{"id":"CVE-2025-38248","summary":"bridge: mcast: Fix use-after-free during router port configuration","details":"In the Linux kernel, the following vulnerability has been resolved:\n\nbridge: mcast: Fix use-after-free during router port configuration\n\nThe bridge maintains a global list of ports behind which a multicast\nrouter resides. The list is consulted during forwarding to ensure\nmulticast packets are forwarded to these ports even if the ports are not\nmember in the matching MDB entry.\n\nWhen per-VLAN multicast snooping is enabled, the per-port multicast\ncontext is disabled on each port and the port is removed from the global\nrouter port list:\n\n # ip link add name br1 up type bridge vlan_filtering 1 mcast_snooping 1\n # ip link add name dummy1 up master br1 type dummy\n # ip link set dev dummy1 type bridge_slave mcast_router 2\n $ bridge -d mdb show | grep router\n router ports on br1: dummy1\n # ip link set dev br1 type bridge mcast_vlan_snooping 1\n $ bridge -d mdb show | grep router\n\nHowever, the port can be re-added to the global list even when per-VLAN\nmulticast snooping is enabled:\n\n # ip link set dev dummy1 type bridge_slave mcast_router 0\n # ip link set dev dummy1 type bridge_slave mcast_router 2\n $ bridge -d mdb show | grep router\n router ports on br1: dummy1\n\nSince commit 4b30ae9adb04 (\"net: bridge: mcast: re-implement\nbr_multicast_{enable, disable}_port functions\"), when per-VLAN multicast\nsnooping is enabled, multicast disablement on a port will disable the\nper-{port, VLAN} multicast contexts and not the per-port one. As a\nresult, a port will remain in the global router port list even after it\nis deleted. This will lead to a use-after-free [1] when the list is\ntraversed (when adding a new port to the list, for example):\n\n # ip link del dev dummy1\n # ip link add name dummy2 up master br1 type dummy\n # ip link set dev dummy2 type bridge_slave mcast_router 2\n\nSimilarly, stale entries can also be found in the per-VLAN router port\nlist. When per-VLAN multicast snooping is disabled, the per-{port, VLAN}\ncontexts are disabled on each port and the port is removed from the\nper-VLAN router port list:\n\n # ip link add name br1 up type bridge vlan_filtering 1 mcast_snooping 1 mcast_vlan_snooping 1\n # ip link add name dummy1 up master br1 type dummy\n # bridge vlan add vid 2 dev dummy1\n # bridge vlan global set vid 2 dev br1 mcast_snooping 1\n # bridge vlan set vid 2 dev dummy1 mcast_router 2\n $ bridge vlan global show dev br1 vid 2 | grep router\n       router ports: dummy1\n # ip link set dev br1 type bridge mcast_vlan_snooping 0\n $ bridge vlan global show dev br1 vid 2 | grep router\n\nHowever, the port can be re-added to the per-VLAN list even when\nper-VLAN multicast snooping is disabled:\n\n # bridge vlan set vid 2 dev dummy1 mcast_router 0\n # bridge vlan set vid 2 dev dummy1 mcast_router 2\n $ bridge vlan global show dev br1 vid 2 | grep router\n       router ports: dummy1\n\nWhen the VLAN is deleted from the port, the per-{port, VLAN} multicast\ncontext will not be disabled since multicast snooping is not enabled\non the VLAN. As a result, the port will remain in the per-VLAN router\nport list even after it is no longer member in the VLAN. This will lead\nto a use-after-free [2] when the list is traversed (when adding a new\nport to the list, for example):\n\n # ip link add name dummy2 up master br1 type dummy\n # bridge vlan add vid 2 dev dummy2\n # bridge vlan del vid 2 dev dummy1\n # bridge vlan set vid 2 dev dummy2 mcast_router 2\n\nFix these issues by removing the port from the relevant (global or\nper-VLAN) router port list in br_multicast_port_ctx_deinit(). The\nfunction is invoked during port deletion with the per-port multicast\ncontext and during VLAN deletion with the per-{port, VLAN} multicast\ncontext.\n\nNote that deleting the multicast router timer is not enough as it only\ntakes care of the temporary multicast router states (1 or 3) and not the\npermanent one (2).\n\n[1]\nBUG: KASAN: slab-out-of-bounds in br_multicast_add_router.part.0+0x3f1/0x560\nWrite of size 8 at addr ffff888004a67328 by task ip/384\n[...]\nCall Trace:\n \u003cTASK\u003e\n dump_stack\n---truncated---","modified":"2026-04-24T06:42:50.482168239Z","published":"2025-07-09T10:42:29.133Z","related":["ALSA-2026:3083","ALSA-2026:3110","CGA-h398-gcwv-83p8","SUSE-SU-2025:02853-1","SUSE-SU-2025:02923-1","SUSE-SU-2025:02969-1","SUSE-SU-2025:02996-1","SUSE-SU-2025:02997-1","SUSE-SU-2025:03011-1","SUSE-SU-2025:03023-1","SUSE-SU-2025:20577-1","SUSE-SU-2025:20586-1","SUSE-SU-2025:20601-1","SUSE-SU-2025:20602-1"],"database_specific":{"cna_assigner":"Linux","osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2025/38xxx/CVE-2025-38248.json"},"references":[{"type":"WEB","url":"https://git.kernel.org/stable/c/4d3c2a1d4c7c33103f1ddfdbc5cfe1ea4f6d0dcd"},{"type":"WEB","url":"https://git.kernel.org/stable/c/7544f3f5b0b58c396f374d060898b5939da31709"},{"type":"WEB","url":"https://git.kernel.org/stable/c/bdced577da71b118b6ed4242ebd47f81bf54d406"},{"type":"WEB","url":"https://git.kernel.org/stable/c/f05a4f9e959e0fc098046044c650acf897ea52d2"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2025/38xxx/CVE-2025-38248.json"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2025-38248"},{"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":"2796d846d74a18cc6563e96eff8bf28c5e06f912"},{"fixed":"4d3c2a1d4c7c33103f1ddfdbc5cfe1ea4f6d0dcd"},{"fixed":"bdced577da71b118b6ed4242ebd47f81bf54d406"},{"fixed":"f05a4f9e959e0fc098046044c650acf897ea52d2"},{"fixed":"7544f3f5b0b58c396f374d060898b5939da31709"}]}],"database_specific":{"source":"https://storage.googleapis.com/osv-test-cve-osv-conversion/osv-output/CVE-2025-38248.json"}}],"schema_version":"1.7.5"}