crypto/tls: add Config.GetConfigForClient

GetConfigForClient allows the tls.Config to be updated on a per-client
basis.

Fixes #16066.
Fixes #15707.
Fixes #15699.

Change-Id: I2c675a443d557f969441226729f98502b38901ea
Reviewed-on: https://go-review.googlesource.com/30790
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

common.go

@@ -303,7 +303,27 @@ type Config struct {
 	// 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)
+	GetCertificate func(*ClientHelloInfo) (*Certificate, error)
+
+	// GetConfigForClient, if not nil, is called after a ClientHello is
+	// received from a client. It may return a non-nil Config in order to
+	// change the Config that will be used to handle this connection. If
+	// the returned Config is nil, the original Config will be used. The
+	// Config returned by this callback may not be subsequently modified.
+	//
+	// If GetConfigForClient is nil, the Config passed to Server() will be
+	// used for all connections.
+	//
+	// Uniquely for the fields in the returned Config, session ticket keys
+	// will be duplicated from the original Config if not set.
+	// Specifically, if SetSessionTicketKeys was called on the original
+	// config but not on the returned config then the ticket keys from the
+	// original config will be copied into the new config before use.
+	// Otherwise, if SessionTicketKey was set in the original config but
+	// not in the returned config then it will be copied into the returned
+	// config before use. If neither of those cases applies then the key
+	// material from the returned config will be used for session tickets.
+	GetConfigForClient func(*ClientHelloInfo) (*Config, error)
 
 	// RootCAs defines the set of root certificate authorities
 	// that clients use when verifying server certificates.
@@ -398,13 +418,17 @@ type Config struct {
 
 	serverInitOnce sync.Once // guards calling (*Config).serverInit
 
-	// mutex protects sessionTicketKeys
+	// mutex protects sessionTicketKeys and originalConfig.
 	mutex sync.RWMutex
 	// sessionTicketKeys contains zero or more ticket keys. If the length
 	// is zero, SessionTicketsDisabled must be true. The first key is used
 	// for new tickets and any subsequent keys can be used to decrypt old
 	// tickets.
 	sessionTicketKeys []ticketKey
+	// originalConfig is set to the Config that was passed to Server if
+	// this Config is returned by a GetConfigForClient callback. It's used
+	// by serverInit in order to copy session ticket keys if needed.
+	originalConfig *Config
 }
 
 // ticketKeyNameLen is the number of bytes of identifier that is prepended to
@@ -434,12 +458,18 @@ func ticketKeyFromBytes(b [32]byte) (key ticketKey) {
 // Clone returns a shallow clone of c.
 // Only the exported fields are copied.
 func (c *Config) Clone() *Config {
+	var sessionTicketKeys []ticketKey
+	c.mutex.RLock()
+	sessionTicketKeys = c.sessionTicketKeys
+	c.mutex.RUnlock()
+
 	return &Config{
 		Rand:                        c.Rand,
 		Time:                        c.Time,
 		Certificates:                c.Certificates,
 		NameToCertificate:           c.NameToCertificate,
 		GetCertificate:              c.GetCertificate,
+		GetConfigForClient:          c.GetConfigForClient,
 		RootCAs:                     c.RootCAs,
 		NextProtos:                  c.NextProtos,
 		ServerName:                  c.ServerName,
@@ -457,6 +487,8 @@ func (c *Config) Clone() *Config {
 		DynamicRecordSizingDisabled: c.DynamicRecordSizingDisabled,
 		Renegotiation:               c.Renegotiation,
 		KeyLogWriter:                c.KeyLogWriter,
+		sessionTicketKeys:           sessionTicketKeys,
+		// originalConfig is deliberately not duplicated.
 	}
 }
 
@@ -465,6 +497,11 @@ func (c *Config) serverInit() {
 		return
 	}
 
+	var originalConfig *Config
+	c.mutex.Lock()
+	originalConfig, c.originalConfig = c.originalConfig, nil
+	c.mutex.Unlock()
+
 	alreadySet := false
 	for _, b := range c.SessionTicketKey {
 		if b != 0 {
@@ -474,13 +511,21 @@ func (c *Config) serverInit() {
 	}
 
 	if !alreadySet {
-		if _, err := io.ReadFull(c.rand(), c.SessionTicketKey[:]); err != nil {
+		if originalConfig != nil {
+			copy(c.SessionTicketKey[:], originalConfig.SessionTicketKey[:])
+		} else if _, err := io.ReadFull(c.rand(), c.SessionTicketKey[:]); err != nil {
 			c.SessionTicketsDisabled = true
 			return
 		}
 	}
 
-	c.sessionTicketKeys = []ticketKey{ticketKeyFromBytes(c.SessionTicketKey)}
+	if originalConfig != nil {
+		originalConfig.mutex.RLock()
+		c.sessionTicketKeys = originalConfig.sessionTicketKeys
+		originalConfig.mutex.RUnlock()
+	} else {
+		c.sessionTicketKeys = []ticketKey{ticketKeyFromBytes(c.SessionTicketKey)}
+	}
 }
 
 func (c *Config) ticketKeys() []ticketKey {

handshake_server.go

@@ -37,11 +37,9 @@ type serverHandshakeState struct {
 // serverHandshake performs a TLS handshake as a server.
 // c.out.Mutex <= L; c.handshakeMutex <= L.
 func (c *Conn) serverHandshake() error {
-	config := c.config
-
 	// If this is the first server handshake, we generate a random key to
 	// encrypt the tickets with.
-	config.serverInitOnce.Do(config.serverInit)
+	c.config.serverInitOnce.Do(c.config.serverInit)
 
 	hs := serverHandshakeState{
 		c: c,
@@ -112,7 +110,6 @@ func (c *Conn) serverHandshake() error {
 // readClientHello reads a ClientHello message from the client and decides
 // whether we will perform session resumption.
 func (hs *serverHandshakeState) readClientHello() (isResume bool, err error) {
-	config := hs.c.config
 	c := hs.c
 
 	msg, err := c.readHandshake()
@@ -125,7 +122,29 @@ func (hs *serverHandshakeState) readClientHello() (isResume bool, err error) {
 		c.sendAlert(alertUnexpectedMessage)
 		return false, unexpectedMessageError(hs.clientHello, msg)
 	}
-	c.vers, ok = config.mutualVersion(hs.clientHello.vers)
+
+	clientHelloInfo := &ClientHelloInfo{
+		CipherSuites:    hs.clientHello.cipherSuites,
+		ServerName:      hs.clientHello.serverName,
+		SupportedCurves: hs.clientHello.supportedCurves,
+		SupportedPoints: hs.clientHello.supportedPoints,
+	}
+
+	if c.config.GetConfigForClient != nil {
+		if newConfig, err := c.config.GetConfigForClient(clientHelloInfo); err != nil {
+			c.sendAlert(alertInternalError)
+			return false, err
+		} else if newConfig != nil {
+			newConfig.mutex.Lock()
+			newConfig.originalConfig = c.config
+			newConfig.mutex.Unlock()
+
+			newConfig.serverInitOnce.Do(newConfig.serverInit)
+			c.config = newConfig
+		}
+	}
+
+	c.vers, ok = c.config.mutualVersion(hs.clientHello.vers)
 	if !ok {
 		c.sendAlert(alertProtocolVersion)
 		return false, fmt.Errorf("tls: client offered an unsupported, maximum protocol version of %x", hs.clientHello.vers)
@@ -135,7 +154,7 @@ func (hs *serverHandshakeState) readClientHello() (isResume bool, err error) {
 	hs.hello = new(serverHelloMsg)
 
 	supportedCurve := false
-	preferredCurves := config.curvePreferences()
+	preferredCurves := c.config.curvePreferences()
 Curves:
 	for _, curve := range hs.clientHello.supportedCurves {
 		for _, supported := range preferredCurves {
@@ -171,7 +190,7 @@ Curves:
 
 	hs.hello.vers = c.vers
 	hs.hello.random = make([]byte, 32)
-	_, err = io.ReadFull(config.rand(), hs.hello.random)
+	_, err = io.ReadFull(c.config.rand(), hs.hello.random)
 	if err != nil {
 		c.sendAlert(alertInternalError)
 		return false, err
@@ -196,20 +215,15 @@ Curves:
 	} else {
 		// Although sending an empty NPN extension is reasonable, Firefox has
 		// had a bug around this. Best to send nothing at all if
-		// config.NextProtos is empty. See
+		// c.config.NextProtos is empty. See
 		// https://golang.org/issue/5445.
-		if hs.clientHello.nextProtoNeg && len(config.NextProtos) > 0 {
+		if hs.clientHello.nextProtoNeg && len(c.config.NextProtos) > 0 {
 			hs.hello.nextProtoNeg = true
-			hs.hello.nextProtos = config.NextProtos
+			hs.hello.nextProtos = c.config.NextProtos
 		}
 	}
 
-	hs.cert, err = config.getCertificate(&ClientHelloInfo{
+	hs.cert, err = c.config.getCertificate(clientHelloInfo)
-		CipherSuites:    hs.clientHello.cipherSuites,
-		ServerName:      hs.clientHello.serverName,
-		SupportedCurves: hs.clientHello.supportedCurves,
-		SupportedPoints: hs.clientHello.supportedPoints,
-	})
 	if err != nil {
 		c.sendAlert(alertInternalError)
 		return false, err
@@ -354,18 +368,17 @@ func (hs *serverHandshakeState) doResumeHandshake() error {
 }
 
 func (hs *serverHandshakeState) doFullHandshake() error {
-	config := hs.c.config
 	c := hs.c
 
 	if hs.clientHello.ocspStapling && len(hs.cert.OCSPStaple) > 0 {
 		hs.hello.ocspStapling = true
 	}
 
-	hs.hello.ticketSupported = hs.clientHello.ticketSupported && !config.SessionTicketsDisabled
+	hs.hello.ticketSupported = hs.clientHello.ticketSupported && !c.config.SessionTicketsDisabled
 	hs.hello.cipherSuite = hs.suite.id
 
 	hs.finishedHash = newFinishedHash(hs.c.vers, hs.suite)
-	if config.ClientAuth == NoClientCert {
+	if c.config.ClientAuth == NoClientCert {
 		// No need to keep a full record of the handshake if client
 		// certificates won't be used.
 		hs.finishedHash.discardHandshakeBuffer()
@@ -394,7 +407,7 @@ func (hs *serverHandshakeState) doFullHandshake() error {
 	}
 
 	keyAgreement := hs.suite.ka(c.vers)
-	skx, err := keyAgreement.generateServerKeyExchange(config, hs.cert, hs.clientHello, hs.hello)
+	skx, err := keyAgreement.generateServerKeyExchange(c.config, hs.cert, hs.clientHello, hs.hello)
 	if err != nil {
 		c.sendAlert(alertHandshakeFailure)
 		return err
@@ -406,7 +419,7 @@ func (hs *serverHandshakeState) doFullHandshake() error {
 		}
 	}
 
-	if config.ClientAuth >= RequestClientCert {
+	if c.config.ClientAuth >= RequestClientCert {
 		// Request a client certificate
 		certReq := new(certificateRequestMsg)
 		certReq.certificateTypes = []byte{
@@ -423,8 +436,8 @@ func (hs *serverHandshakeState) doFullHandshake() error {
 		// to our request. When we know the CAs we trust, then
 		// we can send them down, so that the client can choose
 		// an appropriate certificate to give to us.
-		if config.ClientCAs != nil {
+		if c.config.ClientCAs != nil {
-			certReq.certificateAuthorities = config.ClientCAs.Subjects()
+			certReq.certificateAuthorities = c.config.ClientCAs.Subjects()
 		}
 		hs.finishedHash.Write(certReq.marshal())
 		if _, err := c.writeRecord(recordTypeHandshake, certReq.marshal()); err != nil {
@@ -452,7 +465,7 @@ func (hs *serverHandshakeState) doFullHandshake() error {
 	var ok bool
 	// If we requested a client certificate, then the client must send a
 	// certificate message, even if it's empty.
-	if config.ClientAuth >= RequestClientCert {
+	if c.config.ClientAuth >= RequestClientCert {
 		if certMsg, ok = msg.(*certificateMsg); !ok {
 			c.sendAlert(alertUnexpectedMessage)
 			return unexpectedMessageError(certMsg, msg)
@@ -461,7 +474,7 @@ func (hs *serverHandshakeState) doFullHandshake() error {
 
 		if len(certMsg.certificates) == 0 {
 			// The client didn't actually send a certificate
-			switch config.ClientAuth {
+			switch c.config.ClientAuth {
 			case RequireAnyClientCert, RequireAndVerifyClientCert:
 				c.sendAlert(alertBadCertificate)
 				return errors.New("tls: client didn't provide a certificate")
@@ -487,13 +500,13 @@ func (hs *serverHandshakeState) doFullHandshake() error {
 	}
 	hs.finishedHash.Write(ckx.marshal())
 
-	preMasterSecret, err := keyAgreement.processClientKeyExchange(config, hs.cert, ckx, c.vers)
+	preMasterSecret, err := keyAgreement.processClientKeyExchange(c.config, hs.cert, ckx, c.vers)
 	if err != nil {
 		c.sendAlert(alertHandshakeFailure)
 		return err
 	}
 	hs.masterSecret = masterFromPreMasterSecret(c.vers, hs.suite, preMasterSecret, hs.clientHello.random, hs.hello.random)
-	if err := config.writeKeyLog(hs.clientHello.random, hs.masterSecret); err != nil {
+	if err := c.config.writeKeyLog(hs.clientHello.random, hs.masterSecret); err != nil {
 		c.sendAlert(alertInternalError)
 		return err
 	}

handshake_server_test.go

@@ -1141,6 +1141,141 @@ func TestSNIGivenOnFailure(t *testing.T) {
 	}
 }
 
+var getConfigForClientTests = []struct {
+	setup          func(config *Config)
+	callback       func(clientHello *ClientHelloInfo) (*Config, error)
+	errorSubstring string
+	verify         func(config *Config) error
+}{
+	{
+		nil,
+		func(clientHello *ClientHelloInfo) (*Config, error) {
+			return nil, nil
+		},
+		"",
+		nil,
+	},
+	{
+		nil,
+		func(clientHello *ClientHelloInfo) (*Config, error) {
+			return nil, errors.New("should bubble up")
+		},
+		"should bubble up",
+		nil,
+	},
+	{
+		nil,
+		func(clientHello *ClientHelloInfo) (*Config, error) {
+			config := testConfig.Clone()
+			// Setting a maximum version of TLS 1.1 should cause
+			// the handshake to fail.
+			config.MaxVersion = VersionTLS11
+			return config, nil
+		},
+		"version 301 when expecting version 302",
+		nil,
+	},
+	{
+		func(config *Config) {
+			for i := range config.SessionTicketKey {
+				config.SessionTicketKey[i] = byte(i)
+			}
+			config.sessionTicketKeys = nil
+		},
+		func(clientHello *ClientHelloInfo) (*Config, error) {
+			config := testConfig.Clone()
+			for i := range config.SessionTicketKey {
+				config.SessionTicketKey[i] = 0
+			}
+			config.sessionTicketKeys = nil
+			return config, nil
+		},
+		"",
+		func(config *Config) error {
+			// The value of SessionTicketKey should have been
+			// duplicated into the per-connection Config.
+			for i := range config.SessionTicketKey {
+				if b := config.SessionTicketKey[i]; b != byte(i) {
+					return fmt.Errorf("SessionTicketKey was not duplicated from original Config: byte %d has value %d", i, b)
+				}
+			}
+			return nil
+		},
+	},
+	{
+		func(config *Config) {
+			var dummyKey [32]byte
+			for i := range dummyKey {
+				dummyKey[i] = byte(i)
+			}
+
+			config.SetSessionTicketKeys([][32]byte{dummyKey})
+		},
+		func(clientHello *ClientHelloInfo) (*Config, error) {
+			config := testConfig.Clone()
+			config.sessionTicketKeys = nil
+			return config, nil
+		},
+		"",
+		func(config *Config) error {
+			// The session ticket keys should have been duplicated
+			// into the per-connection Config.
+			if l := len(config.sessionTicketKeys); l != 1 {
+				return fmt.Errorf("got len(sessionTicketKeys) == %d, wanted 1", l)
+			}
+			return nil
+		},
+	},
+}
+
+func TestGetConfigForClient(t *testing.T) {
+	serverConfig := testConfig.Clone()
+	clientConfig := testConfig.Clone()
+	clientConfig.MinVersion = VersionTLS12
+
+	for i, test := range getConfigForClientTests {
+		if test.setup != nil {
+			test.setup(serverConfig)
+		}
+
+		var configReturned *Config
+		serverConfig.GetConfigForClient = func(clientHello *ClientHelloInfo) (*Config, error) {
+			config, err := test.callback(clientHello)
+			configReturned = config
+			return config, err
+		}
+		c, s := net.Pipe()
+		done := make(chan error)
+
+		go func() {
+			defer s.Close()
+			done <- Server(s, serverConfig).Handshake()
+		}()
+
+		clientErr := Client(c, clientConfig).Handshake()
+		c.Close()
+
+		serverErr := <-done
+
+		if len(test.errorSubstring) == 0 {
+			if serverErr != nil || clientErr != nil {
+				t.Errorf("%#d: expected no error but got serverErr: %q, clientErr: %q", i, serverErr, clientErr)
+			}
+			if test.verify != nil {
+				if err := test.verify(configReturned); err != nil {
+					t.Errorf("#%d: verify returned error: %v", i, err)
+				}
+			}
+		} else {
+			if serverErr == nil {
+				t.Errorf("%#d: expected error containing %q but got no error", i, test.errorSubstring)
+			} else if !strings.Contains(serverErr.Error(), test.errorSubstring) {
+				t.Errorf("%#d: expected error to contain %q but it was %q", i, test.errorSubstring, serverErr)
+			}
+		}
+	}
+}
+
 func bigFromString(s string) *big.Int {
 	ret := new(big.Int)
 	ret.SetString(s, 10)

tls_test.go

@@ -477,7 +477,7 @@ func TestClone(t *testing.T) {
 		case "Rand":
 			f.Set(reflect.ValueOf(io.Reader(os.Stdin)))
 			continue
-		case "Time", "GetCertificate":
+		case "Time", "GetCertificate", "GetConfigForClient":
 			// DeepEqual can't compare functions.
 			continue
 		case "Certificates":