mptcp: fix slab-use-after-free in __inet_lookup_established

{
  "id": "11a1b372df4835d1bd6589278b1d619a2ac70e2cd50908513a295d29613eb428",
  "url": "https://osv.dev/vulnerability/CVE-2026-31669",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "mptcp: fix slab-use-after-free in __inet_lookup_established",
    "summary": "In the Linux kernel, the following vulnerability has been resolved: mptcp: fix slab-use-after-free in __inet_lookup_established The ehash table lookups are lockless and rely on SLAB_TYPESAFE_BY_RCU to guarantee socket memory stability during RCU read-side critical sections. Both tcp_prot and tcpv6_prot have their slab caches created with this flag via proto_register(). However, MPTCP's mptcp_subflow_init() copies tcpv6_prot into tcpv6_prot_override during inet_init() (fs_initcall, level 5), before inet6_init() (module_init/device_initcall, level 6) has called proto_register(&tcpv6_prot). At that point, tcpv6_prot.slab is still NULL, so tcpv6_prot_override.slab remains NULL permanently. This causes MPTCP v6 subflow child sockets to be allocated via kmalloc (falling into kmalloc-4k) instead of the TCPv6 slab cache. The kmalloc-4k cache lacks SLAB_TYPESAFE_BY_RCU, so when these sockets are freed without SOCK_RCU_FREE (which is cleared for child sockets by design), the memory can be immediately reused. Concurrent ehash lookups under rcu_read_lock can then access freed memory, triggering a slab-use-after-free in __inet_lookup_established. Fix this by splitting the IPv6-specific initialization out of mptcp_subflow_init() into a new mptcp_subflow_v6_init(), called from mptcp_proto_v6_init() before protocol registration. This ensures tcpv6_prot_override.slab correctly inherits the SLAB_TYPESAFE_BY_RCU slab cache.",
    "evidence": "osv:CVE-2026-31669",
    "products": [
      {
        "name": {
          "value": "Kernel",
          "evidence": "/affected/1/package/name"
        },
        "prerequisites": [],
        "fixed_versions": [
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            "evidence": "/affected/1/ranges"
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            "value": "6.1.169",
            "evidence": "/affected/1/ranges"
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            "value": "6.6.135",
            "evidence": "/affected/1/ranges"
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          {
            "value": "6.12.82",
            "evidence": "/affected/1/ranges"
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            "value": "6.18.23",
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    "severity": "critical",
    "structured": {
      "id": "CVE-2026-31669",
      "aliases": [
        "CVE-2026-31669"
      ],
      "metrics": [
        {
          "type": "CVSS_V3",
          "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H"
        }
      ],
      "affected": [
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              "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
              "type": "GIT",
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              "type": "ECOSYSTEM",
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                  "introduced": "5.12.0"
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          "package": {
            "name": "Kernel",
            "ecosystem": "Linux"
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          "database_specific": {
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      ],
      "modified": "2026-10-28T02:30:28.781934885Z",
      "provider": "osv",
      "references": [
        "https://cert-portal.siemens.com/productcert/html/ssa-019113.html",
        "https://cert-portal.siemens.com/productcert/html/ssa-082556.html",
        "https://cert-portal.siemens.com/productcert/html/ssa-457936.html",
        "https://git.kernel.org/stable/c/15fa9ead4d5e6b6b9c794e84144146c917f2cb62",
        "https://git.kernel.org/stable/c/3fd6547f5b8ac99687be6d937a0321efda760597",
        "https://git.kernel.org/stable/c/9b55b253907e7431210483519c5ad711a37dafa1",
        "https://git.kernel.org/stable/c/b313e9037d98c13938740e5ebda7852929366dff",
        "https://git.kernel.org/stable/c/eb9c6aeb512f877cf397deb1e4526f646c70e4a7",
        "https://git.kernel.org/stable/c/f6e1f25fa5e733570f6d6fe37a4dfed2a0deba47",
        "https://git.kernel.org/stable/c/fb1f54b7d16f393b8b65d328410f78b4beea8fcc",
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/31xxx/CVE-2026-31669.json",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-31669",
        "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git"
      ],
      "provider_metadata": {
        "cna_assigner": "Linux",
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/31xxx/CVE-2026-31669.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-31669",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 495799,
  "published_at": "2026-04-24T14:45:17.295+00:00"
}