CVE-2026-31611

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

ksmbd: require 3 sub-authorities before reading sub_auth[2]

parse_dacl() compares each ACE SID against sid_unix_NFS_mode and on
match reads sid.sub_auth[2] as the file mode.  If sid_unix_NFS_mode is
the prefix S-1-5-88-3 with num_subauth = 2 then compare_sids() compares
only min(num_subauth, 2) sub-authorities so a client SID with
num_subauth = 2 and sub_auth = {88, 3} will match.

If num_subauth = 2 and the ACE is placed at the very end of the security
descriptor, sub_auth[2] will be  4 bytes past end_of_acl.  The
out-of-band bytes will then be masked to the low 9 bits and applied as
the file's POSIX mode, probably not something that is good to have
happen.

Fix this up by forcing the SID to actually carry a third sub-authority
before reading it at all.