cifs: check for bytes_remaining going to zero in CIFS_SessSetup

It's possible that when we go to decode the string area in the
SESSION_SETUP response, that bytes_remaining will be 0. Decrementing it at
that point will mean that it can go "negative" and wrap. Check for a
bytes_remaining value of 0, and don't try to decode the string area if
that's the case.

Cc: stable@kernel.org
Reported-and-Acked-by: NDavid Howells <dhowells@redhat.com>
Signed-off-by: NJeff Layton <jlayton@redhat.com>
Signed-off-by: NSteve French <sfrench@us.ibm.com>

fs/cifs/sess.c

@@ -916,7 +916,9 @@ CIFS_SessSetup(unsigned int xid, struct cifsSesInfo *ses,
 	}
 
 	/* BB check if Unicode and decode strings */
-	if (smb_buf->Flags2 & SMBFLG2_UNICODE) {
+	if (bytes_remaining == 0) {
+		/* no string area to decode, do nothing */
+	} else if (smb_buf->Flags2 & SMBFLG2_UNICODE) {
 		/* unicode string area must be word-aligned */
 		if (((unsigned long) bcc_ptr - (unsigned long) smb_buf) % 2) {
 			++bcc_ptr;