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CVE-2026-43121

Estado: AnalizadaMedia (4.7)—

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:

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.

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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]

Detalles técnicos trazas, registros y código del informe original
  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]

CVSS

Probabilidad de explotación (EPSS)

EPSS (Exploit Prediction Scoring System, de FIRST) estima la probabilidad de que una vulnerabilidad sea explotada en 30 días. Complementa a CVSS (impacto) y a CISA KEV (explotación confirmada).

Tecnologías afectadas (1)

Linux — Linux Kernel

CWE

Referencias

JSON original (NVD)

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  "id": "CVE-2026-43121",
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          "attackVector": "LOCAL",
          "baseSeverity": "MEDIUM",
          "vectorString": "CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H",
          "integrityImpact": "NONE",
          "userInteraction": "NONE",
          "attackComplexity": "HIGH",
          "availabilityImpact": "HIGH",
          "privilegesRequired": "LOW",
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  "published": "2026-05-06T12:16:28.950",
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          "value": "CWE-362"
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  "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'd 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]"
    }
  ],
  "lastModified": "2026-06-17T10:48:58.537",
  "configurations": [
    {
      "nodes": [
        {
          "negate": false,
          "cpeMatch": [
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}