---
id: CVE-2026-80842
title: 'net: bridge: mcast: fix use-after-free of a master VLAN''s multicast context'
summary: |-
  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 befo…
severity: none
vendor: Linux
product: Linux
affected:
  - >-
    Linux >= 7b54aaaf53cb784411426c64482af0435f7c845e <
    3afaaee2f972aec9059110953adb62fa3cf5c4bd
  - >-
    Linux >= 7b54aaaf53cb784411426c64482af0435f7c845e <
    22226a2c3b90f15b0925f1464470d3baa6c5677e
  - >-
    Linux >= 7b54aaaf53cb784411426c64482af0435f7c845e <
    7c54fd8cfbcf371a5ef50db5c53fe6e85fb76686
  - >-
    Linux >= 7b54aaaf53cb784411426c64482af0435f7c845e <
    3a0ad4fcdfa0b7dba1876de14a12cb65c8b5ca50
  - >-
    Linux >= 7b54aaaf53cb784411426c64482af0435f7c845e <
    c069f29da72324697aa4b7cab5b3647a7d24a575
  - >-
    Linux >= 7b54aaaf53cb784411426c64482af0435f7c845e <
    3f4752996735e0628af559aa8da1d872c2fac13b
  - >-
    Linux >= 7b54aaaf53cb784411426c64482af0435f7c845e <
    57f94d3f4dee8b54d63cefddf1112be4656ef9e6
  - >-
    Linux >= 7b54aaaf53cb784411426c64482af0435f7c845e <
    50e5c6605cc9c2dd57bd2d1b3459674d19738983
  - Linux 5.15
published: '2026-09-04'
updated: '2026-10-19'
sourceUpdated: '2026-10-19T08:44:35.389Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-80842'
references:
  - url: 'https://git.kernel.org/stable/c/3afaaee2f972aec9059110953adb62fa3cf5c4bd'
  - url: 'https://git.kernel.org/stable/c/22226a2c3b90f15b0925f1464470d3baa6c5677e'
  - url: 'https://git.kernel.org/stable/c/7c54fd8cfbcf371a5ef50db5c53fe6e85fb76686'
  - url: 'https://git.kernel.org/stable/c/3a0ad4fcdfa0b7dba1876de14a12cb65c8b5ca50'
  - url: 'https://git.kernel.org/stable/c/c069f29da72324697aa4b7cab5b3647a7d24a575'
  - url: 'https://git.kernel.org/stable/c/3f4752996735e0628af559aa8da1d872c2fac13b'
  - url: 'https://git.kernel.org/stable/c/57f94d3f4dee8b54d63cefddf1112be4656ef9e6'
  - url: 'https://git.kernel.org/stable/c/50e5c6605cc9c2dd57bd2d1b3459674d19738983'
tags:
  - cve.org
epss: 0.00177
epssPercentile: 0.0675
ingestedAt: '2026-10-19T09:16:01.498Z'
---

## Overview

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.

## Affected

- `Linux >= 7b54aaaf53cb784411426c64482af0435f7c845e < 3afaaee2f972aec9059110953adb62fa3cf5c4bd`
- `Linux >= 7b54aaaf53cb784411426c64482af0435f7c845e < 22226a2c3b90f15b0925f1464470d3baa6c5677e`
- `Linux >= 7b54aaaf53cb784411426c64482af0435f7c845e < 7c54fd8cfbcf371a5ef50db5c53fe6e85fb76686`
- `Linux >= 7b54aaaf53cb784411426c64482af0435f7c845e < 3a0ad4fcdfa0b7dba1876de14a12cb65c8b5ca50`
- `Linux >= 7b54aaaf53cb784411426c64482af0435f7c845e < c069f29da72324697aa4b7cab5b3647a7d24a575`
- `Linux >= 7b54aaaf53cb784411426c64482af0435f7c845e < 3f4752996735e0628af559aa8da1d872c2fac13b`
- `Linux >= 7b54aaaf53cb784411426c64482af0435f7c845e < 57f94d3f4dee8b54d63cefddf1112be4656ef9e6`
- `Linux >= 7b54aaaf53cb784411426c64482af0435f7c845e < 50e5c6605cc9c2dd57bd2d1b3459674d19738983`
- `Linux 5.15`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.