published September 4, 2026
In the Linux kernel, the following vulnerability has been resolved:
net: bridge: mcast: fix use-after-free of a master VLAN's multicast context
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.
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.