CVE-2026-31706

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.