CVE-2026-68097: ksmbd: validate ACE size against SID sub-authorities

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

ksmbd: validate ACE size against SID sub-authorities

setntacldacl() validates sid.numsubauth before copying an ACE, but does not verify that the declared ACE size contains all sub-authorities described by that field. An undersized ACE can therefore be copied and later make the POSIX ACL deduplication walk inspect data beyond the copied ACE boundary.

The existing initial bound check is also too small. It only ensures that the ACE size field is accessible before setntacldacl() reads sid.numsubauth farther into the input buffer.

Require enough input for the fixed SID header before accessing numsubauth, reject ACEs smaller than that header, and skip ACEs whose declared size cannot contain the complete SID. This makes the validation consistent with the other ACE walk paths.