{"id":"CVE-2026-80842","summary":"net: bridge: mcast: fix use-after-free of a master VLAN's multicast context","details":"In the Linux kernel, the following vulnerability has been resolved:\n\nnet: bridge: mcast: fix use-after-free of a master VLAN's multicast context\n\nbr_multicast_toggle_one_vlan() clears BR_VLFLAG_MCAST_ENABLED under\nbr-\u003emulticast_lock before stopping a VLAN's 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 = &vlan-\u003ebr_mcast_ctx with\npmctx = NULL, so IGMP sent to the bridge device re-arms the context's\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.","modified":"2026-09-06T03:46:53.454594109Z","published":"2026-09-04T15:54:52.459Z","database_specific":{"cna_assigner":"Linux","osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/80xxx/CVE-2026-80842.json"},"references":[{"type":"WEB","url":"https://git.kernel.org/stable/c/22226a2c3b90f15b0925f1464470d3baa6c5677e"},{"type":"WEB","url":"https://git.kernel.org/stable/c/3a0ad4fcdfa0b7dba1876de14a12cb65c8b5ca50"},{"type":"WEB","url":"https://git.kernel.org/stable/c/3afaaee2f972aec9059110953adb62fa3cf5c4bd"},{"type":"WEB","url":"https://git.kernel.org/stable/c/3f4752996735e0628af559aa8da1d872c2fac13b"},{"type":"WEB","url":"https://git.kernel.org/stable/c/50e5c6605cc9c2dd57bd2d1b3459674d19738983"},{"type":"WEB","url":"https://git.kernel.org/stable/c/57f94d3f4dee8b54d63cefddf1112be4656ef9e6"},{"type":"WEB","url":"https://git.kernel.org/stable/c/7c54fd8cfbcf371a5ef50db5c53fe6e85fb76686"},{"type":"WEB","url":"https://git.kernel.org/stable/c/c069f29da72324697aa4b7cab5b3647a7d24a575"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/80xxx/CVE-2026-80842.json"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-80842"},{"type":"PACKAGE","url":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git"}],"affected":[{"ranges":[{"type":"GIT","repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","events":[{"introduced":"7b54aaaf53cb784411426c64482af0435f7c845e"},{"fixed":"3afaaee2f972aec9059110953adb62fa3cf5c4bd"},{"fixed":"22226a2c3b90f15b0925f1464470d3baa6c5677e"},{"fixed":"7c54fd8cfbcf371a5ef50db5c53fe6e85fb76686"},{"fixed":"3a0ad4fcdfa0b7dba1876de14a12cb65c8b5ca50"},{"fixed":"c069f29da72324697aa4b7cab5b3647a7d24a575"},{"fixed":"3f4752996735e0628af559aa8da1d872c2fac13b"},{"fixed":"57f94d3f4dee8b54d63cefddf1112be4656ef9e6"},{"fixed":"50e5c6605cc9c2dd57bd2d1b3459674d19738983"}]}],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-80842.json"}},{"package":{"name":"Kernel","ecosystem":"Linux"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"5.15.0"},{"fixed":"5.15.220"}]},{"type":"ECOSYSTEM","events":[{"introduced":"5.16.0"},{"fixed":"6.1.187"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.2.0"},{"fixed":"6.6.156"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.7.0"},{"fixed":"6.12.108"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.13.0"},{"fixed":"6.18.49"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.19.0"},{"fixed":"7.1.13"}]},{"type":"ECOSYSTEM","events":[{"introduced":"7.2.0"},{"fixed":"7.2.3"}]}],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-80842.json"}}],"schema_version":"1.9.0"}