CVE-2026-80842

In the Linux kernel, the following vulnerability has been resolved:

net: bridge: mcast: fix use-after-free of a master VLANs multicast context

br_multicast_toggle_one_vlan() clears BR_VLFLAG_MCAST_ENABLED under br->multicast_lock before stopping a VLANs 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 contexts 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.

Affected Software

References

• https://git.kernel.org/stable/c/22226a2c3b90f15b0925f1464470d3baa6c5677e
• https://git.kernel.org/stable/c/3a0ad4fcdfa0b7dba1876de14a12cb65c8b5ca50
• https://git.kernel.org/stable/c/3afaaee2f972aec9059110953adb62fa3cf5c4bd
• https://git.kernel.org/stable/c/3f4752996735e0628af559aa8da1d872c2fac13b
• https://git.kernel.org/stable/c/50e5c6605cc9c2dd57bd2d1b3459674d19738983
• https://git.kernel.org/stable/c/57f94d3f4dee8b54d63cefddf1112be4656ef9e6
• https://git.kernel.org/stable/c/7c54fd8cfbcf371a5ef50db5c53fe6e85fb76686
• https://git.kernel.org/stable/c/c069f29da72324697aa4b7cab5b3647a7d24a575