crypto/tls: limit number of consecutive warning alerts

In the current implementation, it is possible for a client to
continuously send warning alerts, which are just dropped on the floor
inside readRecord.

This can enable scenarios in where someone can try to continuously
send warning alerts to the server just to keep it busy.

This CL implements a simple counter that triggers an error if
we hit the warning alert limit.

Fixes #22543

Change-Id: Ief0ca10308cf5a4dea21a5a67d3e8f6501912da6
Reviewed-on: https://go-review.googlesource.com/75750
Reviewed-by: Adam Langley <agl@golang.org>
Reviewed-by: Filippo Valsorda <hi@filippo.io>
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

common.go

@@ -33,6 +33,7 @@ const (
 	maxCiphertext     = 16384 + 2048 // maximum ciphertext payload length
 	recordHeaderLen   = 5            // record header length
 	maxHandshake      = 65536        // maximum handshake we support (protocol max is 16 MB)
+	maxWarnAlertCount = 5            // maximum number of consecutive warning alerts
 
 	minVersion = VersionTLS10
 	maxVersion = VersionTLS12

conn.go

@@ -94,6 +94,10 @@ type Conn struct {
 	bytesSent   int64
 	packetsSent int64
 
+	// warnCount counts the number of consecutive warning alerts received
+	// by Conn.readRecord. Protected by in.Mutex.
+	warnCount int
+
 	// activeCall is an atomic int32; the low bit is whether Close has
 	// been called. the rest of the bits are the number of goroutines
 	// in Conn.Write.
@@ -658,6 +662,11 @@ Again:
 		return c.in.setErrorLocked(err)
 	}
 
+	if typ != recordTypeAlert && len(data) > 0 {
+		// this is a valid non-alert message: reset the count of alerts
+		c.warnCount = 0
+	}
+
 	switch typ {
 	default:
 		c.in.setErrorLocked(c.sendAlert(alertUnexpectedMessage))
@@ -675,6 +684,13 @@ Again:
 		case alertLevelWarning:
 			// drop on the floor
 			c.in.freeBlock(b)
+
+			c.warnCount++
+			if c.warnCount > maxWarnAlertCount {
+				c.sendAlert(alertUnexpectedMessage)
+				return c.in.setErrorLocked(errors.New("tls: too many warn alerts"))
+			}
+
 			goto Again
 		case alertLevelError:
 			c.in.setErrorLocked(&net.OpError{Op: "remote error", Err: alert(data[1])})

tls_test.go

@@ -566,6 +566,58 @@ func TestConnCloseWrite(t *testing.T) {
 	}
 }
 
+func TestWarningAlertFlood(t *testing.T) {
+	ln := newLocalListener(t)
+	defer ln.Close()
+
+	server := func() error {
+		sconn, err := ln.Accept()
+		if err != nil {
+			return fmt.Errorf("accept: %v", err)
+		}
+		defer sconn.Close()
+
+		serverConfig := testConfig.Clone()
+		srv := Server(sconn, serverConfig)
+		if err := srv.Handshake(); err != nil {
+			return fmt.Errorf("handshake: %v", err)
+		}
+		defer srv.Close()
+
+		_, err = ioutil.ReadAll(srv)
+		if err == nil {
+			return errors.New("unexpected lack of error from server")
+		}
+		const expected = "too many warn"
+		if str := err.Error(); !strings.Contains(str, expected) {
+			return fmt.Errorf("expected error containing %q, but saw: %s", expected, str)
+		}
+
+		return nil
+	}
+
+	errChan := make(chan error, 1)
+	go func() { errChan <- server() }()
+
+	clientConfig := testConfig.Clone()
+	conn, err := Dial("tcp", ln.Addr().String(), clientConfig)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer conn.Close()
+	if err := conn.Handshake(); err != nil {
+		t.Fatal(err)
+	}
+
+	for i := 0; i < maxWarnAlertCount+1; i++ {
+		conn.sendAlert(alertNoRenegotiation)
+	}
+
+	if err := <-errChan; err != nil {
+		t.Fatal(err)
+	}
+}
+
 func TestCloneFuncFields(t *testing.T) {
 	const expectedCount = 5
 	called := 0