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CVE-2026-23016 (GCVE-0-2026-23016)

Vulnerability from cvelistv5 – Published: 2026-01-31 11:38 – Updated: 2026-05-11 21:58
VLAI
Title
inet: frags: drop fraglist conntrack references
Summary
In the Linux kernel, the following vulnerability has been resolved: inet: frags: drop fraglist conntrack references Jakub added a warning in nf_conntrack_cleanup_net_list() to make debugging leaked skbs/conntrack references more obvious. syzbot reports this as triggering, and I can also reproduce this via ip_defrag.sh selftest: conntrack cleanup blocked for 60s WARNING: net/netfilter/nf_conntrack_core.c:2512 [..] conntrack clenups gets stuck because there are skbs with still hold nf_conn references via their frag_list. net.core.skb_defer_max=0 makes the hang disappear. Eric Dumazet points out that skb_release_head_state() doesn't follow the fraglist. ip_defrag.sh can only reproduce this problem since commit 6471658dc66c ("udp: use skb_attempt_defer_free()"), but AFAICS this problem could happen with TCP as well if pmtu discovery is off. The relevant problem path for udp is: 1. netns emits fragmented packets 2. nf_defrag_v6_hook reassembles them (in output hook) 3. reassembled skb is tracked (skb owns nf_conn reference) 4. ip6_output refragments 5. refragmented packets also own nf_conn reference (ip6_fragment calls ip6_copy_metadata()) 6. on input path, nf_defrag_v6_hook skips defragmentation: the fragments already have skb->nf_conn attached 7. skbs are reassembled via ipv6_frag_rcv() 8. skb_consume_udp -> skb_attempt_defer_free() -> skb ends up in pcpu freelist, but still has nf_conn reference. Possible solutions: 1 let defrag engine drop nf_conn entry, OR 2 export kick_defer_list_purge() and call it from the conntrack netns exit callback, OR 3 add skb_has_frag_list() check to skb_attempt_defer_free() 2 & 3 also solve ip_defrag.sh hang but share same drawback: Such reassembled skbs, queued to socket, can prevent conntrack module removal until userspace has consumed the packet. While both tcp and udp stack do call nf_reset_ct() before placing skb on socket queue, that function doesn't iterate frag_list skbs. Therefore drop nf_conn entries when they are placed in defrag queue. Keep the nf_conn entry of the first (offset 0) skb so that reassembled skb retains nf_conn entry for sake of TX path. Note that fixes tag is incorrect; it points to the commit introducing the 'ip_defrag.sh reproducible problem': no need to backport this patch to every stable kernel.
Severity
5.5 (Medium)
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
Assigner
References
Impacted products
Vendor Product Version CPE status
Linux Linux Affected: 6471658dc66c670580a7616e75f51b52917e7883 , < 088ca99dbb039c444c3ff987c5412a73f4f0cbf8 (git)
Affected: 6471658dc66c670580a7616e75f51b52917e7883 , < 2ef02ac38d3c17f34a00c4b267d961a8d4b45d1a (git)
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Linux Linux Affected: 6.18
Unaffected: 0 , < 6.18 (semver)
Unaffected: 6.18.6 , ≤ 6.18.* (semver)
Unaffected: 6.19 , ≤ * (original_commit_for_fix)
guessed Create a notification for this product.
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          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\ninet: frags: drop fraglist conntrack references\n\nJakub added a warning in nf_conntrack_cleanup_net_list() to make debugging\nleaked skbs/conntrack references more obvious.\n\nsyzbot reports this as triggering, and I can also reproduce this via\nip_defrag.sh selftest:\n\n conntrack cleanup blocked for 60s\n WARNING: net/netfilter/nf_conntrack_core.c:2512\n [..]\n\nconntrack clenups gets stuck because there are skbs with still hold nf_conn\nreferences via their frag_list.\n\n   net.core.skb_defer_max=0 makes the hang disappear.\n\nEric Dumazet points out that skb_release_head_state() doesn\u0027t follow the\nfraglist.\n\nip_defrag.sh can only reproduce this problem since\ncommit 6471658dc66c (\"udp: use skb_attempt_defer_free()\"), but AFAICS this\nproblem could happen with TCP as well if pmtu discovery is off.\n\nThe relevant problem path for udp is:\n1. netns emits fragmented packets\n2. nf_defrag_v6_hook reassembles them (in output hook)\n3. reassembled skb is tracked (skb owns nf_conn reference)\n4. ip6_output refragments\n5. refragmented packets also own nf_conn reference (ip6_fragment\n   calls ip6_copy_metadata())\n6. on input path, nf_defrag_v6_hook skips defragmentation: the\n   fragments already have skb-\u003enf_conn attached\n7. skbs are reassembled via ipv6_frag_rcv()\n8. skb_consume_udp -\u003e skb_attempt_defer_free() -\u003e skb ends up\n   in pcpu freelist, but still has nf_conn reference.\n\nPossible solutions:\n 1 let defrag engine drop nf_conn entry, OR\n 2 export kick_defer_list_purge() and call it from the conntrack\n   netns exit callback, OR\n 3 add skb_has_frag_list() check to skb_attempt_defer_free()\n\n2 \u0026 3 also solve ip_defrag.sh hang but share same drawback:\n\nSuch reassembled skbs, queued to socket, can prevent conntrack module\nremoval until userspace has consumed the packet. While both tcp and udp\nstack do call nf_reset_ct() before placing skb on socket queue, that\nfunction doesn\u0027t iterate frag_list skbs.\n\nTherefore drop nf_conn entries when they are placed in defrag queue.\nKeep the nf_conn entry of the first (offset 0) skb so that reassembled\nskb retains nf_conn entry for sake of TX path.\n\nNote that fixes tag is incorrect; it points to the commit introducing the\n\u0027ip_defrag.sh reproducible problem\u0027: no need to backport this patch to\nevery stable kernel."
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    "datePublished": "2026-01-31T11:38:59.578Z",
    "dateReserved": "2026-01-13T15:37:45.940Z",
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    "nvd": "{\"cve\":{\"id\":\"CVE-2026-23016\",\"sourceIdentifier\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\",\"published\":\"2026-01-31T12:16:04.900\",\"lastModified\":\"2026-06-17T10:20:42.990\",\"vulnStatus\":\"Analyzed\",\"cveTags\":[],\"descriptions\":[{\"lang\":\"en\",\"value\":\"In the Linux kernel, the following vulnerability has been resolved:\\n\\ninet: frags: drop fraglist conntrack references\\n\\nJakub added a warning in nf_conntrack_cleanup_net_list() to make debugging\\nleaked skbs/conntrack references more obvious.\\n\\nsyzbot reports this as triggering, and I can also reproduce this via\\nip_defrag.sh selftest:\\n\\n conntrack cleanup blocked for 60s\\n WARNING: net/netfilter/nf_conntrack_core.c:2512\\n [..]\\n\\nconntrack clenups gets stuck because there are skbs with still hold nf_conn\\nreferences via their frag_list.\\n\\n   net.core.skb_defer_max=0 makes the hang disappear.\\n\\nEric Dumazet points out that skb_release_head_state() doesn\u0027t follow the\\nfraglist.\\n\\nip_defrag.sh can only reproduce this problem since\\ncommit 6471658dc66c (\\\"udp: use skb_attempt_defer_free()\\\"), but AFAICS this\\nproblem could happen with TCP as well if pmtu discovery is off.\\n\\nThe relevant problem path for udp is:\\n1. netns emits fragmented packets\\n2. nf_defrag_v6_hook reassembles them (in output hook)\\n3. reassembled skb is tracked (skb owns nf_conn reference)\\n4. ip6_output refragments\\n5. refragmented packets also own nf_conn reference (ip6_fragment\\n   calls ip6_copy_metadata())\\n6. on input path, nf_defrag_v6_hook skips defragmentation: the\\n   fragments already have skb-\u003enf_conn attached\\n7. skbs are reassembled via ipv6_frag_rcv()\\n8. skb_consume_udp -\u003e skb_attempt_defer_free() -\u003e skb ends up\\n   in pcpu freelist, but still has nf_conn reference.\\n\\nPossible solutions:\\n 1 let defrag engine drop nf_conn entry, OR\\n 2 export kick_defer_list_purge() and call it from the conntrack\\n   netns exit callback, OR\\n 3 add skb_has_frag_list() check to skb_attempt_defer_free()\\n\\n2 \u0026 3 also solve ip_defrag.sh hang but share same drawback:\\n\\nSuch reassembled skbs, queued to socket, can prevent conntrack module\\nremoval until userspace has consumed the packet. While both tcp and udp\\nstack do call nf_reset_ct() before placing skb on socket queue, that\\nfunction doesn\u0027t iterate frag_list skbs.\\n\\nTherefore drop nf_conn entries when they are placed in defrag queue.\\nKeep the nf_conn entry of the first (offset 0) skb so that reassembled\\nskb retains nf_conn entry for sake of TX path.\\n\\nNote that fixes tag is incorrect; it points to the commit introducing the\\n\u0027ip_defrag.sh reproducible problem\u0027: no need to backport this patch to\\nevery stable kernel.\"},{\"lang\":\"es\",\"value\":\"En el kernel de Linux, la siguiente vulnerabilidad ha sido resuelta:\\n\\ninet: frags: eliminar referencias de conntrack de fraglist\\n\\nJakub a\u00f1adi\u00f3 una advertencia en nf_conntrack_cleanup_net_list() para hacer m\u00e1s obvia la depuraci\u00f3n de skbs/referencias de conntrack filtradas.\\n\\nsyzbot informa que esto se activa, y tambi\u00e9n puedo reproducirlo a trav\u00e9s de la autoprueba ip_defrag.sh:\\n\\nlimpieza de conntrack bloqueada durante 60s\\nADVERTENCIA: net/netfilter/nf_conntrack_core.c:2512\\n[..]\\n\\nLas limpiezas de conntrack se atascan porque hay skbs que a\u00fan mantienen referencias nf_conn a trav\u00e9s de su frag_list.\\n\\nnet.core.skb_defer_max=0 hace que el cuelgue desaparezca.\\n\\nEric Dumazet se\u00f1ala que skb_release_head_state() no sigue la fraglist.\\n\\nip_defrag.sh solo puede reproducir este problema desde el commit 6471658dc66c (\u0027udp: use skb_attempt_defer_free()\u0027), pero AFAICS este problema tambi\u00e9n podr\u00eda ocurrir con TCP si el descubrimiento de pmtu est\u00e1 desactivado.\\n\\nLa ruta del problema relevante para udp es:\\n1. netns emite paquetes fragmentados\\n2. nf_defrag_v6_hook los reensambla (en el hook de salida)\\n3. el skb reensamblado es rastreado (skb posee la referencia nf_conn)\\n4. ip6_output refragmenta\\n5. los paquetes refragmentados tambi\u00e9n poseen la referencia nf_conn (ip6_fragment llama a ip6_copy_metadata())\\n6. en la ruta de entrada, nf_defrag_v6_hook omite la desfragmentaci\u00f3n: los fragmentos ya tienen skb-\u0026gt;nf_conn adjunto\\n7. los skbs son reensamblados a trav\u00e9s de ipv6_frag_rcv()\\n8. skb_consume_udp -\u0026gt; skb_attempt_defer_free() -\u0026gt; skb termina en la freelist de pcpu, pero a\u00fan tiene la referencia nf_conn.\\n\\nPosibles soluciones:\\n1 dejar que el motor de desfragmentaci\u00f3n elimine la entrada nf_conn, O\\n2 exportar kick_defer_list_purge() y llamarlo desde la devoluci\u00f3n de llamada de salida de netns de conntrack, O\\n3 a\u00f1adir una comprobaci\u00f3n skb_has_frag_list() a skb_attempt_defer_free()\\n\\n2 y 3 tambi\u00e9n resuelven el cuelgue de ip_defrag.sh pero comparten el mismo inconveniente:\\n\\nTales skbs reensamblados, encolados al socket, pueden impedir la eliminaci\u00f3n del m\u00f3dulo conntrack hasta que el espacio de usuario haya consumido el paquete. 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    "redhat_vex": {
      "aggregate_severity": "Moderate",
      "current_release_date": "2026-06-28T07:18:09+00:00",
      "cve": "CVE-2026-23016",
      "id": "CVE-2026-23016",
      "initial_release_date": "2026-01-31T00:00:00+00:00",
      "product_status:known_not_affected": "274",
      "source": "Red Hat CSAF VEX",
      "status": "final",
      "title": "kernel: Linux kernel: Denial of Service due to improper network connection tracking reference handling",
      "url": "https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-23016.json",
      "version": "3"
    },
    "suse_vex": {
      "aggregate_severity": "important",
      "current_release_date": "2026-05-13T03:47:09Z",
      "cve": "CVE-2026-23016",
      "id": "CVE-2026-23016",
      "initial_release_date": "2026-02-01T00:24:03Z",
      "product_status:known_not_affected": "261",
      "source": "SUSE CSAF VEX",
      "status": "interim",
      "title": "SUSE CVE CVE-2026-23016",
      "url": "https://ftp.suse.com/pub/projects/security/csaf-vex/cve-2026-23016.json",
      "version": "7"
    }
  }
}



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