CVE-2026-31706 (GCVE-0-2026-31706)
Vulnerability from cvelistv5
Published
2026-05-01 13:56
Modified
2026-08-05 12:24
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
Impacted products
Vendor Product Version
Linux Linux Version: e2f34481b24db2fd634b5edb0a5bd0e4d38cc6e9
Version: e2f34481b24db2fd634b5edb0a5bd0e4d38cc6e9
Version: e2f34481b24db2fd634b5edb0a5bd0e4d38cc6e9
Version: e2f34481b24db2fd634b5edb0a5bd0e4d38cc6e9
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{
  "containers": {
    "cna": {
      "affected": [
        {
          "defaultStatus": "unaffected",
          "product": "Linux",
          "programFiles": [
            "fs/smb/server/smbacl.c"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
            {
              "lessThan": "063a7409b0de46d7c770b65bb0338e6fdb3b1f0a",
              "status": "affected",
              "version": "e2f34481b24db2fd634b5edb0a5bd0e4d38cc6e9",
              "versionType": "git"
            },
            {
              "lessThan": "3e5360b422dd741cb315654a191fa73869a37414",
              "status": "affected",
              "version": "e2f34481b24db2fd634b5edb0a5bd0e4d38cc6e9",
              "versionType": "git"
            },
            {
              "lessThan": "59c32abaaec9cdd6164811c7e864e72f7554b82d",
              "status": "affected",
              "version": "e2f34481b24db2fd634b5edb0a5bd0e4d38cc6e9",
              "versionType": "git"
            },
            {
              "lessThan": "3e4e2ea2a781018ed5d75f969e3e5606beb66e48",
              "status": "affected",
              "version": "e2f34481b24db2fd634b5edb0a5bd0e4d38cc6e9",
              "versionType": "git"
            }
          ]
        },
        {
          "defaultStatus": "affected",
          "product": "Linux",
          "programFiles": [
            "fs/smb/server/smbacl.c"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
            {
              "status": "affected",
              "version": "5.15"
            },
            {
              "lessThan": "5.15",
              "status": "unaffected",
              "version": "0",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "6.12.*",
              "status": "unaffected",
              "version": "6.12.84",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "6.18.*",
              "status": "unaffected",
              "version": "6.18.25",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "7.0.*",
              "status": "unaffected",
              "version": "7.0.2",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "*",
              "status": "unaffected",
              "version": "7.1",
              "versionType": "original_commit_for_fix"
            }
          ]
        }
      ],
      "cpeApplicability": [
        {
          "nodes": [
            {
              "cpeMatch": [
                {
                  "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                  "versionEndExcluding": "6.12.84",
                  "versionStartIncluding": "5.15",
                  "vulnerable": true
                },
                {
                  "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                  "versionEndExcluding": "6.18.25",
                  "versionStartIncluding": "5.15",
                  "vulnerable": true
                },
                {
                  "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                  "versionEndExcluding": "7.0.2",
                  "versionStartIncluding": "5.15",
                  "vulnerable": true
                },
                {
                  "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                  "versionEndExcluding": "7.1",
                  "versionStartIncluding": "5.15",
                  "vulnerable": true
                }
              ],
              "negate": false,
              "operator": "OR"
            }
          ]
        }
      ],
      "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."
        }
      ],
      "metrics": [
        {
          "cvssV3_1": {
            "baseScore": 8.8,
            "baseSeverity": "HIGH",
            "vectorString": "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H",
            "version": "3.1"
          },
          "scenarios": [
            {
              "lang": "en",
              "value": "AV:N - ksmbd processes SMB2 CREATE requests over TCP/445, and the vulnerable `smb_inherit_dacl()` path is reached remotely through the in-kernel SMB server.\nAC:L - An attacker with access to a writable ACL-xattr share can deterministically store or arrange a malformed parent DACL and then create a child object to trigger inheritance; no race or uncontrollable condition is required.\nPR:L - The SMB2 CREATE path requires a valid SMB session and tree connection, and the practical attack requires share write/ACL-setting privileges, but not kernel, root, or administrative privileges.\nUI:N - After the attacker has SMB access, exploitation is driven entirely by attacker-sent SMB requests and does not require victim interaction.\nS:U - The vulnerable component and impacted resource are both within the same kernel/ksmbd security authority; this is not a VM escape or cross-boundary isolation bypass.\nC:H - The hardened ACE walk addresses attacker-controlled malformed DACL data that can drive out-of-bounds reads of kernel heap-backed security descriptor data; under the required high-severity tie-break rule, this is scored as high confidentiality impact.\nI:H - The malformed inherited ACE construction can corrupt kernel-managed ACL/security descriptor state and has plausible heap memory-corruption consequences, so the highest defensible integrity impact is high.\nA:H - The documented reproduction triggers a kernel allocator warning from attacker-controlled SMB activity, and such kernel warnings/oops conditions can crash or destabilize the server, especially with panic-on-warn configurations."
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-08-05T12:24:40.999Z",
        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
      },
      "references": [
        {
          "url": "https://git.kernel.org/stable/c/063a7409b0de46d7c770b65bb0338e6fdb3b1f0a"
        },
        {
          "url": "https://git.kernel.org/stable/c/3e5360b422dd741cb315654a191fa73869a37414"
        },
        {
          "url": "https://git.kernel.org/stable/c/59c32abaaec9cdd6164811c7e864e72f7554b82d"
        },
        {
          "url": "https://git.kernel.org/stable/c/3e4e2ea2a781018ed5d75f969e3e5606beb66e48"
        }
      ],
      "title": "ksmbd: validate num_aces and harden ACE walk in smb_inherit_dacl()",
      "x_generator": {
        "engine": "bippy-1.2.0"
      }
    }
  },
  "cveMetadata": {
    "assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
    "assignerShortName": "Linux",
    "cveId": "CVE-2026-31706",
    "datePublished": "2026-05-01T13:56:04.552Z",
    "dateReserved": "2026-03-09T15:48:24.132Z",
    "dateUpdated": "2026-08-05T12:24:40.999Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2"
}


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