CIFS: avoid using MID 0xFFFF

According to MS-CIFS specification MID 0xFFFF should not be used by the
CIFS client, but we actually do. Besides, this has proven to cause races
leading to oops between SendReceive2/cifs_demultiplex_thread. On SMB1,
MID is a 2 byte value easy to reach in CurrentMid which may conflict with
an oplock break notification request coming from server

Signed-off-by: Roberto Bergantinos Corpas <[email protected]>
Reviewed-by: Ronnie Sahlberg <[email protected]>
Reviewed-by: Aurelien Aptel <[email protected]>
Signed-off-by: Steve French <[email protected]>
CC: Stable <[email protected]>

Author: Roberto Bergantinos Corpas

fs/cifs/smb1ops.c

@@ -171,6 +171,9 @@ cifs_get_next_mid(struct TCP_Server_Info *server)
 	/* we do not want to loop forever */
 	last_mid = cur_mid;
 	cur_mid++;
+	/* avoid 0xFFFF MID */
+	if (cur_mid == 0xffff)
+		cur_mid++;
 
 	/*
 	 * This nested loop looks more expensive than it is.