CIFS: Do not reconnect TCP session in add_credits()

When executing add_credits() we currently call cifs_reconnect()
if the number of credits is zero and there are no requests in
flight. In this case we may call cifs_reconnect() recursively
twice and cause memory corruption given the following sequence
of functions:

mid1.callback() -> add_credits() -> cifs_reconnect() ->
-> mid2.callback() -> add_credits() -> cifs_reconnect().

Fix this by avoiding to call cifs_reconnect() in add_credits()
and checking for zero credits in the demultiplex thread.

Cc: <stable@vger.kernel.org>
Signed-off-by: Pavel Shilovsky <pshilov@microsoft.com>
Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/connect.c

@@ -720,6 +720,21 @@ server_unresponsive(struct TCP_Server_Info *server)
 	return false;
 }
 
+static inline bool
+zero_credits(struct TCP_Server_Info *server)
+{
+	int val;
+
+	spin_lock(&server->req_lock);
+	val = server->credits + server->echo_credits + server->oplock_credits;
+	if (server->in_flight == 0 && val == 0) {
+		spin_unlock(&server->req_lock);
+		return true;
+	}
+	spin_unlock(&server->req_lock);
+	return false;
+}
+
 static int
 cifs_readv_from_socket(struct TCP_Server_Info *server, struct msghdr *smb_msg)
 {
@@ -732,6 +747,12 @@ cifs_readv_from_socket(struct TCP_Server_Info *server, struct msghdr *smb_msg)
 	for (total_read = 0; msg_data_left(smb_msg); total_read += length) {
 		try_to_freeze();
 
+		/* reconnect if no credits and no requests in flight */
+		if (zero_credits(server)) {
+			cifs_reconnect(server);
+			return -ECONNABORTED;
+		}
+
 		if (server_unresponsive(server))
 			return -ECONNABORTED;
 		if (cifs_rdma_enabled(server) && server->smbd_conn)