CVE-2025-38248 (GCVE-0-2025-38248)
Vulnerability from cvelistv5
Published
2025-07-09 10:42
Modified
2026-08-05 12:00
Severity ?
7.8 (High) - CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Summary
In the Linux kernel, the following vulnerability has been resolved: bridge: mcast: Fix use-after-free during router port configuration The bridge maintains a global list of ports behind which a multicast router resides. The list is consulted during forwarding to ensure multicast packets are forwarded to these ports even if the ports are not member in the matching MDB entry. When per-VLAN multicast snooping is enabled, the per-port multicast context is disabled on each port and the port is removed from the global router port list: # ip link add name br1 up type bridge vlan_filtering 1 mcast_snooping 1 # ip link add name dummy1 up master br1 type dummy # ip link set dev dummy1 type bridge_slave mcast_router 2 $ bridge -d mdb show | grep router router ports on br1: dummy1 # ip link set dev br1 type bridge mcast_vlan_snooping 1 $ bridge -d mdb show | grep router However, the port can be re-added to the global list even when per-VLAN multicast snooping is enabled: # ip link set dev dummy1 type bridge_slave mcast_router 0 # ip link set dev dummy1 type bridge_slave mcast_router 2 $ bridge -d mdb show | grep router router ports on br1: dummy1 Since commit 4b30ae9adb04 ("net: bridge: mcast: re-implement br_multicast_{enable, disable}_port functions"), when per-VLAN multicast snooping is enabled, multicast disablement on a port will disable the per-{port, VLAN} multicast contexts and not the per-port one. As a result, a port will remain in the global router port list even after it is deleted. This will lead to a use-after-free [1] when the list is traversed (when adding a new port to the list, for example): # ip link del dev dummy1 # ip link add name dummy2 up master br1 type dummy # ip link set dev dummy2 type bridge_slave mcast_router 2 Similarly, stale entries can also be found in the per-VLAN router port list. When per-VLAN multicast snooping is disabled, the per-{port, VLAN} contexts are disabled on each port and the port is removed from the per-VLAN router port list: # ip link add name br1 up type bridge vlan_filtering 1 mcast_snooping 1 mcast_vlan_snooping 1 # ip link add name dummy1 up master br1 type dummy # bridge vlan add vid 2 dev dummy1 # bridge vlan global set vid 2 dev br1 mcast_snooping 1 # bridge vlan set vid 2 dev dummy1 mcast_router 2 $ bridge vlan global show dev br1 vid 2 | grep router router ports: dummy1 # ip link set dev br1 type bridge mcast_vlan_snooping 0 $ bridge vlan global show dev br1 vid 2 | grep router However, the port can be re-added to the per-VLAN list even when per-VLAN multicast snooping is disabled: # bridge vlan set vid 2 dev dummy1 mcast_router 0 # bridge vlan set vid 2 dev dummy1 mcast_router 2 $ bridge vlan global show dev br1 vid 2 | grep router router ports: dummy1 When the VLAN is deleted from the port, the per-{port, VLAN} multicast context will not be disabled since multicast snooping is not enabled on the VLAN. As a result, the port will remain in the per-VLAN router port list even after it is no longer member in the VLAN. This will lead to a use-after-free [2] when the list is traversed (when adding a new port to the list, for example): # ip link add name dummy2 up master br1 type dummy # bridge vlan add vid 2 dev dummy2 # bridge vlan del vid 2 dev dummy1 # bridge vlan set vid 2 dev dummy2 mcast_router 2 Fix these issues by removing the port from the relevant (global or per-VLAN) router port list in br_multicast_port_ctx_deinit(). The function is invoked during port deletion with the per-port multicast context and during VLAN deletion with the per-{port, VLAN} multicast context. Note that deleting the multicast router timer is not enough as it only takes care of the temporary multicast router states (1 or 3) and not the permanent one (2). [1] BUG: KASAN: slab-out-of-bounds in br_multicast_add_router.part.0+0x3f1/0x560 Write of size 8 at addr ffff888004a67328 by task ip/384 [...] Call Trace: <TASK> dump_stack ---truncated---
References
Impacted products
Vendor Product Version
Linux Linux Version: 2796d846d74a18cc6563e96eff8bf28c5e06f912
Version: 2796d846d74a18cc6563e96eff8bf28c5e06f912
Version: 2796d846d74a18cc6563e96eff8bf28c5e06f912
Version: 2796d846d74a18cc6563e96eff8bf28c5e06f912
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          "value": "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---"
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              "value": "AV:L - The dangling router-port entry is created and triggered through rtnetlink configuration messages (RTM_NEWLINK/RTM_SETLINK with IFLA_BRPORT_MULTICAST_ROUTER and RTM_NEWVLAN options), which require a local socket. Per kernel guidance, netlink-reachable bugs are Local.\nAC:L - The bug is a deterministic state-machine error, not a race: a fixed sequence of `ip link`/`bridge vlan` commands leaves the freed port on the router list and the very next mcast_router set triggers the UAF, as shown by the reproducers and the syzkaller report.\nPR:L - Bridge port and VLAN configuration only requires CAP_NET_ADMIN in the netns owning the device, which any unprivileged user obtains via `unshare -Urn` and then creates the bridge, dummy ports and multicast options entirely inside that namespace.\nUI:N - The attacker performs the entire configuration sequence themselves through their own netlink socket; no victim action is needed at any point.\nS:U - The corruption stays within the kernel\u0027s own memory and security authority, with no crossing of a VM, IOMMU or sandbox boundary.\nC:H - List traversal reads `pmctx-\u003eport` and the `hlist_node` links out of freed slab memory, and the resulting dangling entries are also walked on the multicast forwarding path, giving an attacker who reallocates the freed object a means to read kernel memory and defeat KASLR.\nI:H - `hlist_add_behind_rcu()` writes 8 bytes into the freed (attacker-reallocatable) object and later `hlist_del_init_rcu()` writes through an attacker-controlled `pprev`, yielding a controlled-write primitive; freed ports are additionally used to deliver packets via a dangling `net_device`, enabling control-flow hijack.\nA:H - The use-after-free corrupts the bridge router list and reliably produces a KASAN-confirmed slab-use-after-free/out-of-bounds access, causing kernel oops or panic, and can be repeated at will."
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    "datePublished": "2025-07-09T10:42:29.133Z",
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Per kernel guidance, netlink-reachable bugs are Local.\\nAC:L - The bug is a deterministic state-machine error, not a race: a fixed sequence of `ip link`/`bridge vlan` commands leaves the freed port on the router list and the very next mcast_router set triggers the UAF, as shown by the reproducers and the syzkaller report.\\nPR:L - Bridge port and VLAN configuration only requires CAP_NET_ADMIN in the netns owning the device, which any unprivileged user obtains via `unshare -Urn` and then creates the bridge, dummy ports and multicast options entirely inside that namespace.\\nUI:N - The attacker performs the entire configuration sequence themselves through their own netlink socket; no victim action is needed at any point.\\nS:U - The corruption stays within the kernel\u0027s own memory and security authority, with no crossing of a VM, IOMMU or sandbox boundary.\\nC:H - List traversal reads `pmctx-\u003eport` and the `hlist_node` links out of freed slab memory, and the resulting dangling entries are also walked on the multicast forwarding path, giving an attacker who reallocates the freed object a means to read kernel memory and defeat KASLR.\\nI:H - `hlist_add_behind_rcu()` writes 8 bytes into the freed (attacker-reallocatable) object and later `hlist_del_init_rcu()` writes through an attacker-controlled `pprev`, yielding a controlled-write primitive; freed ports are additionally used to deliver packets via a dangling `net_device`, enabling control-flow hijack.\\nA:H - The use-after-free corrupts the bridge router list and reliably produces a KASAN-confirmed slab-use-after-free/out-of-bounds access, causing kernel oops or panic, and can be repeated at will.\"}]}], \"affected\": [{\"repo\": \"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git\", \"vendor\": \"Linux\", \"product\": \"Linux\", \"versions\": [{\"status\": \"affected\", \"version\": \"2796d846d74a18cc6563e96eff8bf28c5e06f912\", \"lessThan\": \"4d3c2a1d4c7c33103f1ddfdbc5cfe1ea4f6d0dcd\", \"versionType\": \"git\"}, {\"status\": \"affected\", \"version\": \"2796d846d74a18cc6563e96eff8bf28c5e06f912\", \"lessThan\": \"bdced577da71b118b6ed4242ebd47f81bf54d406\", \"versionType\": \"git\"}, {\"status\": \"affected\", \"version\": \"2796d846d74a18cc6563e96eff8bf28c5e06f912\", \"lessThan\": \"f05a4f9e959e0fc098046044c650acf897ea52d2\", \"versionType\": \"git\"}, {\"status\": \"affected\", \"version\": \"2796d846d74a18cc6563e96eff8bf28c5e06f912\", \"lessThan\": \"7544f3f5b0b58c396f374d060898b5939da31709\", \"versionType\": \"git\"}], \"programFiles\": [\"net/bridge/br_multicast.c\"], \"defaultStatus\": \"unaffected\"}, {\"repo\": \"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git\", \"vendor\": \"Linux\", \"product\": \"Linux\", \"versions\": [{\"status\": \"affected\", \"version\": \"5.15\"}, {\"status\": \"unaffected\", \"version\": \"0\", \"lessThan\": \"5.15\", \"versionType\": \"semver\"}, {\"status\": \"unaffected\", \"version\": \"6.6.122\", \"versionType\": \"semver\", \"lessThanOrEqual\": \"6.6.*\"}, {\"status\": \"unaffected\", \"version\": \"6.12.67\", \"versionType\": \"semver\", \"lessThanOrEqual\": \"6.12.*\"}, {\"status\": \"unaffected\", \"version\": \"6.15.5\", \"versionType\": \"semver\", \"lessThanOrEqual\": \"6.15.*\"}, {\"status\": \"unaffected\", \"version\": \"6.16\", \"versionType\": \"original_commit_for_fix\", \"lessThanOrEqual\": \"*\"}], \"programFiles\": [\"net/bridge/br_multicast.c\"], \"defaultStatus\": \"affected\"}], \"references\": [{\"url\": \"https://git.kernel.org/stable/c/4d3c2a1d4c7c33103f1ddfdbc5cfe1ea4f6d0dcd\"}, {\"url\": \"https://git.kernel.org/stable/c/bdced577da71b118b6ed4242ebd47f81bf54d406\"}, {\"url\": \"https://git.kernel.org/stable/c/f05a4f9e959e0fc098046044c650acf897ea52d2\"}, {\"url\": \"https://git.kernel.org/stable/c/7544f3f5b0b58c396f374d060898b5939da31709\"}], \"x_generator\": {\"engine\": \"bippy-1.2.0\"}, \"descriptions\": [{\"lang\": \"en\", \"value\": \"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---\"}], \"cpeApplicability\": [{\"nodes\": [{\"negate\": false, \"cpeMatch\": [{\"criteria\": \"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*\", \"vulnerable\": true, \"versionEndExcluding\": \"6.6.122\", \"versionStartIncluding\": \"5.15\"}, {\"criteria\": \"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*\", \"vulnerable\": true, \"versionEndExcluding\": \"6.12.67\", \"versionStartIncluding\": \"5.15\"}, {\"criteria\": \"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*\", \"vulnerable\": true, \"versionEndExcluding\": \"6.15.5\", \"versionStartIncluding\": \"5.15\"}, {\"criteria\": \"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*\", \"vulnerable\": true, \"versionEndExcluding\": \"6.16\", \"versionStartIncluding\": \"5.15\"}], \"operator\": \"OR\"}]}], \"providerMetadata\": {\"orgId\": \"416baaa9-dc9f-4396-8d5f-8c081fb06d67\", \"shortName\": \"Linux\", \"dateUpdated\": \"2026-08-05T12:00:42.080Z\"}}}",
      "cveMetadata": "{\"cveId\": \"CVE-2025-38248\", \"state\": \"PUBLISHED\", \"dateUpdated\": \"2026-08-05T12:00:42.080Z\", \"dateReserved\": \"2025-04-16T04:51:23.997Z\", \"assignerOrgId\": \"416baaa9-dc9f-4396-8d5f-8c081fb06d67\", \"datePublished\": \"2025-07-09T10:42:29.133Z\", \"assignerShortName\": \"Linux\"}",
      "dataType": "CVE_RECORD",
      "dataVersion": "5.2"
    }
  }
}


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