smb: client: require a full NFS mode SID before reading mode bits

parse_dacl() treats an ACE SID matching sid_unix_NFS_mode as an NFS
mode SID and reads sid.sub_auth[2] to recover the mode bits.

That assumes the ACE carries three subauthorities, but compare_sids()
only compares min(a, b) subauthorities.  A malicious server can return
an ACE with num_subauth = 2 and sub_auth[] = {88, 3}, which still
matches sid_unix_NFS_mode and then drives the sub_auth[2] read four
bytes past the end of the ACE.

Require num_subauth >= 3 before treating the ACE as an NFS mode SID.
This keeps the fix local to the special-SID mode path without changing
compare_sids() semantics for the rest of cifsacl.

Fixes: e2f8fbf ("cifs: get mode bits from special sid on stat")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

Author: mjbommar

fs/smb/client/cifsacl.c

@@ -879,6 +879,7 @@ static void parse_dacl(struct smb_acl *pdacl, char *end_of_acl,
 			dump_ace(ppace[i], end_of_dacl);
 #endif
 			if (mode_from_special_sid &&
+			    ppace[i]->sid.num_subauth >= 3 &&
 			    (compare_sids(&(ppace[i]->sid),
 					  &sid_unix_NFS_mode) == 0)) {
 				/*