FKIE_CVE-2026-72139

Vulnerability from fkie_nvd - Published: 2026-08-15 06:21 - Updated: 2026-08-17 06:18
Severity
9.8 (Critical) - CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Summary
In the Linux kernel, the following vulnerability has been resolved: tcp: defer md5sig_info kfree past RCU grace period in tcp_connect The md5+ao reconciliation in tcp_connect() (net/ipv4/tcp_output.c) has two symmetric branches: if (needs_md5) { tcp_ao_destroy_sock(sk, false); } else if (needs_ao) { tcp_clear_md5_list(sk); kfree(rcu_replace_pointer(tp->md5sig_info, NULL, ...)); } Both branches free a per-socket auth-info object while the socket is in TCP_SYN_SENT and is already on the inet ehash (inserted by inet_hash_connect() in tcp_v4_connect()). Both branches are reachable by softirq RX-path readers that load the corresponding info pointer via implicit RCU before bh_lock_sock_nested() is taken. The needs_md5 branch is fixed in the prior patch by re-introducing the call_rcu() free in tcp_ao_destroy_sock(): the equivalent per-key loop runs inside tcp_ao_info_free_rcu(), the RCU callback, so by the time it frees each tcp_ao_key all softirq readers that captured the container have already completed rcu_read_unlock(). The needs_ao branch is not symmetric in the same way. The container free can be deferred via kfree_rcu(md5sig, rcu) -- struct tcp_md5sig_info already has the required rcu member (include/net/tcp.h:1999-2002), and the rest of the tree already does this in the tcp_md5sig_info_add() rollback paths (net/ipv4/tcp_ipv4.c:1410, 1436). But the per-key teardown is done by tcp_clear_md5_list() in process context BEFORE the container's RCU grace period: it walks &md5sig->head and frees each tcp_md5sig_key with bare hlist_del + kfree. A concurrent softirq reader in __tcp_md5_do_lookup() / __tcp_md5_do_lookup_exact() (tcp_ipv4.c:1253, 1298) walks the same list via hlist_for_each_entry_rcu() and races with that bare kfree on the keys themselves -- a per-key slab use-after-free of the same class as the TCP-AO bug, on the same race window. Fix this in two halves: 1. Convert the bare kfree() in tcp_connect() to kfree_rcu() so the md5sig_info container joins the rest of the md5sig lifecycle. The local-variable lift is mechanical and required because kfree_rcu() is a macro that expects an lvalue. 2. Make tcp_clear_md5_list() RCU-safe by replacing hlist_del + kfree(key) with hlist_del_rcu + kfree_rcu(key, rcu). struct tcp_md5sig_key already carries the rcu member (include/net/tcp.h:1995) and tcp_md5_do_del() (net/ipv4/tcp_ipv4.c:1456) already uses kfree_rcu, so this restores the lifecycle invariant the rest of the file follows rather than introducing a one-off. The other caller of tcp_clear_md5_list() is tcp_md5_destruct_sock() (net/ipv4/tcp.c:412), which runs from the sock destructor when the socket is already unhashed and unreachable; the extra grace period there is unnecessary but harmless. Making the helper unconditionally RCU-safe is the cleaner contract. The needs_ao branch is not reachable by the userns reproducer used to demonstrate the AO-side splat (the repro installs both keys but ends up in the needs_md5 branch because the connect peer matches the MD5 key, not the AO key); however the symmetric race exists and a maintainer touching this code should not have to think about which branch escapes RCU and which one does not. [also credits to Qihang, who found that this races with tcp-diag]
References
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/33a1bee413628378fd036a4f2b17ba86b0bc560c
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/b74cd55038905d5e74c1de109ab78a30b2ea0e1f
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/da48b9bf1eb95a9cfd09d615ca58cfc2b03de369
Impacted products
Vendor Product Version
To clipboard 

{
  "affected": [
    {
      "affectedData": [
        {
          "defaultStatus": "unaffected",
          "product": "Linux",
          "programFiles": [
            "net/ipv4/tcp_ipv4.c",
            "net/ipv4/tcp_output.c"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
            {
              "lessThan": "33a1bee413628378fd036a4f2b17ba86b0bc560c",
              "status": "affected",
              "version": "51e547e8c89c661f6fbede4a28b1d33b13625683",
              "versionType": "git"
            },
            {
              "lessThan": "da48b9bf1eb95a9cfd09d615ca58cfc2b03de369",
              "status": "affected",
              "version": "51e547e8c89c661f6fbede4a28b1d33b13625683",
              "versionType": "git"
            },
            {
              "lessThan": "b74cd55038905d5e74c1de109ab78a30b2ea0e1f",
              "status": "affected",
              "version": "51e547e8c89c661f6fbede4a28b1d33b13625683",
              "versionType": "git"
            }
          ]
        },
        {
          "defaultStatus": "affected",
          "product": "Linux",
          "programFiles": [
            "net/ipv4/tcp_ipv4.c",
            "net/ipv4/tcp_output.c"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
            {
              "status": "affected",
              "version": "6.18"
            },
            {
              "lessThan": "6.18",
              "status": "unaffected",
              "version": "0",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "6.18.*",
              "status": "unaffected",
              "version": "6.18.40",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "7.1.*",
              "status": "unaffected",
              "version": "7.1.5",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "*",
              "status": "unaffected",
              "version": "7.2",
              "versionType": "original_commit_for_fix"
            }
          ]
        }
      ],
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"
    }
  ],
  "cveTags": [],
  "descriptions": [
    {
      "lang": "en",
      "value": "In the Linux kernel, the following vulnerability has been resolved:\n\ntcp: defer md5sig_info kfree past RCU grace period in tcp_connect\n\nThe md5+ao reconciliation in tcp_connect() (net/ipv4/tcp_output.c)\nhas two symmetric branches:\n\n\tif (needs_md5) {\n\t\ttcp_ao_destroy_sock(sk, false);\n\t} else if (needs_ao) {\n\t\ttcp_clear_md5_list(sk);\n\t\tkfree(rcu_replace_pointer(tp-\u003emd5sig_info, NULL, ...));\n\t}\n\nBoth branches free a per-socket auth-info object while the socket is\nin TCP_SYN_SENT and is already on the inet ehash (inserted by\ninet_hash_connect() in tcp_v4_connect()). Both branches are reachable\nby softirq RX-path readers that load the corresponding info pointer\nvia implicit RCU before bh_lock_sock_nested() is taken.\n\nThe needs_md5 branch is fixed in the prior patch by re-introducing\nthe call_rcu() free in tcp_ao_destroy_sock(): the equivalent per-key\nloop runs inside tcp_ao_info_free_rcu(), the RCU callback, so by the\ntime it frees each tcp_ao_key all softirq readers that captured the\ncontainer have already completed rcu_read_unlock().\n\nThe needs_ao branch is not symmetric in the same way. The container\nfree can be deferred via kfree_rcu(md5sig, rcu) -- struct\ntcp_md5sig_info already has the required rcu member\n(include/net/tcp.h:1999-2002), and the rest of the tree already does\nthis in the tcp_md5sig_info_add() rollback paths\n(net/ipv4/tcp_ipv4.c:1410, 1436). But the per-key teardown is done\nby tcp_clear_md5_list() in process context BEFORE the container\u0027s\nRCU grace period: it walks \u0026md5sig-\u003ehead and frees each\ntcp_md5sig_key with bare hlist_del + kfree. A concurrent softirq\nreader in __tcp_md5_do_lookup() / __tcp_md5_do_lookup_exact()\n(tcp_ipv4.c:1253, 1298) walks the same list via\nhlist_for_each_entry_rcu() and races with that bare kfree on the\nkeys themselves -- a per-key slab use-after-free of the same class\nas the TCP-AO bug, on the same race window.\n\nFix this in two halves:\n\n  1. Convert the bare kfree() in tcp_connect() to kfree_rcu() so the\n     md5sig_info container joins the rest of the md5sig lifecycle.\n     The local-variable lift is mechanical and required because\n     kfree_rcu() is a macro that expects an lvalue.\n\n  2. Make tcp_clear_md5_list() RCU-safe by replacing hlist_del +\n     kfree(key) with hlist_del_rcu + kfree_rcu(key, rcu). struct\n     tcp_md5sig_key already carries the rcu member\n     (include/net/tcp.h:1995) and tcp_md5_do_del()\n     (net/ipv4/tcp_ipv4.c:1456) already uses kfree_rcu, so this\n     restores the lifecycle invariant the rest of the file follows\n     rather than introducing a one-off.\n\nThe other caller of tcp_clear_md5_list() is tcp_md5_destruct_sock()\n(net/ipv4/tcp.c:412), which runs from the sock destructor when the\nsocket is already unhashed and unreachable; the extra grace period\nthere is unnecessary but harmless. Making the helper unconditionally\nRCU-safe is the cleaner contract.\n\nThe needs_ao branch is not reachable by the userns reproducer used\nto demonstrate the AO-side splat (the repro installs both keys but\nends up in the needs_md5 branch because the connect peer matches\nthe MD5 key, not the AO key); however the symmetric race exists\nand a maintainer touching this code should not have to think about\nwhich branch escapes RCU and which one does not.\n\n[also credits to Qihang, who found that this races with tcp-diag]"
    }
  ],
  "id": "CVE-2026-72139",
  "lastModified": "2026-08-17T06:18:13.823",
  "metrics": {
    "cvssMetricV31": [
      {
        "cvssData": {
          "attackComplexity": "LOW",
          "attackVector": "NETWORK",
          "availabilityImpact": "HIGH",
          "baseScore": 9.8,
          "baseSeverity": "CRITICAL",
          "confidentialityImpact": "HIGH",
          "integrityImpact": "HIGH",
          "privilegesRequired": "NONE",
          "scope": "UNCHANGED",
          "userInteraction": "NONE",
          "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H",
          "version": "3.1"
        },
        "exploitabilityScore": 3.9,
        "impactScore": 5.9,
        "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "type": "Secondary"
      }
    ]
  },
  "published": "2026-08-15T06:21:31.460",
  "references": [
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/33a1bee413628378fd036a4f2b17ba86b0bc560c"
    },
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/b74cd55038905d5e74c1de109ab78a30b2ea0e1f"
    },
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/da48b9bf1eb95a9cfd09d615ca58cfc2b03de369"
    }
  ],
  "sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
  "vulnStatus": "Received"
}



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