enforce a message size limit

so we do not allocate loads of memory when receiving (accidental or
deliberate) garbage

router/consts.go

@@ -25,6 +25,7 @@ const (
 	MaxDuration         = time.Duration(math.MaxInt64)
 	MaxMissedHeartbeats = 6
 	HeartbeatTimeout    = MaxMissedHeartbeats * SlowHeartbeat
+	MaxTCPMsgSize       = 10 * 1024 * 1024
 )
 
 var (

router/crypto.go

@@ -361,6 +361,9 @@ func NewLengthPrefixTCPSender(writer io.Writer) *LengthPrefixTCPSender {
 
 func (sender *LengthPrefixTCPSender) Send(msg []byte) error {
 	l := len(msg)
+	if l > MaxTCPMsgSize {
+		return fmt.Errorf("outgoing message exceeds maximum size: %d > %d", l, MaxTCPMsgSize)
+	}
 	prefixedMsg := make([]byte, 4+l)
 	binary.BigEndian.PutUint32(prefixedMsg, uint32(l))
 	copy(prefixedMsg[4:], msg)
@@ -417,6 +420,9 @@ func (receiver *LengthPrefixTCPReceiver) Receive() ([]byte, error) {
 		return nil, err
 	}
 	l := binary.BigEndian.Uint32(lenPrefix)
+	if l > MaxTCPMsgSize {
+		return nil, fmt.Errorf("incoming message exceeds maximum size: %d > %d", l, MaxTCPMsgSize)
+	}
 	msg := make([]byte, l)
 	_, err := io.ReadFull(receiver.reader, msg)
 	return msg, err