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>

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