GHSA-8PQ6-4W7P-QW4G
Vulnerability from github – Published: 2026-05-01 15:30 – Updated: 2026-05-03 09:33In 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 kmalloclarge_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:
-
Validating num_aces against pdacl_size using the same formula applied in parse_dacl().
-
Replacing the raw kmalloc(sizeof * num_aces * 2) with kmalloc_array(num_aces * 2, sizeof(...)) for overflow-safe allocation.
-
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.
{
"affected": [],
"aliases": [
"CVE-2026-31706"
],
"database_specific": {
"cwe_ids": [],
"github_reviewed": false,
"github_reviewed_at": null,
"nvd_published_at": "2026-05-01T14:16:20Z",
"severity": "HIGH"
},
"details": "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": "GHSA-8pq6-4w7p-qw4g",
"modified": "2026-05-03T09:33:09Z",
"published": "2026-05-01T15:30:33Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-31706"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/063a7409b0de46d7c770b65bb0338e6fdb3b1f0a"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/3e4e2ea2a781018ed5d75f969e3e5606beb66e48"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/3e5360b422dd741cb315654a191fa73869a37414"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/59c32abaaec9cdd6164811c7e864e72f7554b82d"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H",
"type": "CVSS_V3"
}
]
}
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