CVE-2026-31712

In the Linux kernel, the following vulnerability has been resolved:

ksmbd: require minimum ACE size in smb_check_perm_dacl()

Both ACE-walk loops in smb_check_perm_dacl() only guard against an

under-sized remaining buffer, not against an ACE whose declared

`ace->size` is smaller than the struct it claims to describe:

if (offsetof(struct smb_ace, access_req) > aces_size)

break;

ace_size = le16_to_cpu(ace->size);

if (ace_size > aces_size)

break;

The first check only requires the 4-byte ACE header to be in bounds;

it does not require access_req (4 bytes at offset 4) to be readable.

An attacker who has set a crafted DACL on a file they own can declare

ace->size == 4 with aces_size == 4, pass both checks, and then

granted |= le32_to_cpu(ace->access_req); /* upper loop */

compare_sids(&sid, &ace->sid); /* lower loop */

reads access_req at offset 4 (OOB by up to 4 bytes) and ace->sid at

offset 8 (OOB by up to CIFS_SID_BASE_SIZE + SID_MAX_SUB_AUTHORITIES

* 4 bytes).

Tighten both loops to require

ace_size >= offsetof(struct smb_ace, sid) + CIFS_SID_BASE_SIZE

which is the smallest valid on-wire ACE layout (4-byte header +

4-byte access_req + 8-byte sid base with zero sub-auths). Also

reject ACEs whose sid.num_subauth exceeds SID_MAX_SUB_AUTHORITIES

before letting compare_sids() dereference sub_auth[] entries.

parse_sec_desc() already enforces an equivalent check (lines 441-448);

smb_check_perm_dacl() simply grew weaker validation over time.

Reachability: authenticated SMB client with permission to set an ACL

on a file. On a subsequent CREATE against that file, the kernel

walks the stored DACL via smb_check_perm_dacl() and triggers the

OOB read. Not pre-auth, and the OOB read is not reflected to the

attacker, but KASAN reports and kernel state corruption are

possible.