cmd/stunstamp: validate STUN tx ID in responses (#12339)

Extremely late arriving responses may leak across probing intervals.

Updates tailscale/corp#20344

Signed-off-by: Jordan Whited <jordan@tailscale.com>

cmd/stunstamp/stunstamp.go

@@ -94,27 +94,36 @@ type result struct {
 	rtt             *time.Duration // nil signifies failure, e.g. timeout
 }
 
-func measureRTT(conn io.ReadWriteCloser, dst *net.UDPAddr, req []byte) (resp []byte, rtt time.Duration, err error) {
+func measureRTT(conn io.ReadWriteCloser, dst *net.UDPAddr) (rtt time.Duration, err error) {
 	uconn, ok := conn.(*net.UDPConn)
 	if !ok {
-		return nil, 0, fmt.Errorf("unexpected conn type: %T", conn)
+		return 0, fmt.Errorf("unexpected conn type: %T", conn)
 	}
 	err = uconn.SetReadDeadline(time.Now().Add(time.Second * 2))
 	if err != nil {
-		return nil, 0, fmt.Errorf("error setting read deadline: %w", err)
+		return 0, fmt.Errorf("error setting read deadline: %w", err)
 	}
+	txID := stun.NewTxID()
+	req := stun.Request(txID)
 	txAt := time.Now()
 	_, err = uconn.WriteToUDP(req, dst)
 	if err != nil {
-		return nil, 0, fmt.Errorf("error writing to udp socket: %w", err)
+		return 0, fmt.Errorf("error writing to udp socket: %w", err)
 	}
 	b := make([]byte, 1460)
+	for {
 		n, err := uconn.Read(b)
 		rxAt := time.Now()
 		if err != nil {
-		return nil, 0, fmt.Errorf("error reading from udp socket: %w", err)
+			return 0, fmt.Errorf("error reading from udp socket: %w", err)
 		}
-	return b[:n], rxAt.Sub(txAt), nil
+		gotTxID, _, err := stun.ParseResponse(b[:n])
+		if err != nil || gotTxID != txID {
+			continue
+		}
+		return rxAt.Sub(txAt), nil
+	}
+
 }
 
 func isTemporaryOrTimeoutErr(err error) bool {
@@ -134,7 +143,7 @@ type nodeMeta struct {
 	addr       netip.Addr
 }
 
-type measureFn func(conn io.ReadWriteCloser, dst *net.UDPAddr, req []byte) (resp []byte, rtt time.Duration, err error)
+type measureFn func(conn io.ReadWriteCloser, dst *net.UDPAddr) (rtt time.Duration, err error)
 
 func probe(meta nodeMeta, conn io.ReadWriteCloser, fn measureFn) (*time.Duration, error) {
 	ua := &net.UDPAddr{
@@ -142,25 +151,14 @@ func probe(meta nodeMeta, conn io.ReadWriteCloser, fn measureFn) (*time.Duration
 		Port: 3478,
 	}
 
-	var (
-		resp []byte
-		rtt  time.Duration
-	)
-	txID := stun.NewTxID()
-	req := stun.Request(txID)
 	time.Sleep(time.Millisecond * time.Duration(rand.Intn(200))) // jitter across tx
-	resp, rtt, err := fn(conn, ua, req)
+	rtt, err := fn(conn, ua)
 	if err != nil {
 		if isTemporaryOrTimeoutErr(err) {
 			log.Printf("temp error measuring RTT to %s(%s): %v", meta.hostname, meta.addr, err)
 			return nil, nil
 		}
 	}
-	_, _, err = stun.ParseResponse(resp)
-	if err != nil {
-		log.Printf("invalid stun response from %s: %v", meta.hostname, err)
-		return nil, nil
-	}
 	return &rtt, nil
 }
 

cmd/stunstamp/stunstamp_default.go

@@ -16,8 +16,8 @@ func getConnKernelTimestamp() (io.ReadWriteCloser, error) {
 	return nil, errors.New("unimplemented")
 }
 
-func measureRTTKernel(conn io.ReadWriteCloser, dst *net.UDPAddr, req []byte) (resp []byte, rtt time.Duration, err error) {
+func measureRTTKernel(conn io.ReadWriteCloser, dst *net.UDPAddr) (rtt time.Duration, err error) {
-	return nil, 0, errors.New("unimplemented")
+	return 0, errors.New("unimplemented")
 }
 
 func supportsKernelTS() bool {

cmd/stunstamp/stunstamp_linux.go

@@ -15,6 +15,7 @@ import (
 
 	"github.com/mdlayher/socket"
 	"golang.org/x/sys/unix"
+	"tailscale.com/net/stun"
 )
 
 const (
@@ -55,10 +56,10 @@ func parseTimestampFromCmsgs(oob []byte) (time.Time, error) {
 	return time.Time{}, errors.New("failed to parse timestamp from cmsgs")
 }
 
-func measureRTTKernel(conn io.ReadWriteCloser, dst *net.UDPAddr, req []byte) (resp []byte, rtt time.Duration, err error) {
+func measureRTTKernel(conn io.ReadWriteCloser, dst *net.UDPAddr) (rtt time.Duration, err error) {
 	sconn, ok := conn.(*socket.Conn)
 	if !ok {
-		return nil, 0, fmt.Errorf("conn of unexpected type: %T", conn)
+		return 0, fmt.Errorf("conn of unexpected type: %T", conn)
 	}
 
 	var to unix.Sockaddr
@@ -75,9 +76,12 @@ func measureRTTKernel(conn io.ReadWriteCloser, dst *net.UDPAddr, req []byte) (re
 		copy(to.(*unix.SockaddrInet6).Addr[:], dst.IP)
 	}
 
+	txID := stun.NewTxID()
+	req := stun.Request(txID)
+
 	err = sconn.Sendto(context.Background(), req, 0, to)
 	if err != nil {
-		return nil, 0, fmt.Errorf("sendto error: %v", err) // don't wrap
+		return 0, fmt.Errorf("sendto error: %v", err) // don't wrap
 	}
 
 	txCtx, txCancel := context.WithTimeout(context.Background(), time.Second*2)
@@ -90,7 +94,7 @@ func measureRTTKernel(conn io.ReadWriteCloser, dst *net.UDPAddr, req []byte) (re
 	for {
 		n, oobn, _, _, err := sconn.Recvmsg(txCtx, buf, oob, unix.MSG_ERRQUEUE)
 		if err != nil {
-			return nil, 0, fmt.Errorf("recvmsg (MSG_ERRQUEUE) error: %v", err) // don't wrap
+			return 0, fmt.Errorf("recvmsg (MSG_ERRQUEUE) error: %v", err) // don't wrap
 		}
 
 		buf = buf[:n]
@@ -101,24 +105,37 @@ func measureRTTKernel(conn io.ReadWriteCloser, dst *net.UDPAddr, req []byte) (re
 		}
 		txAt, err = parseTimestampFromCmsgs(oob[:oobn])
 		if err != nil {
-			return nil, 0, fmt.Errorf("failed to get tx timestamp: %v", err) // don't wrap
+			return 0, fmt.Errorf("failed to get tx timestamp: %v", err) // don't wrap
 		}
 		break
 	}
 
 	rxCtx, rxCancel := context.WithTimeout(context.Background(), time.Second*2)
 	defer rxCancel()
+
+	for {
 		n, oobn, _, _, err := sconn.Recvmsg(rxCtx, buf, oob, 0)
 		if err != nil {
-		return nil, 0, fmt.Errorf("recvmsg error: %w", err) // wrap for timeout-related error unwrapping
+			return 0, fmt.Errorf("recvmsg error: %w", err) // wrap for timeout-related error unwrapping
+		}
+
+		gotTxID, _, err := stun.ParseResponse(buf[:n])
+		if err != nil || gotTxID != txID {
+			// Spin until we find the txID we sent. We may end up reading
+			// extremely late arriving responses from previous intervals. As
+			// such, we can't be certain if we're parsing the "current"
+			// response, so spin for parse errors too.
+			continue
 		}
 
 		rxAt, err := parseTimestampFromCmsgs(oob[:oobn])
 		if err != nil {
-		return nil, 0, fmt.Errorf("failed to get rx timestamp: %v", err) // don't wrap
+			return 0, fmt.Errorf("failed to get rx timestamp: %v", err) // don't wrap
+		}
+
+		return rxAt.Sub(txAt), nil
 	}
 
-	return buf[:n], rxAt.Sub(txAt), nil
 }
 
 func supportsKernelTS() bool {