CVE-2026-80842 (GCVE-0-2026-80842)
Vulnerability from cvelistv5
Published
2026-09-04 15:54
Modified
2026-09-04 15:54
Severity ?
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 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.
References
Impacted products
Vendor Product Version
Linux Linux Version: 7b54aaaf53cb784411426c64482af0435f7c845e
Version: 7b54aaaf53cb784411426c64482af0435f7c845e
Version: 7b54aaaf53cb784411426c64482af0435f7c845e
Version: 7b54aaaf53cb784411426c64482af0435f7c845e
Version: 7b54aaaf53cb784411426c64482af0435f7c845e
Version: 7b54aaaf53cb784411426c64482af0435f7c845e
Version: 7b54aaaf53cb784411426c64482af0435f7c845e
Version: 7b54aaaf53cb784411426c64482af0435f7c845e
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{
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          "defaultStatus": "unaffected",
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          "programFiles": [
            "net/bridge/br_multicast.c"
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      "cpeApplicability": [
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      "descriptions": [
        {
          "lang": "en",
          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nnet: bridge: mcast: fix use-after-free of a master VLAN\u0027s multicast context\n\nbr_multicast_toggle_one_vlan() clears BR_VLFLAG_MCAST_ENABLED under\nbr-\u003emulticast_lock before stopping a VLAN\u0027s multicast context.  That is\nthe teardown handshake: lockless readers gate on the flag through\nbr_multicast_ctx_should_use() -\u003e br_multicast_ctx_vlan_disabled(), so\nonce it is cleared under the lock no reader can arm the context again.\n\nFor a master VLAN the handshake never runs.  __vlan_del() clears\nBRIDGE_VLAN_INFO_BRENTRY before calling br_vlan_put_master(), so\nbr_multicast_toggle_one_vlan(masterv, false) returns early on\n!br_vlan_is_brentry(vlan): the flag stays set and br-\u003emulticast_lock is\nnever taken.  br_vlan_put_master() then drains the context in\nbr_multicast_ctx_deinit() and frees the VLAN through call_rcu(), while a\nreader still inside rcu_read_lock() sees the context as enabled and\nre-arms it.  The port and port-VLAN branch of the function has no\nbr_vlan_is_brentry() test and flips the flag under br-\u003emulticast_lock,\nso it is not affected.\n\nThe reader is the bridge transmit path.  For a master VLAN\nbr_multicast_rcv() selects brmctx = \u0026vlan-\u003ebr_mcast_ctx with\npmctx = NULL, so IGMP sent to the bridge device re-arms the context\u0027s\ntimers after br_multicast_ctx_deinit() has already stopped them.\n\n  BUG: KASAN: slab-use-after-free in detach_if_pending+0x412/0x4a0\n  Write of size 8 at addr ffff88810ac39918 by task brmc/601\n   __mod_timer+0x51a/0xc50\n   br_multicast_host_join+0x25b/0x390\n   __br_multicast_add_group+0x468/0x530\n   br_ip4_multicast_add_group+0x1a0/0x260\n   br_multicast_rcv+0x2cda/0x61e0\n   br_dev_xmit+0x6c4/0x1540\n  Allocated by task 610:\n   br_vlan_add+0x111/0xb40\n   br_vlan_info+0x370/0x3e0\n  Freed by task 0:\n   kfree+0x1a7/0x4f0\n   rcu_core+0x7dc/0x10a0\n\nOnly test br_vlan_is_brentry() when enabling, like the\nbr_multicast_ctx_vlan_global_disabled() test next to it.  Disabling then\nalways clears BR_VLFLAG_MCAST_ENABLED under br-\u003emulticast_lock before\nbr_multicast_ctx_deinit() drains the context."
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-09-04T15:54:52.459Z",
        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
      },
      "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"
        }
      ],
      "title": "net: bridge: mcast: fix use-after-free of a master VLAN\u0027s multicast context",
      "x_generator": {
        "engine": "bippy-1.2.0"
      }
    }
  },
  "cveMetadata": {
    "assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
    "assignerShortName": "Linux",
    "cveId": "CVE-2026-80842",
    "datePublished": "2026-09-04T15:54:52.459Z",
    "dateReserved": "2026-08-26T14:34:25.796Z",
    "dateUpdated": "2026-09-04T15:54:52.459Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2"
}


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