CVE-2026-31712 — ksmbd: require minimum ACE size in smb_check_perm_dacl()

Affected: Linux

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 w…

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CVE-2026-31712 — ksmbd: require minimum ACE size in smb_check_perm_dacl()