FKIE_CVE-2026-31706

Vulnerability from fkie_nvd - Published: 2026-05-01 14:16 - Updated: 2026-05-06 20:27
Severity ?
8.8 (High) - CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Summary
In the Linux kernel, the following vulnerability has been resolved: ksmbd: validate num_aces and harden ACE walk in smb_inherit_dacl() smb_inherit_dacl() trusts the on-disk num_aces value from the parent directory's DACL xattr and uses it to size a heap allocation: aces_base = kmalloc(sizeof(struct smb_ace) * num_aces * 2, ...); num_aces is a u16 read from le16_to_cpu(parent_pdacl->num_aces) without checking that it is consistent with the declared pdacl_size. An authenticated client whose parent directory's security.NTACL is tampered (e.g. via offline xattr corruption or a concurrent path that bypasses parse_dacl()) can present num_aces = 65535 with minimal actual ACE data. This causes a ~8 MB allocation (not kzalloc, so uninitialized) that the subsequent loop only partially populates, and may also overflow the three-way size_t multiply on 32-bit kernels. Additionally, the ACE walk loop uses the weaker offsetof(struct smb_ace, access_req) minimum size check rather than the minimum valid on-wire ACE size, and does not reject ACEs whose declared size is below the minimum. Reproduced on UML + KASAN + LOCKDEP against the real ksmbd code path. A legitimate mount.cifs client creates a parent directory over SMB (ksmbd writes a valid security.NTACL xattr), then the NTACL blob on the backing filesystem is rewritten to set num_aces = 0xFFFF while keeping the posix_acl_hash bytes intact so ksmbd_vfs_get_sd_xattr()'s hash check still passes. A subsequent SMB2 CREATE of a child under that parent drives smb2_open() into smb_inherit_dacl() (share has "vfs objects = acl_xattr" set), which fails the page allocator: WARNING: mm/page_alloc.c:5226 at __alloc_frozen_pages_noprof+0x46c/0x9c0 Workqueue: ksmbd-io handle_ksmbd_work __alloc_frozen_pages_noprof+0x46c/0x9c0 ___kmalloc_large_node+0x68/0x130 __kmalloc_large_node_noprof+0x24/0x70 __kmalloc_noprof+0x4c9/0x690 smb_inherit_dacl+0x394/0x2430 smb2_open+0x595d/0xabe0 handle_ksmbd_work+0x3d3/0x1140 With the patch applied the added guard rejects the tampered value with -EINVAL before any large allocation runs, smb2_open() falls back to smb2_create_sd_buffer(), and the child is created with a default SD. No warning, no splat. Fix by: 1. Validating num_aces against pdacl_size using the same formula applied in parse_dacl(). 2. Replacing the raw kmalloc(sizeof * num_aces * 2) with kmalloc_array(num_aces * 2, sizeof(...)) for overflow-safe allocation. 3. Tightening the per-ACE loop guard to require the minimum valid ACE size (offsetof(smb_ace, sid) + CIFS_SID_BASE_SIZE) and rejecting under-sized ACEs, matching the hardening in smb_check_perm_dacl() and parse_dacl(). v1 -> v2: - Replace the synthetic test-module splat in the changelog with a real-path UML + KASAN reproduction driven through mount.cifs and SMB2 CREATE; Namjae flagged the kcifs3_test_inherit_dacl_old name in v1 since it does not exist in ksmbd. - Drop the commit-hash citation from the code comment per Namjae's review; keep the parse_dacl() pointer.
References
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/063a7409b0de46d7c770b65bb0338e6fdb3b1f0aPatch
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/3e4e2ea2a781018ed5d75f969e3e5606beb66e48Patch
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/3e5360b422dd741cb315654a191fa73869a37414Patch
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/59c32abaaec9cdd6164811c7e864e72f7554b82dPatch
Impacted products
Vendor Product Version
linux linux_kernel *
linux linux_kernel *
linux linux_kernel *
To clipboard 

{
  "configurations": [
    {
      "nodes": [
        {
          "cpeMatch": [
            {
              "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
              "matchCriteriaId": "04651641-C387-4546-B02F-17BA989CC253",
              "versionEndExcluding": "6.12.84",
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              "vulnerable": true
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          "negate": false,
          "operator": "OR"
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  ],
  "cveTags": [],
  "descriptions": [
    {
      "lang": "en",
      "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nksmbd: validate num_aces and harden ACE walk in smb_inherit_dacl()\n\nsmb_inherit_dacl() trusts the on-disk num_aces value from the parent\ndirectory\u0027s DACL xattr and uses it to size a heap allocation:\n\n  aces_base = kmalloc(sizeof(struct smb_ace) * num_aces * 2, ...);\n\nnum_aces is a u16 read from le16_to_cpu(parent_pdacl-\u003enum_aces)\nwithout checking that it is consistent with the declared pdacl_size.\nAn authenticated client whose parent directory\u0027s security.NTACL is\ntampered (e.g. via offline xattr corruption or a concurrent path that\nbypasses parse_dacl()) can present num_aces = 65535 with minimal\nactual ACE data.  This causes a ~8 MB allocation (not kzalloc, so\nuninitialized) that the subsequent loop only partially populates, and\nmay also overflow the three-way size_t multiply on 32-bit kernels.\n\nAdditionally, the ACE walk loop uses the weaker\noffsetof(struct smb_ace, access_req) minimum size check rather than\nthe minimum valid on-wire ACE size, and does not reject ACEs whose\ndeclared size is below the minimum.\n\nReproduced on UML + KASAN + LOCKDEP against the real ksmbd code path.\nA legitimate mount.cifs client creates a parent directory over SMB\n(ksmbd writes a valid security.NTACL xattr), then the NTACL blob on\nthe backing filesystem is rewritten to set num_aces = 0xFFFF while\nkeeping the posix_acl_hash bytes intact so ksmbd_vfs_get_sd_xattr()\u0027s\nhash check still passes.  A subsequent SMB2 CREATE of a child under\nthat parent drives smb2_open() into smb_inherit_dacl() (share has\n\"vfs objects = acl_xattr\" set), which fails the page allocator:\n\n  WARNING: mm/page_alloc.c:5226 at __alloc_frozen_pages_noprof+0x46c/0x9c0\n  Workqueue: ksmbd-io handle_ksmbd_work\n   __alloc_frozen_pages_noprof+0x46c/0x9c0\n   ___kmalloc_large_node+0x68/0x130\n   __kmalloc_large_node_noprof+0x24/0x70\n   __kmalloc_noprof+0x4c9/0x690\n   smb_inherit_dacl+0x394/0x2430\n   smb2_open+0x595d/0xabe0\n   handle_ksmbd_work+0x3d3/0x1140\n\nWith the patch applied the added guard rejects the tampered value\nwith -EINVAL before any large allocation runs, smb2_open() falls back\nto smb2_create_sd_buffer(), and the child is created with a default\nSD.  No warning, no splat.\n\nFix by:\n\n  1. Validating num_aces against pdacl_size using the same formula\n     applied in parse_dacl().\n\n  2. Replacing the raw kmalloc(sizeof * num_aces * 2) with\n     kmalloc_array(num_aces * 2, sizeof(...)) for overflow-safe\n     allocation.\n\n  3. Tightening the per-ACE loop guard to require the minimum valid\n     ACE size (offsetof(smb_ace, sid) + CIFS_SID_BASE_SIZE) and\n     rejecting under-sized ACEs, matching the hardening in\n     smb_check_perm_dacl() and parse_dacl().\n\nv1 -\u003e v2:\n  - Replace the synthetic test-module splat in the changelog with a\n    real-path UML + KASAN reproduction driven through mount.cifs and\n    SMB2 CREATE; Namjae flagged the kcifs3_test_inherit_dacl_old name\n    in v1 since it does not exist in ksmbd.\n  - Drop the commit-hash citation from the code comment per Namjae\u0027s\n    review; keep the parse_dacl() pointer."
    }
  ],
  "id": "CVE-2026-31706",
  "lastModified": "2026-05-06T20:27:43.123",
  "metrics": {
    "cvssMetricV31": [
      {
        "cvssData": {
          "attackComplexity": "LOW",
          "attackVector": "NETWORK",
          "availabilityImpact": "HIGH",
          "baseScore": 8.8,
          "baseSeverity": "HIGH",
          "confidentialityImpact": "HIGH",
          "integrityImpact": "HIGH",
          "privilegesRequired": "LOW",
          "scope": "UNCHANGED",
          "userInteraction": "NONE",
          "vectorString": "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H",
          "version": "3.1"
        },
        "exploitabilityScore": 2.8,
        "impactScore": 5.9,
        "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "type": "Secondary"
      }
    ]
  },
  "published": "2026-05-01T14:16:20.597",
  "references": [
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "tags": [
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      "url": "https://git.kernel.org/stable/c/063a7409b0de46d7c770b65bb0338e6fdb3b1f0a"
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      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
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    },
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
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    },
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      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "tags": [
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    }
  ],
  "sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
  "vulnStatus": "Analyzed",
  "weaknesses": [
    {
      "description": [
        {
          "lang": "en",
          "value": "NVD-CWE-noinfo"
        }
      ],
      "source": "nvd@nist.gov",
      "type": "Primary"
    }
  ]
}


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