CVE-2026-98104 (GCVE-0-2026-98104)
Vulnerability from cvelistv5
Published
2026-09-25 10:35
Modified
2026-10-19 09:13
Severity ?
Summary
In the Linux kernel, the following vulnerability has been resolved: net/sched: cls_u32: fix duplicate handle when node ID pool is exhausted gen_new_kid() falls back to returning max (htid | 0xFFF) when both idr_alloc_u32() ranges are full, instead of reporting an error. u32_change() trusts that value and inserts a new knode with a handle that is already live in the hash table, breaking handle uniqueness within the table's node ID space. The handle was never reserved in ht->handle_idr, so every later error path that does idr_remove(&ht->handle_idr, handle) removes the reservation of a different, live knode, which is then reused — one failed add compounds into further duplicates. The 4095 limit is per (table, bucket) — ht->handle_idr is per hash table and the range is derived from htid (bucketid), so a table with divisor 256 can legitimately hold 256*4095 knodes. The sibling helper gen_new_htid() has the same silent in-band failure: it returns 0 when the tp_c handle pool (1..0x7FF) is full, and u32_init() publishes the root hash table with handle 0 without checking. Two root tables with handle 0 alias in u32_lookup_ht(), allowing cross-tcf_proto knode add/lookup/delete. Add the same exhaustion check that the divisor path already has. Return an error so u32_change() fails with ENOSPC/ENOMEM when the node ID space is exhausted, and so u32_init() fails with -ENOMEM when the hash table ID space is exhausted. The extack message distinguishes pool exhaustion (-ENOSPC) from a transient allocation failure (-ENOMEM). Conditions to recreate the bug: - CONFIG_NET_SCHED=y, CONFIG_CLS_U32=y (or =m with module loaded) - Create a clsact qdisc on a device, then add 4095 u32 filters with auto-generated handles to fill the node ID space for the root hash table (single bucket). The 4096th auto-handle filter add triggers the duplicate handle (fh 800::fff reused). Reachable at Level 2 (unshare -Urn, namespace-local CAP_NET_ADMIN). - For gen_new_htid: create 2047 u32 proto entries on the same block to fill the tp_c handle pool, then create one more. The root table gets handle 0 and aliases with other handle-0 root tables.
References
Impacted products
Vendor Product Version
Linux Linux Version: 7801db8aec957fa6610efe0ee26a6c8bc0f1d73b
Version: 7801db8aec957fa6610efe0ee26a6c8bc0f1d73b
Version: 7801db8aec957fa6610efe0ee26a6c8bc0f1d73b
Version: 7801db8aec957fa6610efe0ee26a6c8bc0f1d73b
Version: 7801db8aec957fa6610efe0ee26a6c8bc0f1d73b
Version: 7801db8aec957fa6610efe0ee26a6c8bc0f1d73b
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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_u32: fix duplicate handle when node ID pool is exhausted\n\ngen_new_kid() falls back to returning max (htid | 0xFFF) when both\nidr_alloc_u32() ranges are full, instead of reporting an error.\nu32_change() trusts that value and inserts a new knode with a handle\nthat is already live in the hash table, breaking handle uniqueness\nwithin the table\u0027s node ID space.\n\nThe handle was never reserved in ht-\u003ehandle_idr, so every later error\npath that does idr_remove(\u0026ht-\u003ehandle_idr, handle) removes the\nreservation of a different, live knode, which is then reused \u2014 one\nfailed add compounds into further duplicates.\n\nThe 4095 limit is per (table, bucket) \u2014 ht-\u003ehandle_idr is per hash\ntable and the range is derived from htid (bucketid), so a table with\ndivisor 256 can legitimately hold 256*4095 knodes.\n\nThe sibling helper gen_new_htid() has the same silent in-band failure:\nit returns 0 when the tp_c handle pool (1..0x7FF) is full, and\nu32_init() publishes the root hash table with handle 0 without\nchecking.  Two root tables with handle 0 alias in u32_lookup_ht(),\nallowing cross-tcf_proto knode add/lookup/delete.  Add the same\nexhaustion check that the divisor path already has.\n\nReturn an error so u32_change() fails with ENOSPC/ENOMEM when the\nnode ID space is exhausted, and so u32_init() fails with -ENOMEM\nwhen the hash table ID space is exhausted.  The extack message\ndistinguishes pool exhaustion (-ENOSPC) from a transient allocation\nfailure (-ENOMEM).\n\nConditions to recreate the bug:\n- CONFIG_NET_SCHED=y, CONFIG_CLS_U32=y (or =m with module loaded)\n- Create a clsact qdisc on a device, then add 4095 u32 filters with\n  auto-generated handles to fill the node ID space for the root hash\n  table (single bucket). The 4096th auto-handle filter add triggers\n  the duplicate handle (fh 800::fff reused). Reachable at Level 2\n  (unshare -Urn, namespace-local CAP_NET_ADMIN).\n- For gen_new_htid: create 2047 u32 proto entries on the same block\n  to fill the tp_c handle pool, then create one more. The root table\n  gets handle 0 and aliases with other handle-0 root tables."
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-10-19T09:13:56.808Z",
        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
      },
      "references": [
        {
          "url": "https://git.kernel.org/stable/c/f4ee13ed7f7b22dcd8287fcf477ae37bd84567da"
        },
        {
          "url": "https://git.kernel.org/stable/c/84223baf21bc9b008444ae9bc38d5fb91fe00b9d"
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        {
          "url": "https://git.kernel.org/stable/c/6890e28840bae4f6805e8de981c4ec8e12a4e064"
        },
        {
          "url": "https://git.kernel.org/stable/c/feab9261b537df4ebb8350e4779bc185373059fd"
        },
        {
          "url": "https://git.kernel.org/stable/c/f594f04268d01c5fdc975f3f51fc219ea2159ac6"
        },
        {
          "url": "https://git.kernel.org/stable/c/d7e7e98d23f42a92d9ab7e36302bd96bd9b33b5f"
        }
      ],
      "title": "net/sched: cls_u32: fix duplicate handle when node ID pool is exhausted",
      "x_generator": {
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  "cveMetadata": {
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    "assignerShortName": "Linux",
    "cveId": "CVE-2026-98104",
    "datePublished": "2026-09-25T10:35:54.544Z",
    "dateReserved": "2026-09-25T10:25:14.315Z",
    "dateUpdated": "2026-10-19T09:13:56.808Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2"
}


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