crypto/tls: call GetCertificate if Certificates is empty.

This change causes the GetCertificate callback to be called if
Certificates is empty. Previously this configuration would result in an
error.

This allows people to have servers that depend entirely on dynamic
certificate selection, even when the client doesn't send SNI.

Fixes #9208.

Change-Id: I2f5a5551215958b88b154c64a114590300dfc461
Reviewed-on: https://go-review.googlesource.com/8792Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>

src/crypto/tls/common.go

@@ -10,6 +10,7 @@ import (
 	"crypto/rand"
 	"crypto/sha512"
 	"crypto/x509"
+	"errors"
 	"fmt"
 	"io"
 	"math/big"
@@ -270,10 +271,12 @@ type Config struct {
 	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.
+	// ClientHelloInfo. It will only be called if the client supplies SNI
+	// information or if Certificates is empty.
+	//
+	// 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
@@ -500,13 +503,18 @@ func (c *Config) mutualVersion(vers uint16) (uint16, bool) {
 // 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 {
+	if c.GetCertificate != nil &&
+		(len(c.Certificates) == 0 || len(clientHello.ServerName) > 0) {
 		cert, err := c.GetCertificate(clientHello)
 		if cert != nil || err != nil {
 			return cert, err
 		}
 	}
 
+	if len(c.Certificates) == 0 {
+		return nil, errors.New("crypto/tls: no certificates configured")
+	}
+
 	if len(c.Certificates) == 1 || c.NameToCertificate == nil {
 		// There's only one choice, so no point doing any work.
 		return &c.Certificates[0], nil

src/crypto/tls/handshake_server.go

@@ -189,23 +189,15 @@ Curves:
 		}
 	}
 
-	if len(config.Certificates) == 0 {
-		c.sendAlert(alertInternalError)
-		return false, errors.New("tls: no certificates configured")
-	}
-	hs.cert = &config.Certificates[0]
-	if len(hs.clientHello.serverName) > 0 {
-		chi := &ClientHelloInfo{
+	if hs.cert, err = config.getCertificate(&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 {
+	}); err != nil {
 		c.sendAlert(alertInternalError)
 		return false, err
 	}
-	}
 	if hs.clientHello.scts {
 		hs.hello.scts = hs.cert.SignedCertificateTimestamps
 	}

src/crypto/tls/handshake_server_test.go

@@ -796,6 +796,36 @@ func TestHandshakeServerSNIGetCertificateError(t *testing.T) {
 	testClientHelloFailure(t, &serverConfig, clientHello, errMsg)
 }
 
+// TestHandshakeServerEmptyCertificates tests that GetCertificates is called in
+// the case that Certificates is empty, even without SNI.
+func TestHandshakeServerEmptyCertificates(t *testing.T) {
+	const errMsg = "TestHandshakeServerEmptyCertificates error"
+
+	serverConfig := *testConfig
+	serverConfig.GetCertificate = func(clientHello *ClientHelloInfo) (*Certificate, error) {
+		return nil, errors.New(errMsg)
+	}
+	serverConfig.Certificates = nil
+
+	clientHello := &clientHelloMsg{
+		vers:               0x0301,
+		cipherSuites:       []uint16{TLS_RSA_WITH_RC4_128_SHA},
+		compressionMethods: []uint8{0},
+	}
+	testClientHelloFailure(t, &serverConfig, clientHello, errMsg)
+
+	// With an empty Certificates and a nil GetCertificate, the server
+	// should always return a “no certificates” error.
+	serverConfig.GetCertificate = nil
+
+	clientHello = &clientHelloMsg{
+		vers:               0x0301,
+		cipherSuites:       []uint16{TLS_RSA_WITH_RC4_128_SHA},
+		compressionMethods: []uint8{0},
+	}
+	testClientHelloFailure(t, &serverConfig, clientHello, "no certificates")
+}
+
 // 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) {