crypto/tls: reject SNI values with a trailing dot.

SNI values may not include a trailing dot according to
https://tools.ietf.org/html/rfc6066#section-3. Although crypto/tls
handled this correctly as a client, it didn't reject this as a server.

This change makes sending an SNI value with a trailing dot a fatal
error.

Updates #18114.

Change-Id: Ib7897ab40e98d4a7a4646ff8469a55233621f631
Reviewed-on: https://go-review.googlesource.com/33904
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/crypto/tls/handshake_client.go

@@ -815,7 +815,7 @@ func hostnameInSNI(name string) string {
 	if net.ParseIP(host) != nil {
 		return ""
 	}
-	if len(name) > 0 && name[len(name)-1] == '.' {
+	for len(name) > 0 && name[len(name)-1] == '.' {
 		name = name[:len(name)-1]
 	}
 	return name

src/crypto/tls/handshake_messages.go

@@ -4,7 +4,10 @@
 
 package tls
 
-import "bytes"
+import (
+	"bytes"
+	"strings"
+)
 
 type clientHelloMsg struct {
 	raw                          []byte
@@ -393,6 +396,12 @@ func (m *clientHelloMsg) unmarshal(data []byte) bool {
 				}
 				if nameType == 0 {
 					m.serverName = string(d[:nameLen])
+					// An SNI value may not include a
+					// trailing dot. See
+					// https://tools.ietf.org/html/rfc6066#section-3.
+					if strings.HasSuffix(m.serverName, ".") {
+						return false
+					}
 					break
 				}
 				d = d[nameLen:]

src/crypto/tls/handshake_messages_test.go

@@ -8,6 +8,7 @@ import (
 	"bytes"
 	"math/rand"
 	"reflect"
+	"strings"
 	"testing"
 	"testing/quick"
 )
@@ -123,6 +124,9 @@ func (*clientHelloMsg) Generate(rand *rand.Rand, size int) reflect.Value {
 	}
 	if rand.Intn(10) > 5 {
 		m.serverName = randomString(rand.Intn(255), rand)
+		for strings.HasSuffix(m.serverName, ".") {
+			m.serverName = m.serverName[:len(m.serverName)-1]
+		}
 	}
 	m.ocspStapling = rand.Intn(10) > 5
 	m.supportedPoints = randomBytes(rand.Intn(5)+1, rand)

src/crypto/tls/handshake_server_test.go

@@ -137,6 +137,10 @@ func TestNoRC4ByDefault(t *testing.T) {
 	testClientHelloFailure(t, serverConfig, clientHello, "no cipher suite supported by both client and server")
 }
 
+func TestRejectSNIWithTrailingDot(t *testing.T) {
+	testClientHelloFailure(t, testConfig, &clientHelloMsg{vers: VersionTLS12, serverName: "foo.com."}, "unexpected message")
+}
+
 func TestDontSelectECDSAWithRSAKey(t *testing.T) {
 	// Test that, even when both sides support an ECDSA cipher suite, it
 	// won't be selected if the server's private key doesn't support it.