crypto/tls: advertise correct ciphers in TLS 1.3 only mode

This change updates the makeClientHello logic to only advertise TLS
1.3 ciphers when tls.Config.MinVersion is set to tls.VersionTLS13
(i.e the client only supports TLS 1.3).  Previously, TLS 1.2 ciphers
would be included in the client hello message.

Fixes #57771

Change-Id: Ife4123037b0a4609578ffffb1cdf1e1d4e0a8df6
GitHub-Last-Rev: 45f4275aa9b9550e519e1be5c337b53ab8882007
GitHub-Pull-Request: golang/go#49293
Reviewed-on: https://go-review.googlesource.com/c/go/+/360794
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Marten Seemann <martenseemann@gmail.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

handshake_client.go

@@ -133,6 +133,10 @@ func (c *Conn) makeClientHello() (*clientHelloMsg, *ecdh.PrivateKey, error) {
 
 	var key *ecdh.PrivateKey
 	if hello.supportedVersions[0] == VersionTLS13 {
+		// Reset the list of ciphers when the client only supports TLS 1.3.
+		if len(hello.supportedVersions) == 1 {
+			hello.cipherSuites = nil
+		}
 		if hasAESGCMHardwareSupport {
 			hello.cipherSuites = append(hello.cipherSuites, defaultCipherSuitesTLS13...)
 		} else {

handshake_client_test.go

@@ -2658,3 +2658,66 @@ func TestClientHandshakeContextCancellation(t *testing.T) {
 		t.Error("Client connection was not closed when the context was canceled")
 	}
 }
+
+// TestTLS13OnlyClientHelloCipherSuite tests that when a client states that
+// it only supports TLS 1.3, it correctly advertises only TLS 1.3 ciphers.
+func TestTLS13OnlyClientHelloCipherSuite(t *testing.T) {
+	tls13Tests := []struct {
+		name    string
+		ciphers []uint16
+	}{
+		{
+			name:    "nil",
+			ciphers: nil,
+		},
+		{
+			name:    "empty",
+			ciphers: []uint16{},
+		},
+		{
+			name:    "some TLS 1.2 cipher",
+			ciphers: []uint16{TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256},
+		},
+		{
+			name:    "some TLS 1.3 cipher",
+			ciphers: []uint16{TLS_AES_128_GCM_SHA256},
+		},
+		{
+			name:    "some TLS 1.2 and 1.3 ciphers",
+			ciphers: []uint16{TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384, TLS_AES_256_GCM_SHA384},
+		},
+	}
+	for _, tt := range tls13Tests {
+		tt := tt
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+			testTLS13OnlyClientHelloCipherSuite(t, tt.ciphers)
+		})
+	}
+}
+
+func testTLS13OnlyClientHelloCipherSuite(t *testing.T, ciphers []uint16) {
+	serverConfig := &Config{
+		Certificates: testConfig.Certificates,
+		GetConfigForClient: func(chi *ClientHelloInfo) (*Config, error) {
+			if len(chi.CipherSuites) != len(defaultCipherSuitesTLS13NoAES) {
+				t.Errorf("only TLS 1.3 suites should be advertised, got=%x", chi.CipherSuites)
+			} else {
+				for i := range defaultCipherSuitesTLS13NoAES {
+					if want, got := defaultCipherSuitesTLS13NoAES[i], chi.CipherSuites[i]; want != got {
+						t.Errorf("cipher at index %d does not match, want=%x, got=%x", i, want, got)
+					}
+				}
+			}
+			return nil, nil
+		},
+	}
+	clientConfig := &Config{
+		MinVersion:         VersionTLS13, // client only supports TLS 1.3
+		CipherSuites:       ciphers,
+		InsecureSkipVerify: true,
+	}
+	if _, _, err := testHandshake(t, clientConfig, serverConfig); err != nil {
+		t.Fatalf("handshake failed: %s", err)
+	}
+}