enhance convid security and fix a race of random number generator

sess.go

@@ -20,7 +20,6 @@ import (
 var (
 	errTimeout    = errors.New("i/o timeout")
 	errBrokenPipe = errors.New("broken pipe")
-	rng           = rand.New(rand.NewSource(time.Now().UnixNano()))
 )
 
 const (
@@ -58,6 +57,7 @@ type (
 		ackNoDelay        bool
 		keepAliveInterval time.Duration
 		xmitBuf           sync.Pool
+		rng               *rand.Rand
 	}
 )
 
@@ -76,6 +76,7 @@ func newUDPSession(conv uint32, dataShards, parityShards int, l *Listener, conn
 	sess.l = l
 	sess.block = block
 	sess.fec = newFEC(rxFecLimit, dataShards, parityShards)
+	sess.rng = rand.New(rand.NewSource(time.Now().UnixNano()))
 	sess.xmitBuf.New = func() interface{} {
 		return make([]byte, mtuLimit)
 	}
@@ -452,7 +453,7 @@ func (s *UDPSession) outputTask() {
 				interval := s.keepAliveInterval
 				s.mu.Unlock()
 				if interval > 0 && time.Now().After(lastPing.Add(interval)) {
-					sz := rng.Intn(IKCP_MTU_DEF - s.headerSize - IKCP_OVERHEAD)
+					sz := s.rng.Intn(IKCP_MTU_DEF - s.headerSize - IKCP_OVERHEAD)
 					sz += s.headerSize + IKCP_OVERHEAD
 					ping := make([]byte, sz)
 					io.ReadFull(crand.Reader, ping)
@@ -824,7 +825,10 @@ func DialWithOptions(raddr string, block BlockCrypt, dataShards, parityShards in
 	if err != nil {
 		return nil, err
 	}
-	return newUDPSession(rng.Uint32(), dataShards, parityShards, nil, udpconn, udpaddr, block), nil
+
+	convid := make([]byte, 4)
+	io.ReadFull(crand.Reader, convid)
+	return newUDPSession(binary.LittleEndian.Uint32(convid), dataShards, parityShards, nil, udpconn, udpaddr, block), nil
 }
 
 func currentMs() uint32 {