FKIE_CVE-2026-43121

Vulnerability from fkie_nvd - Published: 2026-05-06 12:16 - Updated: 2026-05-12 21:17
Severity ?
4.7 (Medium) - CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
Summary
In the Linux kernel, the following vulnerability has been resolved: io_uring/zcrx: fix user_ref race between scrub and refill paths The io_zcrx_put_niov_uref() function uses a non-atomic check-then-decrement pattern (atomic_read followed by separate atomic_dec) to manipulate user_refs. This is serialized against other callers by rq_lock, but io_zcrx_scrub() modifies the same counter with atomic_xchg() WITHOUT holding rq_lock. On SMP systems, the following race exists: CPU0 (refill, holds rq_lock) CPU1 (scrub, no rq_lock) put_niov_uref: atomic_read(uref) - 1 // window opens atomic_xchg(uref, 0) - 1 return_niov_freelist(niov) [PUSH #1] // window closes atomic_dec(uref) - wraps to -1 returns true return_niov(niov) return_niov_freelist(niov) [PUSH #2: DOUBLE-FREE] The same niov is pushed to the freelist twice, causing free_count to exceed nr_iovs. Subsequent freelist pushes then perform an out-of-bounds write (a u32 value) past the kvmalloc'd freelist array into the adjacent slab object. Fix this by replacing the non-atomic read-then-dec in io_zcrx_put_niov_uref() with an atomic_try_cmpxchg loop that atomically tests and decrements user_refs. This makes the operation safe against concurrent atomic_xchg from scrub without requiring scrub to acquire rq_lock. [pavel: removed a warning and a comment]
References
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/003049b1c4fb8aabb93febb7d1e49004f6ad653bPatch
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/485dc691257b96e6d3bdc25b0eff2daadcc5c46cPatch
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/a94f096e28bfc7975163a6b80f1c8f323efe317aPatch
Impacted products
Vendor Product Version
linux linux_kernel *
linux linux_kernel *
To clipboard 

{
  "configurations": [
    {
      "nodes": [
        {
          "cpeMatch": [
            {
              "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
              "matchCriteriaId": "5B5F989F-E891-48E4-9EC9-3C5EFD3DB9DC",
              "versionEndExcluding": "6.18.16",
              "versionStartIncluding": "6.15",
              "vulnerable": true
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              "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
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  "cveTags": [],
  "descriptions": [
    {
      "lang": "en",
      "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nio_uring/zcrx: fix user_ref race between scrub and refill paths\n\nThe io_zcrx_put_niov_uref() function uses a non-atomic\ncheck-then-decrement pattern (atomic_read followed by separate\natomic_dec) to manipulate user_refs. This is serialized against other\ncallers by rq_lock, but io_zcrx_scrub() modifies the same counter with\natomic_xchg() WITHOUT holding rq_lock.\n\nOn SMP systems, the following race exists:\n\n  CPU0 (refill, holds rq_lock)          CPU1 (scrub, no rq_lock)\n  put_niov_uref:\n    atomic_read(uref) - 1\n    // window opens\n                                        atomic_xchg(uref, 0) - 1\n                                        return_niov_freelist(niov) [PUSH #1]\n    // window closes\n    atomic_dec(uref) - wraps to -1\n    returns true\n    return_niov(niov)\n    return_niov_freelist(niov)           [PUSH #2: DOUBLE-FREE]\n\nThe same niov is pushed to the freelist twice, causing free_count to\nexceed nr_iovs. Subsequent freelist pushes then perform an out-of-bounds\nwrite (a u32 value) past the kvmalloc\u0027d freelist array into the adjacent\nslab object.\n\nFix this by replacing the non-atomic read-then-dec in\nio_zcrx_put_niov_uref() with an atomic_try_cmpxchg loop that atomically\ntests and decrements user_refs. This makes the operation safe against\nconcurrent atomic_xchg from scrub without requiring scrub to acquire\nrq_lock.\n\n[pavel: removed a warning and a comment]"
    }
  ],
  "id": "CVE-2026-43121",
  "lastModified": "2026-05-12T21:17:31.950",
  "metrics": {
    "cvssMetricV31": [
      {
        "cvssData": {
          "attackComplexity": "HIGH",
          "attackVector": "LOCAL",
          "availabilityImpact": "HIGH",
          "baseScore": 4.7,
          "baseSeverity": "MEDIUM",
          "confidentialityImpact": "NONE",
          "integrityImpact": "NONE",
          "privilegesRequired": "LOW",
          "scope": "UNCHANGED",
          "userInteraction": "NONE",
          "vectorString": "CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H",
          "version": "3.1"
        },
        "exploitabilityScore": 1.0,
        "impactScore": 3.6,
        "source": "nvd@nist.gov",
        "type": "Primary"
      }
    ]
  },
  "published": "2026-05-06T12:16:28.950",
  "references": [
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "tags": [
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      "url": "https://git.kernel.org/stable/c/003049b1c4fb8aabb93febb7d1e49004f6ad653b"
    },
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
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  "sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
  "vulnStatus": "Analyzed",
  "weaknesses": [
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      "description": [
        {
          "lang": "en",
          "value": "CWE-362"
        }
      ],
      "source": "nvd@nist.gov",
      "type": "Primary"
    }
  ]
}


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