CVE-2026-63825

Estado: Pendiente de análisisCrítica (9.8)—

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

gcov: use atomic counter updates to fix concurrent access crashes

GCC's GCOV instrumentation can merge global branch counters with loop induction variables as an optimization. In inflate_fast(), the inner copy loops get transformed so that the GCOV counter value is loaded multiple times to compute the loop base address, start index, and end bound. Since GCOV counters are global (not per-CPU), concurrent execution on different CPUs causes the counter to change between loads, producing inconsistent values and out-of-bounds memory writes.

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The crash manifests during IPComp (IP Payload Compression) processing when inflate_fast() runs concurrently on multiple CPUs:

At the crash point, the compiler generated three loads from the same global GCOV counter (__gcov0.inflate_fast+216) to compute base, start, and end for an indexed loop. Another CPU modified the counter between loads, making the values inconsistent - the write went 3.4 MB past a 65 KB buffer.

Add -fprofile-update=prefer-atomic to CFLAGS_GCOV at the global level in the top-level Makefile, guarded by a try-run compile test. The test compiles a minimal program with and without -fprofile-update=prefer-atomic using the full KBUILD_CFLAGS, then compares undefined symbols in the resulting object files. If prefer-atomic introduces new undefined references (such as __atomic_fetch_add_8 on i386 or __aarch64_ldadd8_relax on arm64 with outline-atomics), the flag is not added -- the kernel does not link against libatomic.

On architectures where GCC inlines 64-bit atomic counter updates (x86_64, s390, ...) the test passes and the flag is enabled, preventing the compiler from merging counters with loop induction variables and fixing the observed concurrent-access crash.

On architectures where the flag would introduce libatomic dependencies, it is silently omitted and behaviour is no worse than before this patch.

Move the CFLAGS_GCOV block from its original position (before the arch Makefile include) to after the core KBUILD_CFLAGS assignments but before the scripts/Makefile.gcc-plugins include. This placement ensures the try-run test sees arch-specific flags (-m32, -march=, -mno-outline-atomics) while avoiding GCC plugin flags (-fplugin=) that would break the test on clean builds when plugin shared objects do not yet exist.

Detalles técnicos trazas, registros y código del informe original

  BUG: unable to handle page fault for address: ffffd0a3c0902ffa
  RIP: inflate_fast+1431
  Call Trace:
   zlib_inflate
   __deflate_decompress
   crypto_comp_decompress
   ipcomp_decompress [xfrm_ipcomp]
   ipcomp_input [xfrm_ipcomp]
   xfrm_input

CVSS

Versión: 3.1
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Puntuación base: 9.8

Probabilidad de explotación (EPSS)

Probabilidad de explotación en los próximos 30 días: 0.44%
Percentil entre todas las CVEs puntuadas: 36
Fecha de la puntuación: 6/10/2026

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).

🎯 Técnicas ATT&CK

Cómo se explota esta vulnerabilidad y qué consigue el atacante, en el lenguaje de MITRE ATT&CK.

ExplotaciónT1190 Exploit Public-Facing Applicationinitial access75 %
Impacto principalT1499.004 Application or System Exploitationimpact70 %
Impacto secundarioT1565.001 Stored Data Manipulationimpact60 %

Vulnerabilidad de kernel remota (AV:N, PR:N, UI:N) explotable vía IPComp; impacto: crash (DoS) y corrupción de memoria por race condition en GCOV counters.

Inferido por nuestro agente de análisis a partir de la descripción oficial, el vector CVSS y la CWE, y comprobado por un supervisor. Puede contener errores.

🛡️ Mitigaciones ATT&CK que cubren estas técnicas

Filter Network Traffic Vulnerability Scanning Restrict File and Directory Permissions Privileged Account Management Remote Data Storage Network Segmentation

Tecnologías afectadas (1)

⚠ Inferidas por IA a partir de la descripción — NVD aún no ha analizado esta CVE; no son CPE verificados.

Linux — Linux Kernel^AI

Referencias

JSON original (NVD)

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{
  "id": "CVE-2026-63825",
  "cveTags": [],
  "metrics": {
    "cvssMetricV31": [
      {
        "type": "Secondary",
        "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "cvssData": {
          "scope": "UNCHANGED",
          "version": "3.1",
          "baseScore": 9.8,
          "attackVector": "NETWORK",
          "baseSeverity": "CRITICAL",
          "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H",
          "integrityImpact": "HIGH",
          "userInteraction": "NONE",
          "attackComplexity": "LOW",
          "availabilityImpact": "HIGH",
          "privilegesRequired": "NONE",
          "confidentialityImpact": "HIGH"
        },
        "impactScore": 5.9,
        "exploitabilityScore": 3.9
      }
    ]
  },
  "affected": [
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "affectedData": [
        {
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "product": "Linux",
          "versions": [
            {
              "status": "affected",
              "version": "2521f2c228ad750701ba4702484e31d876dbc386",
              "lessThan": "49d893b9cbcfc5802a32e53a64c6c6956670d65b",
              "versionType": "git"
            },
            {
              "status": "affected",
              "version": "2521f2c228ad750701ba4702484e31d876dbc386",
              "lessThan": "5b959c1dbb4522b9e3ac4e26ad638b8784869841",
              "versionType": "git"
            },
            {
              "status": "affected",
              "version": "2521f2c228ad750701ba4702484e31d876dbc386",
              "lessThan": "56cb9b7d96b28a1173a510ab25354b6599ad3a33",
              "versionType": "git"
            }
          ],
          "programFiles": [
            "Makefile"
          ],
          "defaultStatus": "unaffected"
        },
        {
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "product": "Linux",
          "versions": [
            {
              "status": "affected",
              "version": "2.6.31"
            },
            {
              "status": "unaffected",
              "version": "0",
              "lessThan": "2.6.31",
              "versionType": "semver"
            },
            {
              "status": "unaffected",
              "version": "6.18.38",
              "versionType": "semver",
              "lessThanOrEqual": "6.18.*"
            },
            {
              "status": "unaffected",
              "version": "7.1.3",
              "versionType": "semver",
              "lessThanOrEqual": "7.1.*"
            },
            {
              "status": "unaffected",
              "version": "7.2",
              "versionType": "original_commit_for_fix",
              "lessThanOrEqual": "*"
            }
          ],
          "programFiles": [
            "Makefile"
          ],
          "defaultStatus": "affected"
        }
      ]
    }
  ],
  "published": "2026-07-19T12:16:55.720",
  "references": [
    {
      "url": "https://git.kernel.org/stable/c/49d893b9cbcfc5802a32e53a64c6c6956670d65b",
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"
    },
    {
      "url": "https://git.kernel.org/stable/c/56cb9b7d96b28a1173a510ab25354b6599ad3a33",
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"
    },
    {
      "url": "https://git.kernel.org/stable/c/5b959c1dbb4522b9e3ac4e26ad638b8784869841",
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"
    }
  ],
  "vulnStatus": "Awaiting Analysis",
  "descriptions": [
    {
      "lang": "en",
      "value": "In the Linux kernel, the following vulnerability has been resolved:\n\ngcov: use atomic counter updates to fix concurrent access crashes\n\nGCC's GCOV instrumentation can merge global branch counters with loop\ninduction variables as an optimization.  In inflate_fast(), the inner copy\nloops get transformed so that the GCOV counter value is loaded multiple\ntimes to compute the loop base address, start index, and end bound.  Since\nGCOV counters are global (not per-CPU), concurrent execution on different\nCPUs causes the counter to change between loads, producing inconsistent\nvalues and out-of-bounds memory writes.\n\nThe crash manifests during IPComp (IP Payload Compression) processing when\ninflate_fast() runs concurrently on multiple CPUs:\n\n  BUG: unable to handle page fault for address: ffffd0a3c0902ffa\n  RIP: inflate_fast+1431\n  Call Trace:\n   zlib_inflate\n   __deflate_decompress\n   crypto_comp_decompress\n   ipcomp_decompress [xfrm_ipcomp]\n   ipcomp_input [xfrm_ipcomp]\n   xfrm_input\n\nAt the crash point, the compiler generated three loads from the same\nglobal GCOV counter (__gcov0.inflate_fast+216) to compute base, start, and\nend for an indexed loop.  Another CPU modified the counter between loads,\nmaking the values inconsistent - the write went 3.4 MB past a 65 KB\nbuffer.\n\nAdd -fprofile-update=prefer-atomic to CFLAGS_GCOV at the global level in\nthe top-level Makefile, guarded by a try-run compile test.  The test\ncompiles a minimal program with and without -fprofile-update=prefer-atomic\nusing the full KBUILD_CFLAGS, then compares undefined symbols in the\nresulting object files.  If prefer-atomic introduces new undefined\nreferences (such as __atomic_fetch_add_8 on i386 or __aarch64_ldadd8_relax\non arm64 with outline-atomics), the flag is not added -- the kernel does\nnot link against libatomic.\n\nOn architectures where GCC inlines 64-bit atomic counter updates (x86_64,\ns390, ...) the test passes and the flag is enabled, preventing the\ncompiler from merging counters with loop induction variables and fixing\nthe observed concurrent-access crash.\n\nOn architectures where the flag would introduce libatomic dependencies, it\nis silently omitted and behaviour is no worse than before this patch.\n\nMove the CFLAGS_GCOV block from its original position (before the arch\nMakefile include) to after the core KBUILD_CFLAGS assignments but before\nthe scripts/Makefile.gcc-plugins include.  This placement ensures the\ntry-run test sees arch-specific flags (-m32, -march=,\n-mno-outline-atomics) while avoiding GCC plugin flags (-fplugin=) that\nwould break the test on clean builds when plugin shared objects do not yet\nexist."
    }
  ],
  "lastModified": "2026-08-17T05:17:20.090",
  "sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"
}