CVE-2025-39756 (GCVE-0-2025-39756)

Vulnerability from cvelistv5 – Published: 2025-09-11 16:52 – Updated: 2026-10-15 01:08
VLAI
Title
fs: Prevent file descriptor table allocations exceeding INT_MAX
Summary
In the Linux kernel, the following vulnerability has been resolved: fs: Prevent file descriptor table allocations exceeding INT_MAX When sysctl_nr_open is set to a very high value (for example, 1073741816 as set by systemd), processes attempting to use file descriptors near the limit can trigger massive memory allocation attempts that exceed INT_MAX, resulting in a WARNING in mm/slub.c: WARNING: CPU: 0 PID: 44 at mm/slub.c:5027 __kvmalloc_node_noprof+0x21a/0x288 This happens because kvmalloc_array() and kvmalloc() check if the requested size exceeds INT_MAX and emit a warning when the allocation is not flagged with __GFP_NOWARN. Specifically, when nr_open is set to 1073741816 (0x3ffffff8) and a process calls dup2(oldfd, 1073741880), the kernel attempts to allocate: - File descriptor array: 1073741880 * 8 bytes = 8,589,935,040 bytes - Multiple bitmaps: ~400MB - Total allocation size: > 8GB (exceeding INT_MAX = 2,147,483,647) Reproducer: 1. Set /proc/sys/fs/nr_open to 1073741816: # echo 1073741816 > /proc/sys/fs/nr_open 2. Run a program that uses a high file descriptor: #include <unistd.h> #include <sys/resource.h> int main() { struct rlimit rlim = {1073741824, 1073741824}; setrlimit(RLIMIT_NOFILE, &rlim); dup2(2, 1073741880); // Triggers the warning return 0; } 3. Observe WARNING in dmesg at mm/slub.c:5027 systemd commit a8b627a introduced automatic bumping of fs.nr_open to the maximum possible value. The rationale was that systems with memory control groups (memcg) no longer need separate file descriptor limits since memory is properly accounted. However, this change overlooked that: 1. The kernel's allocation functions still enforce INT_MAX as a maximum size regardless of memcg accounting 2. Programs and tests that legitimately test file descriptor limits can inadvertently trigger massive allocations 3. The resulting allocations (>8GB) are impractical and will always fail systemd's algorithm starts with INT_MAX and keeps halving the value until the kernel accepts it. On most systems, this results in nr_open being set to 1073741816 (0x3ffffff8), which is just under 1GB of file descriptors. While processes rarely use file descriptors near this limit in normal operation, certain selftests (like tools/testing/selftests/core/unshare_test.c) and programs that test file descriptor limits can trigger this issue. Fix this by adding a check in alloc_fdtable() to ensure the requested allocation size does not exceed INT_MAX. This causes the operation to fail with -EMFILE instead of triggering a kernel warning and avoids the impractical >8GB memory allocation request.
Severity
5.5 (Medium)
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
Assigner
Linux CNA under the mitre root
CNA scorecard D 53/100 over 7061 records in the last 180 days details
References
Impacted products
Vendor Product Version CPE status
Linux Linux Affected: 9cfe015aa424b3c003baba3841a60dd9b5ad319b , < b4159c5a90c03f8acd3de345a7f5fc63b0909818 (git)
Affected: 9cfe015aa424b3c003baba3841a60dd9b5ad319b , < f95638a8f22eba307dceddf5aef9ae2326bbcf98 (git)
Affected: 9cfe015aa424b3c003baba3841a60dd9b5ad319b , < 749528086620f8012b83ae032a80f6ffa80c45cd (git)
Affected: 9cfe015aa424b3c003baba3841a60dd9b5ad319b , < 628fc28f42d979f36dbf75a6129ac7730e30c04e (git)
Affected: 9cfe015aa424b3c003baba3841a60dd9b5ad319b , < 237e416eb62101f21b28c9e6e564d10efe1ecc6f (git)
Affected: 9cfe015aa424b3c003baba3841a60dd9b5ad319b , < d4f9351243c17865a8cdbe6b3ccd09d0b13a7bcc (git)
Affected: 9cfe015aa424b3c003baba3841a60dd9b5ad319b , < 9f61fa6a2a89a610120bc4e5d24379c667314b5c (git)
Affected: 9cfe015aa424b3c003baba3841a60dd9b5ad319b , < dfd1f4ea98c3bd3a03d12169b5b2daa1f0a3e4ae (git)
Affected: 9cfe015aa424b3c003baba3841a60dd9b5ad319b , < 04a2c4b4511d186b0fce685da21085a5d4acd370 (git)
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Linux Linux Affected: 2.6.25
Unaffected: 0 , < 2.6.25 (semver)
Unaffected: 5.4.297 , ≤ 5.4.* (semver)
Unaffected: 5.10.241 , ≤ 5.10.* (semver)
Unaffected: 5.15.190 , ≤ 5.15.* (semver)
Unaffected: 6.1.149 , ≤ 6.1.* (semver)
Unaffected: 6.6.103 , ≤ 6.6.* (semver)
Unaffected: 6.12.43 , ≤ 6.12.* (semver)
Unaffected: 6.15.11 , ≤ 6.15.* (semver)
Unaffected: 6.16.2 , ≤ 6.16.* (semver)
Unaffected: 6.17 , ≤ * (original_commit_for_fix)
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Siemens SIMATIC CN 4100 Affected: 0 , < V5.0 (custom)
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Siemens SIMATIC S7-1500 CPU 1518-4 PN/DP MFP Affected: V3.1.5 , < * (custom)
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Siemens SIMATIC S7-1500 CPU 1518-4 PN/DP MFP Affected: V3.1.6 , < * (custom)
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Siemens SIMATIC S7-1500 CPU 1518-4 PN/DP MFP Affected: V3.1.7 , < * (custom)
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Siemens SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP Affected: V3.1.5 , < * (custom)
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Siemens SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP Affected: V3.1.6 , < * (custom)
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Siemens SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP Affected: V3.1.7 , < * (custom)
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Siemens SIPLUS S7-1500 CPU 1518-4 PN/DP MFP Affected: V3.1.5 , < * (custom)
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Siemens SIPLUS S7-1500 CPU 1518-4 PN/DP MFP Affected: V3.1.6 , < * (custom)
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Siemens SIPLUS S7-1500 CPU 1518-4 PN/DP MFP Affected: V3.1.7 , < * (custom)
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          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nfs: Prevent file descriptor table allocations exceeding INT_MAX\n\nWhen sysctl_nr_open is set to a very high value (for example, 1073741816\nas set by systemd), processes attempting to use file descriptors near\nthe limit can trigger massive memory allocation attempts that exceed\nINT_MAX, resulting in a WARNING in mm/slub.c:\n\n  WARNING: CPU: 0 PID: 44 at mm/slub.c:5027 __kvmalloc_node_noprof+0x21a/0x288\n\nThis happens because kvmalloc_array() and kvmalloc() check if the\nrequested size exceeds INT_MAX and emit a warning when the allocation is\nnot flagged with __GFP_NOWARN.\n\nSpecifically, when nr_open is set to 1073741816 (0x3ffffff8) and a\nprocess calls dup2(oldfd, 1073741880), the kernel attempts to allocate:\n- File descriptor array: 1073741880 * 8 bytes = 8,589,935,040 bytes\n- Multiple bitmaps: ~400MB\n- Total allocation size: \u003e 8GB (exceeding INT_MAX = 2,147,483,647)\n\nReproducer:\n1. Set /proc/sys/fs/nr_open to 1073741816:\n   # echo 1073741816 \u003e /proc/sys/fs/nr_open\n\n2. Run a program that uses a high file descriptor:\n   #include \u003cunistd.h\u003e\n   #include \u003csys/resource.h\u003e\n\n   int main() {\n       struct rlimit rlim = {1073741824, 1073741824};\n       setrlimit(RLIMIT_NOFILE, \u0026rlim);\n       dup2(2, 1073741880);  // Triggers the warning\n       return 0;\n   }\n\n3. Observe WARNING in dmesg at mm/slub.c:5027\n\nsystemd commit a8b627a introduced automatic bumping of fs.nr_open to the\nmaximum possible value. The rationale was that systems with memory\ncontrol groups (memcg) no longer need separate file descriptor limits\nsince memory is properly accounted. However, this change overlooked\nthat:\n\n1. The kernel\u0027s allocation functions still enforce INT_MAX as a maximum\n   size regardless of memcg accounting\n2. Programs and tests that legitimately test file descriptor limits can\n   inadvertently trigger massive allocations\n3. The resulting allocations (\u003e8GB) are impractical and will always fail\n\nsystemd\u0027s algorithm starts with INT_MAX and keeps halving the value\nuntil the kernel accepts it. On most systems, this results in nr_open\nbeing set to 1073741816 (0x3ffffff8), which is just under 1GB of file\ndescriptors.\n\nWhile processes rarely use file descriptors near this limit in normal\noperation, certain selftests (like\ntools/testing/selftests/core/unshare_test.c) and programs that test file\ndescriptor limits can trigger this issue.\n\nFix this by adding a check in alloc_fdtable() to ensure the requested\nallocation size does not exceed INT_MAX. This causes the operation to\nfail with -EMFILE instead of triggering a kernel warning and avoids the\nimpractical \u003e8GB memory allocation request."
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            "lang": "en",
            "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nfs: Prevent file descriptor table allocations exceeding INT_MAX\n\nWhen sysctl_nr_open is set to a very high value (for example, 1073741816\nas set by systemd), processes attempting to use file descriptors near\nthe limit can trigger massive memory allocation attempts that exceed\nINT_MAX, resulting in a WARNING in mm/slub.c:\n\n  WARNING: CPU: 0 PID: 44 at mm/slub.c:5027 __kvmalloc_node_noprof+0x21a/0x288\n\nThis happens because kvmalloc_array() and kvmalloc() check if the\nrequested size exceeds INT_MAX and emit a warning when the allocation is\nnot flagged with __GFP_NOWARN.\n\nSpecifically, when nr_open is set to 1073741816 (0x3ffffff8) and a\nprocess calls dup2(oldfd, 1073741880), the kernel attempts to allocate:\n- File descriptor array: 1073741880 * 8 bytes = 8,589,935,040 bytes\n- Multiple bitmaps: ~400MB\n- Total allocation size: \u003e 8GB (exceeding INT_MAX = 2,147,483,647)\n\nReproducer:\n1. Set /proc/sys/fs/nr_open to 1073741816:\n   # echo 1073741816 \u003e /proc/sys/fs/nr_open\n\n2. Run a program that uses a high file descriptor:\n   #include \u003cunistd.h\u003e\n   #include \u003csys/resource.h\u003e\n\n   int main() {\n       struct rlimit rlim = {1073741824, 1073741824};\n       setrlimit(RLIMIT_NOFILE, \u0026rlim);\n       dup2(2, 1073741880);  // Triggers the warning\n       return 0;\n   }\n\n3. Observe WARNING in dmesg at mm/slub.c:5027\n\nsystemd commit a8b627a introduced automatic bumping of fs.nr_open to the\nmaximum possible value. The rationale was that systems with memory\ncontrol groups (memcg) no longer need separate file descriptor limits\nsince memory is properly accounted. However, this change overlooked\nthat:\n\n1. The kernel\u0027s allocation functions still enforce INT_MAX as a maximum\n   size regardless of memcg accounting\n2. Programs and tests that legitimately test file descriptor limits can\n   inadvertently trigger massive allocations\n3. The resulting allocations (\u003e8GB) are impractical and will always fail\n\nsystemd\u0027s algorithm starts with INT_MAX and keeps halving the value\nuntil the kernel accepts it. On most systems, this results in nr_open\nbeing set to 1073741816 (0x3ffffff8), which is just under 1GB of file\ndescriptors.\n\nWhile processes rarely use file descriptors near this limit in normal\noperation, certain selftests (like\ntools/testing/selftests/core/unshare_test.c) and programs that test file\ndescriptor limits can trigger this issue.\n\nFix this by adding a check in alloc_fdtable() to ensure the requested\nallocation size does not exceed INT_MAX. This causes the operation to\nfail with -EMFILE instead of triggering a kernel warning and avoids the\nimpractical \u003e8GB memory allocation request."
          },
          {
            "lang": "es",
            "value": "En el kernel de Linux, la siguiente vulnerabilidad ha sido resuelta:\n\nfs: Evitar asignaciones de tablas de descriptores de archivo que excedan INT_MAX\n\nCuando sysctl_nr_open se establece en un valor muy alto (por ejemplo, 1073741816 como lo establece systemd), los procesos que intentan usar descriptores de archivo cerca del l\u00edmite pueden desencadenar intentos masivos de asignaci\u00f3n de memoria que exceden INT_MAX, lo que resulta en una ADVERTENCIA en mm/slub.c:\n\n  WARNING: CPU: 0 PID: 44 at mm/slub.c:5027 __kvmalloc_node_noprof+0x21a/0x288\n\nEsto sucede porque kvmalloc_array() y kvmalloc() verifican si el tama\u00f1o solicitado excede INT_MAX y emiten una advertencia cuando la asignaci\u00f3n no est\u00e1 marcada con __GFP_NOWARN.\n\nEspec\u00edficamente, cuando nr_open se establece en 1073741816 (0x3ffffff8) y un proceso llama a dup2(oldfd, 1073741880), el kernel intenta asignar:\n- Matriz de descriptores de archivo: 1073741880 * 8 bytes = 8,589,935,040 bytes\n- M\u00faltiples mapas de bits: ~400MB\n- Tama\u00f1o total de asignaci\u00f3n: \u003e 8GB (excediendo INT_MAX = 2,147,483,647)\n\nReproductor:\n1. Establecer /proc/sys/fs/nr_open en 1073741816:\n   # echo 1073741816 \u003e /proc/sys/fs/nr_open\n\n2. Ejecutar un programa que usa un descriptor de archivo alto:\n   #include \u003cunistd.h\u003e\n   #include \u003csys/resource\u003e\n\n   int main() {\n       struct rlimit rlim = {1073741824, 1073741824};\n       setrlimit(RLIMIT_NOFILE, \u0026rlim);\n       dup2(2, 1073741880);  // Desencadena la advertencia\n       return 0;\n   }\n\n3. Observar ADVERTENCIA en dmesg en mm/slub.c:5027\n\nEl commit a8b627a de systemd introdujo el aumento autom\u00e1tico de fs.nr_open al valor m\u00e1ximo posible. La raz\u00f3n era que los sistemas con grupos de control de memoria (memcg) ya no necesitan l\u00edmites de descriptores de archivo separados, ya que la memoria se contabiliza correctamente. Sin embargo, este cambio pas\u00f3 por alto que:\n\n1. Las funciones de asignaci\u00f3n del kernel a\u00fan imponen INT_MAX como tama\u00f1o m\u00e1ximo, independientemente de la contabilidad de memcg\n2. Los programas y pruebas que prueban leg\u00edtimamente los l\u00edmites de descriptores de archivo pueden desencadenar inadvertidamente asignaciones masivas\n3. Las asignaciones resultantes (\u003e8GB) son poco pr\u00e1cticas y siempre fallar\u00e1n\n\nEl algoritmo de systemd comienza con INT_MAX y sigue reduciendo el valor a la mitad hasta que el kernel lo acepta. En la mayor\u00eda de los sistemas, esto resulta en que nr_open se establece en 1073741816 (0x3ffffff8), lo que equivale a poco menos de 1GB de descriptores de archivo.\n\nSi bien los procesos rara vez usan descriptores de archivo cerca de este l\u00edmite en operaci\u00f3n normal, ciertas autopruebas (como tools/testing/selftests/core/unshare_test.c) y programas que prueban los l\u00edmites de descriptores de archivo pueden desencadenar este problema.\n\nSolucione esto agregando una verificaci\u00f3n en alloc_fdtable() para asegurar que el tama\u00f1o de asignaci\u00f3n solicitado no exceda INT_MAX. Esto hace que la operaci\u00f3n falle con -EMFILE en lugar de desencadenar una advertencia del kernel y evita la solicitud de asignaci\u00f3n de memoria poco pr\u00e1ctica de \u003e8GB."
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        "id": "CVE-2025-39756",
        "lastModified": "2026-10-19T21:10:00.160",
        "metrics": {
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                "attackComplexity": "LOW",
                "attackVector": "LOCAL",
                "availabilityImpact": "HIGH",
                "baseScore": 5.5,
                "baseSeverity": "MEDIUM",
                "confidentialityImpact": "NONE",
                "integrityImpact": "NONE",
                "privilegesRequired": "LOW",
                "scope": "UNCHANGED",
                "userInteraction": "NONE",
                "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H",
                "version": "3.1"
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              "exploitabilityScore": 1.8,
              "impactScore": 3.6,
              "source": "nvd@nist.gov",
              "type": "Primary"
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        "published": "2025-09-11T17:15:39.343",
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          {
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            "url": "https://lists.debian.org/debian-lts-announce/2025/10/msg00007.html"
          },
          {
            "source": "af854a3a-2127-422b-91ae-364da2661108",
            "tags": [
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              "Third Party Advisory"
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          },
          {
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          {
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          }
        ],
        "sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "vulnStatus": "Modified",
        "weaknesses": [
          {
            "description": [
              {
                "lang": "en",
                "value": "CWE-401"
              }
            ],
            "source": "nvd@nist.gov",
            "type": "Primary"
          }
        ]
      }
    },
    "redhat_vex": {
      "aggregate_severity": "Moderate",
      "current_release_date": "2026-06-30T01:19:29+00:00",
      "cve": "CVE-2025-39756",
      "id": "CVE-2025-39756",
      "initial_release_date": "2025-09-11T00:00:00+00:00",
      "product_status:known_affected": "260",
      "product_status:known_not_affected": "14",
      "source": "Red Hat CSAF VEX",
      "status": "final",
      "title": "kernel: Linux kernel: Denial of Service via excessive file descriptor table allocation",
      "url": "https://security.access.redhat.com/data/csaf/v2/vex/2025/cve-2025-39756.json",
      "version": "3"
    },
    "suse_vex": {
      "aggregate_severity": "moderate",
      "current_release_date": "2026-10-15T16:33:40Z",
      "cve": "CVE-2025-39756",
      "id": "CVE-2025-39756",
      "initial_release_date": "2025-09-12T23:24:20Z",
      "product_status:known_affected": "731",
      "product_status:recommended": "532",
      "source": "SUSE CSAF VEX",
      "status": "interim",
      "title": "SUSE CVE CVE-2025-39756",
      "url": "https://ftp.suse.com/pub/projects/security/csaf-vex/2025/cve-2025-39756.json",
      "version": "48"
    }
  }
}



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