crypto/tls: Added dynamic alternative to NameToCertificate map for SNI

Revised version of https://golang.org/cl/81260045/

LGTM=agl
R=golang-codereviews, gobot, agl, ox
CC=golang-codereviews
https://golang.org/cl/107400043

src/pkg/crypto/tls/common.go

@@ -202,6 +202,32 @@ type ClientSessionCache interface {
 	Put(sessionKey string, cs *ClientSessionState)
 }
 
+// ClientHelloInfo contains information from a ClientHello message in order to
+// guide certificate selection in the GetCertificate callback.
+type ClientHelloInfo struct {
+	// CipherSuites lists the CipherSuites supported by the client (e.g.
+	// TLS_RSA_WITH_RC4_128_SHA).
+	CipherSuites []uint16
+
+	// ServerName indicates the name of the server requested by the client
+	// in order to support virtual hosting. ServerName is only set if the
+	// client is using SNI (see
+	// http://tools.ietf.org/html/rfc4366#section-3.1).
+	ServerName string
+
+	// SupportedCurves lists the elliptic curves supported by the client.
+	// SupportedCurves is set only if the Supported Elliptic Curves
+	// Extension is being used (see
+	// http://tools.ietf.org/html/rfc4492#section-5.1.1).
+	SupportedCurves []CurveID
+
+	// SupportedPoints lists the point formats supported by the client.
+	// SupportedPoints is set only if the Supported Point Formats Extension
+	// is being used (see
+	// http://tools.ietf.org/html/rfc4492#section-5.1.2).
+	SupportedPoints []uint8
+}
+
 // A Config structure is used to configure a TLS client or server.
 // After one has been passed to a TLS function it must not be
 // modified. A Config may be reused; the tls package will also not
@@ -230,6 +256,13 @@ type Config struct {
 	// for all connections.
 	NameToCertificate map[string]*Certificate
 
+	// GetCertificate returns a Certificate based on the given
+	// ClientHelloInfo. If GetCertificate is nil or returns nil, then the
+	// certificate is retrieved from NameToCertificate. If
+	// NameToCertificate is nil, the first element of Certificates will be
+	// used.
+	GetCertificate func(clientHello *ClientHelloInfo) (*Certificate, error)
+
 	// RootCAs defines the set of root certificate authorities
 	// that clients use when verifying server certificates.
 	// If RootCAs is nil, TLS uses the host's root CA set.
@@ -384,22 +417,28 @@ func (c *Config) mutualVersion(vers uint16) (uint16, bool) {
 	return vers, true
 }
 
-// getCertificateForName returns the best certificate for the given name,
-// defaulting to the first element of c.Certificates if there are no good
-// options.
-func (c *Config) getCertificateForName(name string) *Certificate {
-	if len(c.Certificates) == 1 || c.NameToCertificate == nil {
-		// There's only one choice, so no point doing any work.
-		return &c.Certificates[0]
+// getCertificate returns the best certificate for the given ClientHelloInfo,
+// defaulting to the first element of c.Certificates.
+func (c *Config) getCertificate(clientHello *ClientHelloInfo) (*Certificate, error) {
+	if c.GetCertificate != nil {
+		cert, err := c.GetCertificate(clientHello)
+		if cert != nil || err != nil {
+			return cert, err
+		}
 	}
 
-	name = strings.ToLower(name)
+	if len(c.Certificates) == 1 || c.NameToCertificate == nil {
+		// There's only one choice, so no point doing any work.
+		return &c.Certificates[0], nil
+	}
+
+	name := strings.ToLower(clientHello.ServerName)
 	for len(name) > 0 && name[len(name)-1] == '.' {
 		name = name[:len(name)-1]
 	}
 
 	if cert, ok := c.NameToCertificate[name]; ok {
-		return cert
+		return cert, nil
 	}
 
 	// try replacing labels in the name with wildcards until we get a
@@ -409,12 +448,12 @@ func (c *Config) getCertificateForName(name string) *Certificate {
 		labels[i] = "*"
 		candidate := strings.Join(labels, ".")
 		if cert, ok := c.NameToCertificate[candidate]; ok {
-			return cert
+			return cert, nil
 		}
 	}
 
 	// If nothing matches, return the first certificate.
-	return &c.Certificates[0]
+	return &c.Certificates[0], nil
 }
 
 // BuildNameToCertificate parses c.Certificates and builds c.NameToCertificate

src/pkg/crypto/tls/conn_test.go

@@ -88,19 +88,31 @@ func TestCertificateSelection(t *testing.T) {
 		return -1
 	}
 
-	if n := pointerToIndex(config.getCertificateForName("example.com")); n != 0 {
+	certificateForName := func(name string) *Certificate {
+		clientHello := &ClientHelloInfo{
+			ServerName: name,
+		}
+		if cert, err := config.getCertificate(clientHello); err != nil {
+			t.Errorf("unable to get certificate for name '%s': %s", name, err)
+			return nil
+		} else {
+			return cert
+		}
+	}
+
+	if n := pointerToIndex(certificateForName("example.com")); n != 0 {
 		t.Errorf("example.com returned certificate %d, not 0", n)
 	}
-	if n := pointerToIndex(config.getCertificateForName("bar.example.com")); n != 1 {
+	if n := pointerToIndex(certificateForName("bar.example.com")); n != 1 {
 		t.Errorf("bar.example.com returned certificate %d, not 1", n)
 	}
-	if n := pointerToIndex(config.getCertificateForName("foo.example.com")); n != 2 {
+	if n := pointerToIndex(certificateForName("foo.example.com")); n != 2 {
 		t.Errorf("foo.example.com returned certificate %d, not 2", n)
 	}
-	if n := pointerToIndex(config.getCertificateForName("foo.bar.example.com")); n != 3 {
+	if n := pointerToIndex(certificateForName("foo.bar.example.com")); n != 3 {
 		t.Errorf("foo.bar.example.com returned certificate %d, not 3", n)
 	}
-	if n := pointerToIndex(config.getCertificateForName("foo.bar.baz.example.com")); n != 0 {
+	if n := pointerToIndex(certificateForName("foo.bar.baz.example.com")); n != 0 {
 		t.Errorf("foo.bar.baz.example.com returned certificate %d, not 0", n)
 	}
 }

src/pkg/crypto/tls/handshake_server.go

@@ -186,7 +186,16 @@ Curves:
 	}
 	hs.cert = &config.Certificates[0]
 	if len(hs.clientHello.serverName) > 0 {
-		hs.cert = config.getCertificateForName(hs.clientHello.serverName)
+		chi := &ClientHelloInfo{
+			CipherSuites:    hs.clientHello.cipherSuites,
+			ServerName:      hs.clientHello.serverName,
+			SupportedCurves: hs.clientHello.supportedCurves,
+			SupportedPoints: hs.clientHello.supportedPoints,
+		}
+		if hs.cert, err = config.getCertificate(chi); err != nil {
+			c.sendAlert(alertInternalError)
+			return false, err
+		}
 	}
 
 	_, hs.ecdsaOk = hs.cert.PrivateKey.(*ecdsa.PrivateKey)

src/pkg/crypto/tls/handshake_server_test.go

@@ -257,6 +257,9 @@ type serverTest struct {
 	expectedPeerCerts []string
 	// config, if not nil, contains a custom Config to use for this test.
 	config *Config
+	// expectAlert, if true, indicates that a fatal alert should be returned
+	// when handshaking with the server.
+	expectAlert bool
 	// validate, if not nil, is a function that will be called with the
 	// ConnectionState of the resulting connection. It returns false if the
 	// ConnectionState is unacceptable.
@@ -370,7 +373,9 @@ func (test *serverTest) run(t *testing.T, write bool) {
 	if !write {
 		flows, err := test.loadData()
 		if err != nil {
-			t.Fatalf("%s: failed to load data from %s", test.name, test.dataPath())
+			if !test.expectAlert {
+				t.Fatalf("%s: failed to load data from %s", test.name, test.dataPath())
+			}
 		}
 		for i, b := range flows {
 			if i%2 == 0 {
@@ -379,11 +384,17 @@ func (test *serverTest) run(t *testing.T, write bool) {
 			}
 			bb := make([]byte, len(b))
 			n, err := io.ReadFull(clientConn, bb)
-			if err != nil {
-				t.Fatalf("%s #%d: %s\nRead %d, wanted %d, got %x, wanted %x\n", test.name, i+1, err, n, len(bb), bb[:n], b)
-			}
-			if !bytes.Equal(b, bb) {
-				t.Fatalf("%s #%d: mismatch on read: got:%x want:%x", test.name, i+1, bb, b)
+			if test.expectAlert {
+				if err == nil {
+					t.Fatal("Expected read failure but read succeeded")
+				}
+			} else {
+				if err != nil {
+					t.Fatalf("%s #%d: %s\nRead %d, wanted %d, got %x, wanted %x\n", test.name, i+1, err, n, len(bb), bb[:n], b)
+				}
+				if !bytes.Equal(b, bb) {
+					t.Fatalf("%s #%d: mismatch on read: got:%x want:%x", test.name, i+1, bb, b)
+				}
 			}
 		}
 		clientConn.Close()
@@ -562,6 +573,61 @@ func TestHandshakeServerSNI(t *testing.T) {
 	runServerTestTLS12(t, test)
 }
 
+// TestHandshakeServerSNICertForName is similar to TestHandshakeServerSNI, but
+// tests the dynamic GetCertificate method
+func TestHandshakeServerSNIGetCertificate(t *testing.T) {
+	config := *testConfig
+
+	// Replace the NameToCertificate map with a GetCertificate function
+	nameToCert := config.NameToCertificate
+	config.NameToCertificate = nil
+	config.GetCertificate = func(clientHello *ClientHelloInfo) (*Certificate, error) {
+		cert, _ := nameToCert[clientHello.ServerName]
+		return cert, nil
+	}
+	test := &serverTest{
+		name:    "SNI",
+		command: []string{"openssl", "s_client", "-no_ticket", "-cipher", "AES128-SHA", "-servername", "snitest.com"},
+		config:  &config,
+	}
+	runServerTestTLS12(t, test)
+}
+
+// TestHandshakeServerSNICertForNameNotFound is similar to
+// TestHandshakeServerSNICertForName, but tests to make sure that when the
+// GetCertificate method doesn't return a cert, we fall back to what's in
+// the NameToCertificate map.
+func TestHandshakeServerSNIGetCertificateNotFound(t *testing.T) {
+	config := *testConfig
+
+	config.GetCertificate = func(clientHello *ClientHelloInfo) (*Certificate, error) {
+		return nil, nil
+	}
+	test := &serverTest{
+		name:    "SNI",
+		command: []string{"openssl", "s_client", "-no_ticket", "-cipher", "AES128-SHA", "-servername", "snitest.com"},
+		config:  &config,
+	}
+	runServerTestTLS12(t, test)
+}
+
+// TestHandshakeServerSNICertForNameError tests to make sure that errors in
+// GetCertificate result in a tls alert.
+func TestHandshakeServerSNIGetCertificateError(t *testing.T) {
+	config := *testConfig
+
+	config.GetCertificate = func(clientHello *ClientHelloInfo) (*Certificate, error) {
+		return nil, fmt.Errorf("Test error in GetCertificate")
+	}
+	test := &serverTest{
+		name:        "SNI",
+		command:     []string{"openssl", "s_client", "-no_ticket", "-cipher", "AES128-SHA", "-servername", "snitest.com"},
+		config:      &config,
+		expectAlert: true,
+	}
+	runServerTestTLS12(t, test)
+}
+
 // TestCipherSuiteCertPreferance ensures that we select an RSA ciphersuite with
 // an RSA certificate and an ECDSA ciphersuite with an ECDSA certificate.
 func TestCipherSuiteCertPreferenceECDSA(t *testing.T) {