crypto/tls: set default minimum client version to TLS 1.2

Updates #45428

Change-Id: I5d70066d4091196ec6f8bfc2edf3d78fdc0520c1
Reviewed-on: https://go-review.googlesource.com/c/go/+/359779
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>

common.go

@@ -18,6 +18,7 @@ import (
 	"crypto/x509"
 	"errors"
 	"fmt"
+	"internal/godebug"
 	"io"
 	"net"
 	"strings"
@@ -682,11 +683,20 @@ type Config struct {
 	ClientSessionCache ClientSessionCache
 
 	// MinVersion contains the minimum TLS version that is acceptable.
-	// If zero, TLS 1.0 is currently taken as the minimum.
+	//
+	// By default, TLS 1.2 is currently used as the minimum when acting as a
+	// client, and TLS 1.0 when acting as a server. TLS 1.0 is the minimum
+	// supported by this package, both as a client and as a server.
+	//
+	// The client-side default can temporarily be reverted to TLS 1.0 by
+	// including the value "x509sha1=1" in the GODEBUG environment variable.
+	// Note that this option will be removed in Go 1.19 (but it will still be
+	// possible to set this field to VersionTLS10 explicitly).
 	MinVersion uint16
 
 	// MaxVersion contains the maximum TLS version that is acceptable.
-	// If zero, the maximum version supported by this package is used,
+	//
+	// By default, the maximum version supported by this package is used,
 	// which is currently TLS 1.3.
 	MaxVersion uint16
 
@@ -964,9 +974,21 @@ var supportedVersions = []uint16{
 	VersionTLS10,
 }
 
-func (c *Config) supportedVersions() []uint16 {
+// debugEnableTLS10 enables TLS 1.0. See issue 45428.
+var debugEnableTLS10 = godebug.Get("tls10default") == "1"
+
+// roleClient and roleServer are meant to call supportedVersions and parents
+// with more readability at the callsite.
+const roleClient = true
+const roleServer = false
+
+func (c *Config) supportedVersions(isClient bool) []uint16 {
 	versions := make([]uint16, 0, len(supportedVersions))
 	for _, v := range supportedVersions {
+		if (c == nil || c.MinVersion == 0) && !debugEnableTLS10 &&
+			isClient && v < VersionTLS12 {
+			continue
+		}
 		if c != nil && c.MinVersion != 0 && v < c.MinVersion {
 			continue
 		}
@@ -978,8 +1000,8 @@ func (c *Config) supportedVersions() []uint16 {
 	return versions
 }
 
-func (c *Config) maxSupportedVersion() uint16 {
-	supportedVersions := c.supportedVersions()
+func (c *Config) maxSupportedVersion(isClient bool) uint16 {
+	supportedVersions := c.supportedVersions(isClient)
 	if len(supportedVersions) == 0 {
 		return 0
 	}
@@ -1020,8 +1042,8 @@ func (c *Config) supportsCurve(curve CurveID) bool {
 
 // mutualVersion returns the protocol version to use given the advertised
 // versions of the peer. Priority is given to the peer preference order.
-func (c *Config) mutualVersion(peerVersions []uint16) (uint16, bool) {
-	supportedVersions := c.supportedVersions()
+func (c *Config) mutualVersion(isClient bool, peerVersions []uint16) (uint16, bool) {
+	supportedVersions := c.supportedVersions(isClient)
 	for _, peerVersion := range peerVersions {
 		for _, v := range supportedVersions {
 			if v == peerVersion {
@@ -1100,7 +1122,7 @@ func (chi *ClientHelloInfo) SupportsCertificate(c *Certificate) error {
 	if config == nil {
 		config = &Config{}
 	}
-	vers, ok := config.mutualVersion(chi.SupportedVersions)
+	vers, ok := config.mutualVersion(roleServer, chi.SupportedVersions)
 	if !ok {
 		return errors.New("no mutually supported protocol versions")
 	}

handshake_client.go

@@ -52,12 +52,12 @@ func (c *Conn) makeClientHello() (*clientHelloMsg, ecdheParameters, error) {
 		return nil, nil, errors.New("tls: NextProtos values too large")
 	}
 
-	supportedVersions := config.supportedVersions()
+	supportedVersions := config.supportedVersions(roleClient)
 	if len(supportedVersions) == 0 {
 		return nil, nil, errors.New("tls: no supported versions satisfy MinVersion and MaxVersion")
 	}
 
-	clientHelloVersion := config.maxSupportedVersion()
+	clientHelloVersion := config.maxSupportedVersion(roleClient)
 	// The version at the beginning of the ClientHello was capped at TLS 1.2
 	// for compatibility reasons. The supported_versions extension is used
 	// to negotiate versions now. See RFC 8446, Section 4.2.1.
@@ -194,7 +194,7 @@ func (c *Conn) clientHandshake(ctx context.Context) (err error) {
 	// If we are negotiating a protocol version that's lower than what we
 	// support, check for the server downgrade canaries.
 	// See RFC 8446, Section 4.1.3.
-	maxVers := c.config.maxSupportedVersion()
+	maxVers := c.config.maxSupportedVersion(roleClient)
 	tls12Downgrade := string(serverHello.random[24:]) == downgradeCanaryTLS12
 	tls11Downgrade := string(serverHello.random[24:]) == downgradeCanaryTLS11
 	if maxVers == VersionTLS13 && c.vers <= VersionTLS12 && (tls12Downgrade || tls11Downgrade) ||
@@ -362,7 +362,7 @@ func (c *Conn) pickTLSVersion(serverHello *serverHelloMsg) error {
 		peerVersion = serverHello.supportedVersion
 	}
 
-	vers, ok := c.config.mutualVersion([]uint16{peerVersion})
+	vers, ok := c.config.mutualVersion(roleClient, []uint16{peerVersion})
 	if !ok {
 		c.sendAlert(alertProtocolVersion)
 		return fmt.Errorf("tls: server selected unsupported protocol version %x", peerVersion)

handshake_server.go

@@ -156,7 +156,7 @@ func (c *Conn) readClientHello(ctx context.Context) (*clientHelloMsg, error) {
 	if len(clientHello.supportedVersions) == 0 {
 		clientVersions = supportedVersionsFromMax(clientHello.vers)
 	}
-	c.vers, ok = c.config.mutualVersion(clientVersions)
+	c.vers, ok = c.config.mutualVersion(roleServer, clientVersions)
 	if !ok {
 		c.sendAlert(alertProtocolVersion)
 		return nil, fmt.Errorf("tls: client offered only unsupported versions: %x", clientVersions)
@@ -191,7 +191,7 @@ func (hs *serverHandshakeState) processClientHello() error {
 	hs.hello.random = make([]byte, 32)
 	serverRandom := hs.hello.random
 	// Downgrade protection canaries. See RFC 8446, Section 4.1.3.
-	maxVers := c.config.maxSupportedVersion()
+	maxVers := c.config.maxSupportedVersion(roleServer)
 	if maxVers >= VersionTLS12 && c.vers < maxVers || testingOnlyForceDowngradeCanary {
 		if c.vers == VersionTLS12 {
 			copy(serverRandom[24:], downgradeCanaryTLS12)
@@ -354,7 +354,7 @@ func (hs *serverHandshakeState) pickCipherSuite() error {
 	for _, id := range hs.clientHello.cipherSuites {
 		if id == TLS_FALLBACK_SCSV {
 			// The client is doing a fallback connection. See RFC 7507.
-			if hs.clientHello.vers < c.config.maxSupportedVersion() {
+			if hs.clientHello.vers < c.config.maxSupportedVersion(roleServer) {
 				c.sendAlert(alertInappropriateFallback)
 				return errors.New("tls: client using inappropriate protocol fallback")
 			}

handshake_server_test.go

@@ -385,13 +385,30 @@ func TestVersion(t *testing.T) {
 	}
 	clientConfig := &Config{
 		InsecureSkipVerify: true,
+		MinVersion:         VersionTLS10,
 	}
 	state, _, err := testHandshake(t, clientConfig, serverConfig)
 	if err != nil {
 		t.Fatalf("handshake failed: %s", err)
 	}
 	if state.Version != VersionTLS11 {
-		t.Fatalf("Incorrect version %x, should be %x", state.Version, VersionTLS11)
+		t.Fatalf("incorrect version %x, should be %x", state.Version, VersionTLS11)
+	}
+
+	clientConfig.MinVersion = 0
+	_, _, err = testHandshake(t, clientConfig, serverConfig)
+	if err == nil {
+		t.Fatalf("expected failure to connect with TLS 1.0/1.1")
+	}
+
+	defer func(old bool) { debugEnableTLS10 = old }(debugEnableTLS10)
+	debugEnableTLS10 = true
+	_, _, err = testHandshake(t, clientConfig, serverConfig)
+	if err != nil {
+		t.Fatalf("handshake failed: %s", err)
+	}
+	if state.Version != VersionTLS11 {
+		t.Fatalf("incorrect version %x, should be %x", state.Version, VersionTLS11)
 	}
 }
 
@@ -472,6 +489,7 @@ func testCrossVersionResume(t *testing.T, version uint16) {
 		InsecureSkipVerify: true,
 		ClientSessionCache: NewLRUClientSessionCache(1),
 		ServerName:         "servername",
+		MinVersion:         VersionTLS10,
 	}
 
 	// Establish a session at TLS 1.1.

handshake_server_tls13.go

@@ -110,7 +110,7 @@ func (hs *serverHandshakeStateTLS13) processClientHello() error {
 		if id == TLS_FALLBACK_SCSV {
 			// Use c.vers instead of max(supported_versions) because an attacker
 			// could defeat this by adding an arbitrary high version otherwise.
-			if c.vers < c.config.maxSupportedVersion() {
+			if c.vers < c.config.maxSupportedVersion(roleServer) {
 				c.sendAlert(alertInappropriateFallback)
 				return errors.New("tls: client using inappropriate protocol fallback")
 			}

handshake_test.go

@@ -363,6 +363,8 @@ func runMain(m *testing.M) int {
 		Certificates:       make([]Certificate, 2),
 		InsecureSkipVerify: true,
 		CipherSuites:       allCipherSuites(),
+		MinVersion:         VersionTLS10,
+		MaxVersion:         VersionTLS13,
 	}
 	testConfig.Certificates[0].Certificate = [][]byte{testRSACertificate}
 	testConfig.Certificates[0].PrivateKey = testRSAPrivateKey