smb3: prevent races updating CurrentMid

There was one place where we weren't locking CurrentMid, and although
likely to be safe since even without the lock since it is during
negotiate protocol, it is more consistent to lock it in this last remaining
place, and avoids confusing Coverity warning.

Addresses-Coverity: 1486665 ("Data race condition")
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/cifsglob.h

@@ -621,7 +621,7 @@ struct TCP_Server_Info {
 	/* SMB_COM_WRITE_RAW or SMB_COM_READ_RAW. */
 	unsigned int capabilities; /* selective disabling of caps by smb sess */
 	int timeAdj;  /* Adjust for difference in server time zone in sec */
-	__u64 CurrentMid;         /* multiplex id - rotating counter */
+	__u64 CurrentMid;         /* multiplex id - rotating counter, protected by GlobalMid_Lock */
 	char cryptkey[CIFS_CRYPTO_KEY_SIZE]; /* used by ntlm, ntlmv2 etc */
 	/* 16th byte of RFC1001 workstation name is always null */
 	char workstation_RFC1001_name[RFC1001_NAME_LEN_WITH_NULL];
@@ -1786,6 +1786,7 @@ require use of the stronger protocol */
  *      updates to XID counters, multiplex id  and SMB sequence numbers
  *      list operations on global DnotifyReqList
  *      updates to ses->status
+ *      updates to server->CurrentMid
  *  tcp_ses_lock protects:
  *	list operations on tcp and SMB session lists
  *  tcon->open_file_lock protects the list of open files hanging off the tcon

fs/cifs/smb2ops.c

@@ -388,7 +388,9 @@ smb2_negotiate(const unsigned int xid, struct cifs_ses *ses)
 {
 	int rc;
 
+	spin_lock(&GlobalMid_Lock);
 	cifs_ses_server(ses)->CurrentMid = 0;
+	spin_unlock(&GlobalMid_Lock);
 	rc = SMB2_negotiate(xid, ses);
 	/* BB we probably don't need to retry with modern servers */
 	if (rc == -EAGAIN)