CVE-2026-98811 (GCVE-0-2026-98811)
Vulnerability from cvelistv5
Published
2026-10-18 15:48
Modified
2026-10-30 12:39
Severity ?
Summary
In the Linux kernel, the following vulnerability has been resolved: net/sched: cls_api: reclaim an empty proto on the error path Two racing tc filter add requests on the same chain/prio of an unlocked classifier both run change() on the shared proto and both can fail: the winner's tcf_chain_tp_delete_empty() attempt gives up because the loser's handle is still in the idr, and the loser's error path drops only its own reference without a second reclamation attempt. The empty proto stays linked in the chain, holding the chain reference, a block reference and the classifier module reference until the chain or block is torn down. Reclaim the proto on the error path of any failed request that holds a proto reference. The reclamation is emptiness-gated: tcf_chain_tp_delete_empty() unlinks the proto only when delete_empty() admits it is empty, so a live shared proto is never unlinked. A proto the request created is reclaimed unconditionally - it is the only owner, so marking it for deletion is safe even without a delete_empty callback. Classifiers without one (the check marks the proto unconditionally) are rtnl-serialized, so the raced window this guard closes cannot arise for them. This is a follow-up to commit d4e359b3608a ("net/sched: cls_api: fix teardown of an adopted proto on insert-race loss"), which stopped the loser of the insert race from unlinking the winner's live proto but left the empty-proto residual in place. Conditions to recreate: - CONFIG_NET_CLS_FLOWER=y; veth pair - tc qdisc add dev veth0 ingress - two concurrent `tc filter add dev veth0 ingress protocol ip pref 1 flower skip_sw ... action drop` (both fail in fl_hw_replace_filter after publishing their handle in the idr); repeat in a loop - an empty flower tp stays linked after both requests fail; visible as a bare `filter protocol ip pref 1 flower chain 0` header in `tc filter show` with no filter entries - CAP_NET_ADMIN (namespace-local via unshare -Urn suffices)
References
Impacted products
Vendor Product Version
Linux Linux Version: 8b64678e0af8f4d62a40149baedebe78503a5255
Version: 8b64678e0af8f4d62a40149baedebe78503a5255
Version: 8b64678e0af8f4d62a40149baedebe78503a5255
Version: 8b64678e0af8f4d62a40149baedebe78503a5255
Version: 8b64678e0af8f4d62a40149baedebe78503a5255
Version: 8b64678e0af8f4d62a40149baedebe78503a5255
Version: 8b64678e0af8f4d62a40149baedebe78503a5255
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{
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      "descriptions": [
        {
          "lang": "en",
          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nnet/sched: cls_api: reclaim an empty proto on the error path\n\nTwo racing tc filter add requests on the same chain/prio of an\nunlocked classifier both run change() on the shared proto and both\ncan fail: the winner\u0027s tcf_chain_tp_delete_empty() attempt gives up\nbecause the loser\u0027s handle is still in the idr, and the loser\u0027s\nerror path drops only its own reference without a second reclamation\nattempt. The empty proto stays linked in the chain, holding the\nchain reference, a block reference and the classifier module\nreference until the chain or block is torn down.\n\nReclaim the proto on the error path of any failed request that holds\na proto reference. The reclamation is emptiness-gated:\ntcf_chain_tp_delete_empty() unlinks the proto only when\ndelete_empty() admits it is empty, so a live shared proto is never\nunlinked. A proto the request created is reclaimed unconditionally -\nit is the only owner, so marking it for deletion is safe even without\na delete_empty callback. Classifiers without one (the check marks the\nproto unconditionally) are rtnl-serialized, so the raced window this\nguard closes cannot arise for them.\n\nThis is a follow-up to commit d4e359b3608a (\"net/sched: cls_api: fix\nteardown of an adopted proto on insert-race loss\"), which stopped the\nloser of the insert race from unlinking the winner\u0027s live proto but\nleft the empty-proto residual in place.\n\nConditions to recreate:\n- CONFIG_NET_CLS_FLOWER=y; veth pair\n- tc qdisc add dev veth0 ingress\n- two concurrent `tc filter add dev veth0 ingress protocol ip pref 1\n  flower skip_sw ... action drop` (both fail in fl_hw_replace_filter\n  after publishing their handle in the idr); repeat in a loop\n- an empty flower tp stays linked after both requests fail; visible\n  as a bare `filter protocol ip pref 1 flower chain 0` header in\n  `tc filter show` with no filter entries\n- CAP_NET_ADMIN (namespace-local via unshare -Urn suffices)"
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-10-30T12:39:57.018Z",
        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
      },
      "references": [
        {
          "url": "https://git.kernel.org/stable/c/0f005c9b8b2e4c018e598cbaf490f27bebfd3d1f"
        },
        {
          "url": "https://git.kernel.org/stable/c/00ad96ee96760d2f12f8c4b3a43f6c6b742c452f"
        },
        {
          "url": "https://git.kernel.org/stable/c/0fc74e7d97d232095ce64cb45d25f0919df7ad4a"
        },
        {
          "url": "https://git.kernel.org/stable/c/315e4fe65cf02ef052de5e6298c3088e448b7517"
        },
        {
          "url": "https://git.kernel.org/stable/c/ebc9611ab154c0cd9f1320e2a73f5e936d7535a9"
        },
        {
          "url": "https://git.kernel.org/stable/c/570141625d212fbbcffb9665cb45c0f04610e61a"
        },
        {
          "url": "https://git.kernel.org/stable/c/0bb8eab29b55045281a4963d4557f2776cdf8cfa"
        }
      ],
      "title": "net/sched: cls_api: reclaim an empty proto on the error path",
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  "cveMetadata": {
    "assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
    "assignerShortName": "Linux",
    "cveId": "CVE-2026-98811",
    "datePublished": "2026-10-18T15:48:57.588Z",
    "dateReserved": "2026-09-25T10:25:14.371Z",
    "dateUpdated": "2026-10-30T12:39:57.018Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2"
}


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