GHSA-QM4P-6QR9-F3W6

Vulnerability from github – Published: 2026-08-21 18:34 – Updated: 2026-08-25 06:31
VLAI
Details

In the Linux kernel, the following vulnerability has been resolved:

net/sched: cls_route: fix fastmap use-after-free on filter

The route4 classifier maintains a 16-slot fastmap cache that stores raw struct route4_filter pointers indexed by (id, iif). The reader (route4_classify) populates this cache via route4_set_fastmap() for every classified packet that hits a filter. The writer (route4_delete, route4_change) clears the cache via route4_reset_fastmap() before RCU-deferred kfree of the filter.

This creates a UAF race: 1. Reader walks the RCU-protected bucket chain, finds filter f 2. Writer unlinks f, calls route4_reset_fastmap(), then tcf_queue_work() 3. Reader calls route4_set_fastmap() and writes f into the cache after the writer's reset, caching a pointer about to be freed 4. After the RCU grace period, kfree(f) executes 5. Next classified packet on the same (id, iif) tuple hits the stale fastmap entry and reads f->res from freed memory

Reproduced with an mdelay(100) accelerator in route4_set_fastmap() and a concurrent add/delete stress test (provided by both zdi and Santosh). Both triggered KASAN slab-use-after-free reports in the route4 fastmap paths.

Fix: Introduce a per-filter boolean dying flag to suppress stale fastmap republishing by in-flight readers.

Severity
7.8 (High)
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
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{
  "affected": [],
  "aliases": [
    "CVE-2026-74583"
  ],
  "database_specific": {
    "cwe_ids": [],
    "github_reviewed": false,
    "github_reviewed_at": null,
    "nvd_published_at": "2026-08-21T17:16:44Z",
    "severity": "HIGH"
  },
  "details": "In the Linux kernel, the following vulnerability has been resolved:\n\nnet/sched: cls_route: fix fastmap use-after-free on filter\n\nThe route4 classifier maintains a 16-slot fastmap cache that stores raw\nstruct route4_filter pointers indexed by (id, iif). The reader\n(route4_classify) populates this cache via route4_set_fastmap() for every\nclassified packet that hits a filter. The writer (route4_delete,\nroute4_change) clears the cache via route4_reset_fastmap() before\nRCU-deferred kfree of the filter.\n\nThis creates a UAF race:\n 1. Reader walks the RCU-protected bucket chain, finds filter f\n 2. Writer unlinks f, calls route4_reset_fastmap(), then tcf_queue_work()\n 3. Reader calls route4_set_fastmap() and writes f into the cache\n    *after* the writer\u0027s reset, caching a pointer about to be freed\n 4. After the RCU grace period, kfree(f) executes\n 5. Next classified packet on the same (id, iif) tuple hits the stale\n    fastmap entry and reads f-\u003eres from freed memory\n\nReproduced with an mdelay(100) accelerator in route4_set_fastmap() and a\nconcurrent add/delete stress test (provided by both zdi and Santosh).\nBoth triggered KASAN slab-use-after-free reports in the route4 fastmap\npaths.\n\nFix:\nIntroduce a per-filter boolean dying flag to suppress stale fastmap\nrepublishing by in-flight readers.",
  "id": "GHSA-qm4p-6qr9-f3w6",
  "modified": "2026-08-25T06:31:21Z",
  "published": "2026-08-21T18:34:58Z",
  "references": [
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-74583"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/0e7a8cf8895b06d07c7311f028eba16ad742b9bc"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/47d7f7051253bdc02b1d245d87e38f16d31a74df"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/5ec9001be6d0eb527251125632ec8fe88278897f"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/7897198b26445b4009a057bda1986b94a99e5d5f"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/820f083c294ad6d319c02a7d43294f2ed2565139"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/a17f636c9330eac879822ce29f998e5abd1b72c1"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/ae9aff87025219005a2d16b4fe83d6f24643e50d"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/b969984b2bdc85d721ce4047cd270cd37ec705a2"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H",
      "type": "CVSS_V3"
    }
  ]
}



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