploitation triggers a kernel warning (WARN_ON) that may result in a denial of service. The warning could cause system log flooding or, if panic_on_warn is set, a kernel panic. There is no evidence of privilege escalation or data leakage; the impact is limited to availability.
Mitigation
The fix is implemented in Linux kernel commit d0fdad1bdd21 ("net: bridge: mcast: always update mdb_n_entries for vlan contexts") [1]. Users should update to a kernel version that includes this commit. If patching is not immediately possible, administrators should avoid the sequence of operations described in the exploitation section, especially toggling mcast_vlan_snooping on a bridge that already has MDB entries configured.
AI Insight generated on May 27, 2026. Synthesized from this CVE's description and the cited reference URLs; citations are validated against the source bundle.
Affected products
1Patches
10fae260fc84e1net: bridge: mcast: always update mdb_n_entries for vlan contexts
1 file changed · +18 −28
net/bridge/br_multicast.c+18 −28 modifieddiff --git a/net/bridge/br_multicast.c b/net/bridge/br_multicast.c index 22d12e54596685..5855eb0502085f 100644 --- a/net/bridge/br_multicast.c +++ b/net/bridge/br_multicast.c @@ -244,14 +244,11 @@ br_multicast_port_vid_to_port_ctx(struct net_bridge_port *port, u16 vid) lockdep_assert_held_once(&port->br->multicast_lock); - if (!br_opt_get(port->br, BROPT_MCAST_VLAN_SNOOPING_ENABLED)) - return NULL; - /* Take RCU to access the vlan. */ rcu_read_lock(); vlan = br_vlan_find(nbp_vlan_group_rcu(port), vid); - if (vlan && !br_multicast_port_ctx_vlan_disabled(&vlan->port_mcast_ctx)) + if (vlan) pmctx = &vlan->port_mcast_ctx; rcu_read_unlock(); @@ -701,7 +698,10 @@ br_multicast_port_ngroups_inc_one(struct net_bridge_mcast_port *pmctx, u32 max = READ_ONCE(pmctx->mdb_max_entries); u32 n = READ_ONCE(pmctx->mdb_n_entries); - if (max && n >= max) { + /* enforce the max limit when it's a port pmctx or a port-vlan pmctx + * with snooping enabled + */ + if (!br_multicast_port_ctx_vlan_disabled(pmctx) && max && n >= max) { NL_SET_ERR_MSG_FMT_MOD(extack, "%s is already in %u groups, and mcast_max_groups=%u", what, n, max); return -E2BIG; @@ -736,9 +736,7 @@ static int br_multicast_port_ngroups_inc(struct net_bridge_port *port, return err; } - /* Only count on the VLAN context if VID is given, and if snooping on - * that VLAN is enabled. - */ + /* Only count on the VLAN context if VID is given */ if (!group->vid) return 0; @@ -2011,6 +2009,18 @@ void br_multicast_port_ctx_init(struct net_bridge_port *port, timer_setup(&pmctx->ip6_own_query.timer, br_ip6_multicast_port_query_expired, 0); #endif + /* initialize mdb_n_entries if a new port vlan is being created */ + if (vlan) { + struct net_bridge_port_group *pg; + u32 n = 0; + + spin_lock_bh(&port->br->multicast_lock); + hlist_for_each_entry(pg, &port->mglist, mglist) + if (pg->key.addr.vid == vlan->vid) + n++; + WRITE_ONCE(pmctx->mdb_n_entries, n); + spin_unlock_bh(&port->br->multicast_lock); + } } void br_multicast_port_ctx_deinit(struct net_bridge_mcast_port *pmctx) @@ -2094,25 +2104,6 @@ static void __br_multicast_enable_port_ctx(struct net_bridge_mcast_port *pmctx) br_ip4_multicast_add_router(brmctx, pmctx); br_ip6_multicast_add_router(brmctx, pmctx); } - - if (br_multicast_port_ctx_is_vlan(pmctx)) { - struct net_bridge_port_group *pg; - u32 n = 0; - - /* The mcast_n_groups counter might be wrong. First, - * BR_VLFLAG_MCAST_ENABLED is toggled before temporary entries - * are flushed, thus mcast_n_groups after the toggle does not - * reflect the true values. And second, permanent entries added - * while BR_VLFLAG_MCAST_ENABLED was disabled, are not reflected - * either. Thus we have to refresh the counter. - */ - - hlist_for_each_entry(pg, &pmctx->port->mglist, mglist) { - if (pg->key.addr.vid == pmctx->vlan->vid) - n++; - } - WRITE_ONCE(pmctx->mdb_n_entries, n); - } } static void br_multicast_enable_port_ctx(struct net_bridge_mcast_port *pmctx) -- cgit 1.3-korg
45525fdfd4cbnet: bridge: mcast: always update mdb_n_entries for vlan contexts
1 file changed · +18 −28
net/bridge/br_multicast.c+18 −28 modifieddiff --git a/net/bridge/br_multicast.c b/net/bridge/br_multicast.c index d55a4ab87837f7..e9a7e653040177 100644 --- a/net/bridge/br_multicast.c +++ b/net/bridge/br_multicast.c @@ -244,14 +244,11 @@ br_multicast_port_vid_to_port_ctx(struct net_bridge_port *port, u16 vid) lockdep_assert_held_once(&port->br->multicast_lock); - if (!br_opt_get(port->br, BROPT_MCAST_VLAN_SNOOPING_ENABLED)) - return NULL; - /* Take RCU to access the vlan. */ rcu_read_lock(); vlan = br_vlan_find(nbp_vlan_group_rcu(port), vid); - if (vlan && !br_multicast_port_ctx_vlan_disabled(&vlan->port_mcast_ctx)) + if (vlan) pmctx = &vlan->port_mcast_ctx; rcu_read_unlock(); @@ -701,7 +698,10 @@ br_multicast_port_ngroups_inc_one(struct net_bridge_mcast_port *pmctx, u32 max = READ_ONCE(pmctx->mdb_max_entries); u32 n = READ_ONCE(pmctx->mdb_n_entries); - if (max && n >= max) { + /* enforce the max limit when it's a port pmctx or a port-vlan pmctx + * with snooping enabled + */ + if (!br_multicast_port_ctx_vlan_disabled(pmctx) && max && n >= max) { NL_SET_ERR_MSG_FMT_MOD(extack, "%s is already in %u groups, and mcast_max_groups=%u", what, n, max); return -E2BIG; @@ -736,9 +736,7 @@ static int br_multicast_port_ngroups_inc(struct net_bridge_port *port, return err; } - /* Only count on the VLAN context if VID is given, and if snooping on - * that VLAN is enabled. - */ + /* Only count on the VLAN context if VID is given */ if (!group->vid) return 0; @@ -2011,6 +2009,18 @@ void br_multicast_port_ctx_init(struct net_bridge_port *port, timer_setup(&pmctx->ip6_own_query.timer, br_ip6_multicast_port_query_expired, 0); #endif + /* initialize mdb_n_entries if a new port vlan is being created */ + if (vlan) { + struct net_bridge_port_group *pg; + u32 n = 0; + + spin_lock_bh(&port->br->multicast_lock); + hlist_for_each_entry(pg, &port->mglist, mglist) + if (pg->key.addr.vid == vlan->vid) + n++; + WRITE_ONCE(pmctx->mdb_n_entries, n); + spin_unlock_bh(&port->br->multicast_lock); + } } void br_multicast_port_ctx_deinit(struct net_bridge_mcast_port *pmctx) @@ -2094,25 +2104,6 @@ static void __br_multicast_enable_port_ctx(struct net_bridge_mcast_port *pmctx) br_ip4_multicast_add_router(brmctx, pmctx); br_ip6_multicast_add_router(brmctx, pmctx); } - - if (br_multicast_port_ctx_is_vlan(pmctx)) { - struct net_bridge_port_group *pg; - u32 n = 0; - - /* The mcast_n_groups counter might be wrong. First, - * BR_VLFLAG_MCAST_ENABLED is toggled before temporary entries - * are flushed, thus mcast_n_groups after the toggle does not - * reflect the true values. And second, permanent entries added - * while BR_VLFLAG_MCAST_ENABLED was disabled, are not reflected - * either. Thus we have to refresh the counter. - */ - - hlist_for_each_entry(pg, &pmctx->port->mglist, mglist) { - if (pg->key.addr.vid == pmctx->vlan->vid) - n++; - } - WRITE_ONCE(pmctx->mdb_n_entries, n); - } } static void br_multicast_enable_port_ctx(struct net_bridge_mcast_port *pmctx) -- cgit 1.3-korg
8b769e311a86net: bridge: mcast: always update mdb_n_entries for vlan contexts
1 file changed · +18 −28
net/bridge/br_multicast.c+18 −28 modifieddiff --git a/net/bridge/br_multicast.c b/net/bridge/br_multicast.c index dccae08b4f4c40..b6a5147886ca41 100644 --- a/net/bridge/br_multicast.c +++ b/net/bridge/br_multicast.c @@ -244,14 +244,11 @@ br_multicast_port_vid_to_port_ctx(struct net_bridge_port *port, u16 vid) lockdep_assert_held_once(&port->br->multicast_lock); - if (!br_opt_get(port->br, BROPT_MCAST_VLAN_SNOOPING_ENABLED)) - return NULL; - /* Take RCU to access the vlan. */ rcu_read_lock(); vlan = br_vlan_find(nbp_vlan_group_rcu(port), vid); - if (vlan && !br_multicast_port_ctx_vlan_disabled(&vlan->port_mcast_ctx)) + if (vlan) pmctx = &vlan->port_mcast_ctx; rcu_read_unlock(); @@ -701,7 +698,10 @@ br_multicast_port_ngroups_inc_one(struct net_bridge_mcast_port *pmctx, u32 max = READ_ONCE(pmctx->mdb_max_entries); u32 n = READ_ONCE(pmctx->mdb_n_entries); - if (max && n >= max) { + /* enforce the max limit when it's a port pmctx or a port-vlan pmctx + * with snooping enabled + */ + if (!br_multicast_port_ctx_vlan_disabled(pmctx) && max && n >= max) { NL_SET_ERR_MSG_FMT_MOD(extack, "%s is already in %u groups, and mcast_max_groups=%u", what, n, max); return -E2BIG; @@ -736,9 +736,7 @@ static int br_multicast_port_ngroups_inc(struct net_bridge_port *port, return err; } - /* Only count on the VLAN context if VID is given, and if snooping on - * that VLAN is enabled. - */ + /* Only count on the VLAN context if VID is given */ if (!group->vid) return 0; @@ -2011,6 +2009,18 @@ void br_multicast_port_ctx_init(struct net_bridge_port *port, timer_setup(&pmctx->ip6_own_query.timer, br_ip6_multicast_port_query_expired, 0); #endif + /* initialize mdb_n_entries if a new port vlan is being created */ + if (vlan) { + struct net_bridge_port_group *pg; + u32 n = 0; + + spin_lock_bh(&port->br->multicast_lock); + hlist_for_each_entry(pg, &port->mglist, mglist) + if (pg->key.addr.vid == vlan->vid) + n++; + WRITE_ONCE(pmctx->mdb_n_entries, n); + spin_unlock_bh(&port->br->multicast_lock); + } } void br_multicast_port_ctx_deinit(struct net_bridge_mcast_port *pmctx) @@ -2094,25 +2104,6 @@ static void __br_multicast_enable_port_ctx(struct net_bridge_mcast_port *pmctx) br_ip4_multicast_add_router(brmctx, pmctx); br_ip6_multicast_add_router(brmctx, pmctx); } - - if (br_multicast_port_ctx_is_vlan(pmctx)) { - struct net_bridge_port_group *pg; - u32 n = 0; - - /* The mcast_n_groups counter might be wrong. First, - * BR_VLFLAG_MCAST_ENABLED is toggled before temporary entries - * are flushed, thus mcast_n_groups after the toggle does not - * reflect the true values. And second, permanent entries added - * while BR_VLFLAG_MCAST_ENABLED was disabled, are not reflected - * either. Thus we have to refresh the counter. - */ - - hlist_for_each_entry(pg, &pmctx->port->mglist, mglist) { - if (pg->key.addr.vid == pmctx->vlan->vid) - n++; - } - WRITE_ONCE(pmctx->mdb_n_entries, n); - } } static void br_multicast_enable_port_ctx(struct net_bridge_mcast_port *pmctx) -- cgit 1.3-korg
d0fdad1bdd21net: bridge: mcast: always update mdb_n_entries for vlan contexts
1 file changed · +18 −28
net/bridge/br_multicast.c+18 −28 modifieddiff --git a/net/bridge/br_multicast.c b/net/bridge/br_multicast.c index 4a2d94e8717e6f..4e75ec75c70210 100644 --- a/net/bridge/br_multicast.c +++ b/net/bridge/br_multicast.c @@ -243,14 +243,11 @@ br_multicast_port_vid_to_port_ctx(struct net_bridge_port *port, u16 vid) lockdep_assert_held_once(&port->br->multicast_lock); - if (!br_opt_get(port->br, BROPT_MCAST_VLAN_SNOOPING_ENABLED)) - return NULL; - /* Take RCU to access the vlan. */ rcu_read_lock(); vlan = br_vlan_find(nbp_vlan_group_rcu(port), vid); - if (vlan && !br_multicast_port_ctx_vlan_disabled(&vlan->port_mcast_ctx)) + if (vlan) pmctx = &vlan->port_mcast_ctx; rcu_read_unlock(); @@ -700,7 +697,10 @@ br_multicast_port_ngroups_inc_one(struct net_bridge_mcast_port *pmctx, u32 max = READ_ONCE(pmctx->mdb_max_entries); u32 n = READ_ONCE(pmctx->mdb_n_entries); - if (max && n >= max) { + /* enforce the max limit when it's a port pmctx or a port-vlan pmctx + * with snooping enabled + */ + if (!br_multicast_port_ctx_vlan_disabled(pmctx) && max && n >= max) { NL_SET_ERR_MSG_FMT_MOD(extack, "%s is already in %u groups, and mcast_max_groups=%u", what, n, max); return -E2BIG; @@ -735,9 +735,7 @@ static int br_multicast_port_ngroups_inc(struct net_bridge_port *port, return err; } - /* Only count on the VLAN context if VID is given, and if snooping on - * that VLAN is enabled. - */ + /* Only count on the VLAN context if VID is given */ if (!group->vid) return 0; @@ -2009,6 +2007,18 @@ void br_multicast_port_ctx_init(struct net_bridge_port *port, timer_setup(&pmctx->ip6_own_query.timer, br_ip6_multicast_port_query_expired, 0); #endif + /* initialize mdb_n_entries if a new port vlan is being created */ + if (vlan) { + struct net_bridge_port_group *pg; + u32 n = 0; + + spin_lock_bh(&port->br->multicast_lock); + hlist_for_each_entry(pg, &port->mglist, mglist) + if (pg->key.addr.vid == vlan->vid) + n++; + WRITE_ONCE(pmctx->mdb_n_entries, n); + spin_unlock_bh(&port->br->multicast_lock); + } } void br_multicast_port_ctx_deinit(struct net_bridge_mcast_port *pmctx) @@ -2092,25 +2102,6 @@ static void __br_multicast_enable_port_ctx(struct net_bridge_mcast_port *pmctx) br_ip4_multicast_add_router(brmctx, pmctx); br_ip6_multicast_add_router(brmctx, pmctx); } - - if (br_multicast_port_ctx_is_vlan(pmctx)) { - struct net_bridge_port_group *pg; - u32 n = 0; - - /* The mcast_n_groups counter might be wrong. First, - * BR_VLFLAG_MCAST_ENABLED is toggled before temporary entries - * are flushed, thus mcast_n_groups after the toggle does not - * reflect the true values. And second, permanent entries added - * while BR_VLFLAG_MCAST_ENABLED was disabled, are not reflected - * either. Thus we have to refresh the counter. - */ - - hlist_for_each_entry(pg, &pmctx->port->mglist, mglist) { - if (pg->key.addr.vid == pmctx->vlan->vid) - n++; - } - WRITE_ONCE(pmctx->mdb_n_entries, n); - } } static void br_multicast_enable_port_ctx(struct net_bridge_mcast_port *pmctx) -- cgit 1.3-korg
724a405ce030net: bridge: mcast: always update mdb_n_entries for vlan contexts
1 file changed · +18 −28
net/bridge/br_multicast.c+18 −28 modifieddiff --git a/net/bridge/br_multicast.c b/net/bridge/br_multicast.c index 4227894e357920..9bd2914006df70 100644 --- a/net/bridge/br_multicast.c +++ b/net/bridge/br_multicast.c @@ -244,14 +244,11 @@ br_multicast_port_vid_to_port_ctx(struct net_bridge_port *port, u16 vid) lockdep_assert_held_once(&port->br->multicast_lock); - if (!br_opt_get(port->br, BROPT_MCAST_VLAN_SNOOPING_ENABLED)) - return NULL; - /* Take RCU to access the vlan. */ rcu_read_lock(); vlan = br_vlan_find(nbp_vlan_group_rcu(port), vid); - if (vlan && !br_multicast_port_ctx_vlan_disabled(&vlan->port_mcast_ctx)) + if (vlan) pmctx = &vlan->port_mcast_ctx; rcu_read_unlock(); @@ -701,7 +698,10 @@ br_multicast_port_ngroups_inc_one(struct net_bridge_mcast_port *pmctx, u32 max = READ_ONCE(pmctx->mdb_max_entries); u32 n = READ_ONCE(pmctx->mdb_n_entries); - if (max && n >= max) { + /* enforce the max limit when it's a port pmctx or a port-vlan pmctx + * with snooping enabled + */ + if (!br_multicast_port_ctx_vlan_disabled(pmctx) && max && n >= max) { NL_SET_ERR_MSG_FMT_MOD(extack, "%s is already in %u groups, and mcast_max_groups=%u", what, n, max); return -E2BIG; @@ -736,9 +736,7 @@ static int br_multicast_port_ngroups_inc(struct net_bridge_port *port, return err; } - /* Only count on the VLAN context if VID is given, and if snooping on - * that VLAN is enabled. - */ + /* Only count on the VLAN context if VID is given */ if (!group->vid) return 0; @@ -2010,6 +2008,18 @@ void br_multicast_port_ctx_init(struct net_bridge_port *port, timer_setup(&pmctx->ip6_own_query.timer, br_ip6_multicast_port_query_expired, 0); #endif + /* initialize mdb_n_entries if a new port vlan is being created */ + if (vlan) { + struct net_bridge_port_group *pg; + u32 n = 0; + + spin_lock_bh(&port->br->multicast_lock); + hlist_for_each_entry(pg, &port->mglist, mglist) + if (pg->key.addr.vid == vlan->vid) + n++; + WRITE_ONCE(pmctx->mdb_n_entries, n); + spin_unlock_bh(&port->br->multicast_lock); + } } void br_multicast_port_ctx_deinit(struct net_bridge_mcast_port *pmctx) @@ -2093,25 +2103,6 @@ static void __br_multicast_enable_port_ctx(struct net_bridge_mcast_port *pmctx) br_ip4_multicast_add_router(brmctx, pmctx); br_ip6_multicast_add_router(brmctx, pmctx); } - - if (br_multicast_port_ctx_is_vlan(pmctx)) { - struct net_bridge_port_group *pg; - u32 n = 0; - - /* The mcast_n_groups counter might be wrong. First, - * BR_VLFLAG_MCAST_ENABLED is toggled before temporary entries - * are flushed, thus mcast_n_groups after the toggle does not - * reflect the true values. And second, permanent entries added - * while BR_VLFLAG_MCAST_ENABLED was disabled, are not reflected - * either. Thus we have to refresh the counter. - */ - - hlist_for_each_entry(pg, &pmctx->port->mglist, mglist) { - if (pg->key.addr.vid == pmctx->vlan->vid) - n++; - } - WRITE_ONCE(pmctx->mdb_n_entries, n); - } } static void br_multicast_enable_port_ctx(struct net_bridge_mcast_port *pmctx) -- cgit 1.3-korg
45525fdfd4cbnet: bridge: mcast: always update mdb_n_entries for vlan contexts
1 file changed · +18 −28
net/bridge/br_multicast.c+18 −28 modifieddiff --git a/net/bridge/br_multicast.c b/net/bridge/br_multicast.c index d55a4ab87837f7..e9a7e653040177 100644 --- a/net/bridge/br_multicast.c +++ b/net/bridge/br_multicast.c @@ -244,14 +244,11 @@ br_multicast_port_vid_to_port_ctx(struct net_bridge_port *port, u16 vid) lockdep_assert_held_once(&port->br->multicast_lock); - if (!br_opt_get(port->br, BROPT_MCAST_VLAN_SNOOPING_ENABLED)) - return NULL; - /* Take RCU to access the vlan. */ rcu_read_lock(); vlan = br_vlan_find(nbp_vlan_group_rcu(port), vid); - if (vlan && !br_multicast_port_ctx_vlan_disabled(&vlan->port_mcast_ctx)) + if (vlan) pmctx = &vlan->port_mcast_ctx; rcu_read_unlock(); @@ -701,7 +698,10 @@ br_multicast_port_ngroups_inc_one(struct net_bridge_mcast_port *pmctx, u32 max = READ_ONCE(pmctx->mdb_max_entries); u32 n = READ_ONCE(pmctx->mdb_n_entries); - if (max && n >= max) { + /* enforce the max limit when it's a port pmctx or a port-vlan pmctx + * with snooping enabled + */ + if (!br_multicast_port_ctx_vlan_disabled(pmctx) && max && n >= max) { NL_SET_ERR_MSG_FMT_MOD(extack, "%s is already in %u groups, and mcast_max_groups=%u", what, n, max); return -E2BIG; @@ -736,9 +736,7 @@ static int br_multicast_port_ngroups_inc(struct net_bridge_port *port, return err; } - /* Only count on the VLAN context if VID is given, and if snooping on - * that VLAN is enabled. - */ + /* Only count on the VLAN context if VID is given */ if (!group->vid) return 0; @@ -2011,6 +2009,18 @@ void br_multicast_port_ctx_init(struct net_bridge_port *port, timer_setup(&pmctx->ip6_own_query.timer, br_ip6_multicast_port_query_expired, 0); #endif + /* initialize mdb_n_entries if a new port vlan is being created */ + if (vlan) { + struct net_bridge_port_group *pg; + u32 n = 0; + + spin_lock_bh(&port->br->multicast_lock); + hlist_for_each_entry(pg, &port->mglist, mglist) + if (pg->key.addr.vid == vlan->vid) + n++; + WRITE_ONCE(pmctx->mdb_n_entries, n); + spin_unlock_bh(&port->br->multicast_lock); + } } void br_multicast_port_ctx_deinit(struct net_bridge_mcast_port *pmctx) @@ -2094,25 +2104,6 @@ static void __br_multicast_enable_port_ctx(struct net_bridge_mcast_port *pmctx) br_ip4_multicast_add_router(brmctx, pmctx); br_ip6_multicast_add_router(brmctx, pmctx); } - - if (br_multicast_port_ctx_is_vlan(pmctx)) { - struct net_bridge_port_group *pg; - u32 n = 0; - - /* The mcast_n_groups counter might be wrong. First, - * BR_VLFLAG_MCAST_ENABLED is toggled before temporary entries - * are flushed, thus mcast_n_groups after the toggle does not - * reflect the true values. And second, permanent entries added - * while BR_VLFLAG_MCAST_ENABLED was disabled, are not reflected - * either. Thus we have to refresh the counter. - */ - - hlist_for_each_entry(pg, &pmctx->port->mglist, mglist) { - if (pg->key.addr.vid == pmctx->vlan->vid) - n++; - } - WRITE_ONCE(pmctx->mdb_n_entries, n); - } } static void br_multicast_enable_port_ctx(struct net_bridge_mcast_port *pmctx) -- cgit 1.3-korg
724a405ce030net: bridge: mcast: always update mdb_n_entries for vlan contexts
1 file changed · +18 −28
net/bridge/br_multicast.c+18 −28 modifieddiff --git a/net/bridge/br_multicast.c b/net/bridge/br_multicast.c index 4227894e357920..9bd2914006df70 100644 --- a/net/bridge/br_multicast.c +++ b/net/bridge/br_multicast.c @@ -244,14 +244,11 @@ br_multicast_port_vid_to_port_ctx(struct net_bridge_port *port, u16 vid) lockdep_assert_held_once(&port->br->multicast_lock); - if (!br_opt_get(port->br, BROPT_MCAST_VLAN_SNOOPING_ENABLED)) - return NULL; - /* Take RCU to access the vlan. */ rcu_read_lock(); vlan = br_vlan_find(nbp_vlan_group_rcu(port), vid); - if (vlan && !br_multicast_port_ctx_vlan_disabled(&vlan->port_mcast_ctx)) + if (vlan) pmctx = &vlan->port_mcast_ctx; rcu_read_unlock(); @@ -701,7 +698,10 @@ br_multicast_port_ngroups_inc_one(struct net_bridge_mcast_port *pmctx, u32 max = READ_ONCE(pmctx->mdb_max_entries); u32 n = READ_ONCE(pmctx->mdb_n_entries); - if (max && n >= max) { + /* enforce the max limit when it's a port pmctx or a port-vlan pmctx + * with snooping enabled + */ + if (!br_multicast_port_ctx_vlan_disabled(pmctx) && max && n >= max) { NL_SET_ERR_MSG_FMT_MOD(extack, "%s is already in %u groups, and mcast_max_groups=%u", what, n, max); return -E2BIG; @@ -736,9 +736,7 @@ static int br_multicast_port_ngroups_inc(struct net_bridge_port *port, return err; } - /* Only count on the VLAN context if VID is given, and if snooping on - * that VLAN is enabled. - */ + /* Only count on the VLAN context if VID is given */ if (!group->vid) return 0; @@ -2010,6 +2008,18 @@ void br_multicast_port_ctx_init(struct net_bridge_port *port, timer_setup(&pmctx->ip6_own_query.timer, br_ip6_multicast_port_query_expired, 0); #endif + /* initialize mdb_n_entries if a new port vlan is being created */ + if (vlan) { + struct net_bridge_port_group *pg; + u32 n = 0; + + spin_lock_bh(&port->br->multicast_lock); + hlist_for_each_entry(pg, &port->mglist, mglist) + if (pg->key.addr.vid == vlan->vid) + n++; + WRITE_ONCE(pmctx->mdb_n_entries, n); + spin_unlock_bh(&port->br->multicast_lock); + } } void br_multicast_port_ctx_deinit(struct net_bridge_mcast_port *pmctx) @@ -2093,25 +2103,6 @@ static void __br_multicast_enable_port_ctx(struct net_bridge_mcast_port *pmctx) br_ip4_multicast_add_router(brmctx, pmctx); br_ip6_multicast_add_router(brmctx, pmctx); } - - if (br_multicast_port_ctx_is_vlan(pmctx)) { - struct net_bridge_port_group *pg; - u32 n = 0; - - /* The mcast_n_groups counter might be wrong. First, - * BR_VLFLAG_MCAST_ENABLED is toggled before temporary entries - * are flushed, thus mcast_n_groups after the toggle does not - * reflect the true values. And second, permanent entries added - * while BR_VLFLAG_MCAST_ENABLED was disabled, are not reflected - * either. Thus we have to refresh the counter. - */ - - hlist_for_each_entry(pg, &pmctx->port->mglist, mglist) { - if (pg->key.addr.vid == pmctx->vlan->vid) - n++; - } - WRITE_ONCE(pmctx->mdb_n_entries, n); - } } static void br_multicast_enable_port_ctx(struct net_bridge_mcast_port *pmctx) -- cgit 1.3-korg
8b769e311a86net: bridge: mcast: always update mdb_n_entries for vlan contexts
1 file changed · +18 −28
net/bridge/br_multicast.c+18 −28 modifieddiff --git a/net/bridge/br_multicast.c b/net/bridge/br_multicast.c index dccae08b4f4c40..b6a5147886ca41 100644 --- a/net/bridge/br_multicast.c +++ b/net/bridge/br_multicast.c @@ -244,14 +244,11 @@ br_multicast_port_vid_to_port_ctx(struct net_bridge_port *port, u16 vid) lockdep_assert_held_once(&port->br->multicast_lock); - if (!br_opt_get(port->br, BROPT_MCAST_VLAN_SNOOPING_ENABLED)) - return NULL; - /* Take RCU to access the vlan. */ rcu_read_lock(); vlan = br_vlan_find(nbp_vlan_group_rcu(port), vid); - if (vlan && !br_multicast_port_ctx_vlan_disabled(&vlan->port_mcast_ctx)) + if (vlan) pmctx = &vlan->port_mcast_ctx; rcu_read_unlock(); @@ -701,7 +698,10 @@ br_multicast_port_ngroups_inc_one(struct net_bridge_mcast_port *pmctx, u32 max = READ_ONCE(pmctx->mdb_max_entries); u32 n = READ_ONCE(pmctx->mdb_n_entries); - if (max && n >= max) { + /* enforce the max limit when it's a port pmctx or a port-vlan pmctx + * with snooping enabled + */ + if (!br_multicast_port_ctx_vlan_disabled(pmctx) && max && n >= max) { NL_SET_ERR_MSG_FMT_MOD(extack, "%s is already in %u groups, and mcast_max_groups=%u", what, n, max); return -E2BIG; @@ -736,9 +736,7 @@ static int br_multicast_port_ngroups_inc(struct net_bridge_port *port, return err; } - /* Only count on the VLAN context if VID is given, and if snooping on - * that VLAN is enabled. - */ + /* Only count on the VLAN context if VID is given */ if (!group->vid) return 0; @@ -2011,6 +2009,18 @@ void br_multicast_port_ctx_init(struct net_bridge_port *port, timer_setup(&pmctx->ip6_own_query.timer, br_ip6_multicast_port_query_expired, 0); #endif + /* initialize mdb_n_entries if a new port vlan is being created */ + if (vlan) { + struct net_bridge_port_group *pg; + u32 n = 0; + + spin_lock_bh(&port->br->multicast_lock); + hlist_for_each_entry(pg, &port->mglist, mglist) + if (pg->key.addr.vid == vlan->vid) + n++; + WRITE_ONCE(pmctx->mdb_n_entries, n); + spin_unlock_bh(&port->br->multicast_lock); + } } void br_multicast_port_ctx_deinit(struct net_bridge_mcast_port *pmctx) @@ -2094,25 +2104,6 @@ static void __br_multicast_enable_port_ctx(struct net_bridge_mcast_port *pmctx) br_ip4_multicast_add_router(brmctx, pmctx); br_ip6_multicast_add_router(brmctx, pmctx); } - - if (br_multicast_port_ctx_is_vlan(pmctx)) { - struct net_bridge_port_group *pg; - u32 n = 0; - - /* The mcast_n_groups counter might be wrong. First, - * BR_VLFLAG_MCAST_ENABLED is toggled before temporary entries - * are flushed, thus mcast_n_groups after the toggle does not - * reflect the true values. And second, permanent entries added - * while BR_VLFLAG_MCAST_ENABLED was disabled, are not reflected - * either. Thus we have to refresh the counter. - */ - - hlist_for_each_entry(pg, &pmctx->port->mglist, mglist) { - if (pg->key.addr.vid == pmctx->vlan->vid) - n++; - } - WRITE_ONCE(pmctx->mdb_n_entries, n); - } } static void br_multicast_enable_port_ctx(struct net_bridge_mcast_port *pmctx) -- cgit 1.3-korg
d0fdad1bdd21net: bridge: mcast: always update mdb_n_entries for vlan contexts
1 file changed · +18 −28
net/bridge/br_multicast.c+18 −28 modifieddiff --git a/net/bridge/br_multicast.c b/net/bridge/br_multicast.c index 4a2d94e8717e6f..4e75ec75c70210 100644 --- a/net/bridge/br_multicast.c +++ b/net/bridge/br_multicast.c @@ -243,14 +243,11 @@ br_multicast_port_vid_to_port_ctx(struct net_bridge_port *port, u16 vid) lockdep_assert_held_once(&port->br->multicast_lock); - if (!br_opt_get(port->br, BROPT_MCAST_VLAN_SNOOPING_ENABLED)) - return NULL; - /* Take RCU to access the vlan. */ rcu_read_lock(); vlan = br_vlan_find(nbp_vlan_group_rcu(port), vid); - if (vlan && !br_multicast_port_ctx_vlan_disabled(&vlan->port_mcast_ctx)) + if (vlan) pmctx = &vlan->port_mcast_ctx; rcu_read_unlock(); @@ -700,7 +697,10 @@ br_multicast_port_ngroups_inc_one(struct net_bridge_mcast_port *pmctx, u32 max = READ_ONCE(pmctx->mdb_max_entries); u32 n = READ_ONCE(pmctx->mdb_n_entries); - if (max && n >= max) { + /* enforce the max limit when it's a port pmctx or a port-vlan pmctx + * with snooping enabled + */ + if (!br_multicast_port_ctx_vlan_disabled(pmctx) && max && n >= max) { NL_SET_ERR_MSG_FMT_MOD(extack, "%s is already in %u groups, and mcast_max_groups=%u", what, n, max); return -E2BIG; @@ -735,9 +735,7 @@ static int br_multicast_port_ngroups_inc(struct net_bridge_port *port, return err; } - /* Only count on the VLAN context if VID is given, and if snooping on - * that VLAN is enabled. - */ + /* Only count on the VLAN context if VID is given */ if (!group->vid) return 0; @@ -2009,6 +2007,18 @@ void br_multicast_port_ctx_init(struct net_bridge_port *port, timer_setup(&pmctx->ip6_own_query.timer, br_ip6_multicast_port_query_expired, 0); #endif + /* initialize mdb_n_entries if a new port vlan is being created */ + if (vlan) { + struct net_bridge_port_group *pg; + u32 n = 0; + + spin_lock_bh(&port->br->multicast_lock); + hlist_for_each_entry(pg, &port->mglist, mglist) + if (pg->key.addr.vid == vlan->vid) + n++; + WRITE_ONCE(pmctx->mdb_n_entries, n); + spin_unlock_bh(&port->br->multicast_lock); + } } void br_multicast_port_ctx_deinit(struct net_bridge_mcast_port *pmctx) @@ -2092,25 +2102,6 @@ static void __br_multicast_enable_port_ctx(struct net_bridge_mcast_port *pmctx) br_ip4_multicast_add_router(brmctx, pmctx); br_ip6_multicast_add_router(brmctx, pmctx); } - - if (br_multicast_port_ctx_is_vlan(pmctx)) { - struct net_bridge_port_group *pg; - u32 n = 0; - - /* The mcast_n_groups counter might be wrong. First, - * BR_VLFLAG_MCAST_ENABLED is toggled before temporary entries - * are flushed, thus mcast_n_groups after the toggle does not - * reflect the true values. And second, permanent entries added - * while BR_VLFLAG_MCAST_ENABLED was disabled, are not reflected - * either. Thus we have to refresh the counter. - */ - - hlist_for_each_entry(pg, &pmctx->port->mglist, mglist) { - if (pg->key.addr.vid == pmctx->vlan->vid) - n++; - } - WRITE_ONCE(pmctx->mdb_n_entries, n); - } } static void br_multicast_enable_port_ctx(struct net_bridge_mcast_port *pmctx) -- cgit 1.3-korg
fae260fc84e1net: bridge: mcast: always update mdb_n_entries for vlan contexts
1 file changed · +18 −28
net/bridge/br_multicast.c+18 −28 modifieddiff --git a/net/bridge/br_multicast.c b/net/bridge/br_multicast.c index 22d12e54596685..5855eb0502085f 100644 --- a/net/bridge/br_multicast.c +++ b/net/bridge/br_multicast.c @@ -244,14 +244,11 @@ br_multicast_port_vid_to_port_ctx(struct net_bridge_port *port, u16 vid) lockdep_assert_held_once(&port->br->multicast_lock); - if (!br_opt_get(port->br, BROPT_MCAST_VLAN_SNOOPING_ENABLED)) - return NULL; - /* Take RCU to access the vlan. */ rcu_read_lock(); vlan = br_vlan_find(nbp_vlan_group_rcu(port), vid); - if (vlan && !br_multicast_port_ctx_vlan_disabled(&vlan->port_mcast_ctx)) + if (vlan) pmctx = &vlan->port_mcast_ctx; rcu_read_unlock(); @@ -701,7 +698,10 @@ br_multicast_port_ngroups_inc_one(struct net_bridge_mcast_port *pmctx, u32 max = READ_ONCE(pmctx->mdb_max_entries); u32 n = READ_ONCE(pmctx->mdb_n_entries); - if (max && n >= max) { + /* enforce the max limit when it's a port pmctx or a port-vlan pmctx + * with snooping enabled + */ + if (!br_multicast_port_ctx_vlan_disabled(pmctx) && max && n >= max) { NL_SET_ERR_MSG_FMT_MOD(extack, "%s is already in %u groups, and mcast_max_groups=%u", what, n, max); return -E2BIG; @@ -736,9 +736,7 @@ static int br_multicast_port_ngroups_inc(struct net_bridge_port *port, return err; } - /* Only count on the VLAN context if VID is given, and if snooping on - * that VLAN is enabled. - */ + /* Only count on the VLAN context if VID is given */ if (!group->vid) return 0; @@ -2011,6 +2009,18 @@ void br_multicast_port_ctx_init(struct net_bridge_port *port, timer_setup(&pmctx->ip6_own_query.timer, br_ip6_multicast_port_query_expired, 0); #endif + /* initialize mdb_n_entries if a new port vlan is being created */ + if (vlan) { + struct net_bridge_port_group *pg; + u32 n = 0; + + spin_lock_bh(&port->br->multicast_lock); + hlist_for_each_entry(pg, &port->mglist, mglist) + if (pg->key.addr.vid == vlan->vid) + n++; + WRITE_ONCE(pmctx->mdb_n_entries, n); + spin_unlock_bh(&port->br->multicast_lock); + } } void br_multicast_port_ctx_deinit(struct net_bridge_mcast_port *pmctx) @@ -2094,25 +2104,6 @@ static void __br_multicast_enable_port_ctx(struct net_bridge_mcast_port *pmctx) br_ip4_multicast_add_router(brmctx, pmctx); br_ip6_multicast_add_router(brmctx, pmctx); } - - if (br_multicast_port_ctx_is_vlan(pmctx)) { - struct net_bridge_port_group *pg; - u32 n = 0; - - /* The mcast_n_groups counter might be wrong. First, - * BR_VLFLAG_MCAST_ENABLED is toggled before temporary entries - * are flushed, thus mcast_n_groups after the toggle does not - * reflect the true values. And second, permanent entries added - * while BR_VLFLAG_MCAST_ENABLED was disabled, are not reflected - * either. Thus we have to refresh the counter. - */ - - hlist_for_each_entry(pg, &pmctx->port->mglist, mglist) { - if (pg->key.addr.vid == pmctx->vlan->vid) - n++; - } - WRITE_ONCE(pmctx->mdb_n_entries, n); - } } static void br_multicast_enable_port_ctx(struct net_bridge_mcast_port *pmctx) -- cgit 1.3-korg
Vulnerability mechanics
Root cause
"Inconsistent conditional updates to mdb_n_entries for VLAN multicast contexts cause the counter to become negative when entries are deleted without a prior paired increment."
Attack vector
An attacker with local access to configure a bridge can trigger the warning by adding a permanent MDB entry on a VLAN, taking the bridge down, enabling per-VLAN multicast snooping while the bridge is down (which skips the counter refresh in __br_multicast_enable_port_ctx() due to the !netif_running check), and then flushing the MDB entries. The flush decrements mdb_n_entries that was never incremented, causing a WARN_ON. The attack requires CAP_NET_ADMIN privileges to issue bridge and netlink commands [patch_id=2661423].
Affected code
The vulnerability is in net/bridge/br_multicast.c. The functions br_multicast_port_vid_to_port_ctx(), br_multicast_port_ngroups_inc_one(), br_multicast_port_ngroups_inc(), br_multicast_port_ctx_init(), and __br_multicast_enable_port_ctx() are all modified by the patch [patch_id=2661423].
What the fix does
The patch removes the conditional counting logic that only updated mdb_n_entries when VLAN snooping was enabled. Instead, it initializes the counter at port-VLAN context creation time in br_multicast_port_ctx_init() by scanning the port's existing multicast group list, and always increments/decrements the counter regardless of snooping state. The mdb limit enforcement is now gated by br_multicast_port_ctx_vlan_disabled() so that the limit only applies when the context is a port or a port-VLAN with snooping enabled [patch_id=2661423]. This ensures the counter never goes negative because every decrement is paired with a prior increment.
Preconditions
- authCAP_NET_ADMIN capability to configure bridge and MDB entries
- configBridge must have vlan_filtering and mcast_snooping enabled
- inputA permanent MDB entry must exist on a VLAN before per-VLAN snooping is enabled
Generated on May 27, 2026. Inputs: CWE entries + fix-commit diffs from this CVE's patches. Citations validated against bundle.
References
5- git.kernel.org/stable/c/45525fdfd4cb612d7b414dd5cfa1f43892a7cd71nvd
- git.kernel.org/stable/c/724a405ce0309676f1e993c173382b4c4a022bebnvd
- git.kernel.org/stable/c/8b769e311a86bb9d15c5658ad283b86fc8f080a2nvd
- git.kernel.org/stable/c/d0fdad1bdd21a358cc2c85da3681ae27b86ce6cenvd
- git.kernel.org/stable/c/fae260fc84e1eae8f590c7907e53e8768df2d986nvd
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