CVE-2026-63909: ksmbd: OOB read regression in smb_check_perm_dacl() ACE-walk loops

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

ksmbd: OOB read regression in smbcheckpermdacl() ACE-walk loops

Commit d07b26f39246 ("ksmbd: require minimum ACE size in smbcheckpermdacl()") introduced a transposed bounds check:

if (offsetof(struct smbace, sid) + acessize < CIFSSIDBASESIZE)

Since offsetof(..sid) is 8 and CIFSSIDBASESIZE is 8, this evaluates to acessize < 0. Because acessize is always non-negative, this check becomes dead code and never breaks the loop.

Worse, that commit removed the old 4-byte guard, meaning the loop now reads ace->size (offset 2) even when acessize is 0-3 bytes. This re-opens a 2-byte heap out-of-bounds (OOB) read past the pntsd allocation during subsequent SMB2CREATE operations.

Fix this by properly transposing the comparison to require at least 16 bytes (8-byte offset + 8-byte SID base), matching the correct form used in smbinheritdacl().