Re: [PATCH net] net: bridge: mcast: fix use-after-free of a master VLAN's multicast context

On 23/08/2026 14:58, Norbert Szetei wrote:
> br_multicast_toggle_one_vlan() clears BR_VLFLAG_MCAST_ENABLED under
> br->multicast_lock before stopping a VLAN's multicast context.  That is
> the teardown handshake: lockless readers gate on the flag through
> br_multicast_ctx_should_use() -> br_multicast_ctx_vlan_disabled(), so
> once it is cleared under the lock no reader can arm the context again.
> 
> For a master VLAN the handshake never runs.  __vlan_del() clears
> BRIDGE_VLAN_INFO_BRENTRY before calling br_vlan_put_master(), so
> br_multicast_toggle_one_vlan(masterv, false) returns early on
> !br_vlan_is_brentry(vlan): the flag stays set and br->multicast_lock is
> never taken.  br_vlan_put_master() then drains the context in
> br_multicast_ctx_deinit() and frees the VLAN through call_rcu(), while a
> reader still inside rcu_read_lock() sees the context as enabled and
> re-arms it.  The port and port-VLAN branch of the function has no
> br_vlan_is_brentry() test and flips the flag under br->multicast_lock,
> so it is not affected.
> 
> The reader is the bridge transmit path.  For a master VLAN
> br_multicast_rcv() selects brmctx = &vlan->br_mcast_ctx with
> pmctx = NULL, so IGMP sent to the bridge device re-arms the context's
> timers after br_multicast_ctx_deinit() has already stopped them.
> 
> Any user able to create a bridge can trigger this, which includes an
> unprivileged user in a user namespace, by deleting and re-adding the
> master VLAN while sending IGMP to the bridge device:
> 

So much AI slop... at least drop this paragraph which is completely useless.

>    BUG: KASAN: slab-use-after-free in detach_if_pending+0x412/0x4a0
>    Write of size 8 at addr ffff88810ac39918 by task brmc/601
>     __mod_timer+0x51a/0xc50
>     br_multicast_host_join+0x25b/0x390
>     __br_multicast_add_group+0x468/0x530
>     br_ip4_multicast_add_group+0x1a0/0x260
>     br_multicast_rcv+0x2cda/0x61e0
>     br_dev_xmit+0x6c4/0x1540
>    Allocated by task 610:
>     br_vlan_add+0x111/0xb40
>     br_vlan_info+0x370/0x3e0
>    Freed by task 0:
>     kfree+0x1a7/0x4f0
>     rcu_core+0x7dc/0x10a0
> 
> Only test br_vlan_is_brentry() when enabling, like the
> br_multicast_ctx_vlan_global_disabled() test next to it.  Disabling then
> always clears BR_VLFLAG_MCAST_ENABLED under br->multicast_lock before
> br_multicast_ctx_deinit() drains the context.
> 
> Fixes: 7b54aaaf53cb ("net: bridge: multicast: add vlan state initialization and control")
> Cc: [email protected]
> Assisted-by: Claude:claude-opus-5
> Signed-off-by: Norbert Szetei <[email protected]>
> ---
>   net/bridge/br_multicast.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/net/bridge/br_multicast.c b/net/bridge/br_multicast.c
> index 75e1e2a8fc83..3ef5d8bbf552 100644
> --- a/net/bridge/br_multicast.c
> +++ b/net/bridge/br_multicast.c
> @@ -4377,8 +4377,8 @@ void br_multicast_toggle_one_vlan(struct net_bridge_vlan *vlan, bool on)
>   	if (br_vlan_is_master(vlan)) {
>   		br = vlan->br;
>   
> -		if (!br_vlan_is_brentry(vlan) ||
> -		    (on &&
> +		if (on &&
> +		    (!br_vlan_is_brentry(vlan) ||
>   		     br_multicast_ctx_vlan_global_disabled(&vlan->br_mcast_ctx)))
>   			return;
>