refactor: move TLS config creation to ConnectionInfo

This refactor moves all TLS configuration into the connection info type
and leaves room for altering the configuration based on the request
connection path, rather than hiding the configuration deep within the
code that retrieves the ephemeral certificate.

dialer.go

@@ -268,7 +268,7 @@ func (d *Dialer) Dial(ctx context.Context, icn string, opts ...DialOption) (conn
 	// The TLS handshake will not fail on an expired client certificate. It's
 	// not until the first read where the client cert error will be surfaced.
 	// So check that the certificate is valid before proceeding.
-	if !validClientCert(cn, d.logger, ci.Conf) {
+	if !validClientCert(cn, d.logger, ci.Expiration) {
 		d.logger.Debugf("[%v] Refreshing certificate now", cn.String())
 		i.ForceRefresh()
 		// Block on refreshed connection info
@@ -313,7 +313,7 @@ func (d *Dialer) Dial(ctx context.Context, icn string, opts ...DialOption) (conn
 		}
 	}
 
-	tlsConn := tls.Client(conn, ci.Conf)
+	tlsConn := tls.Client(conn, ci.TLSConfig())
 	err = tlsConn.HandshakeContext(ctx)
 	if err != nil {
 		d.logger.Debugf("[%v] TLS handshake failed: %v", cn.String(), err)
@@ -339,28 +339,18 @@ func (d *Dialer) Dial(ctx context.Context, icn string, opts ...DialOption) (conn
 // validClientCert checks that the ephemeral client certificate retrieved from
 // the cache is unexpired. The time comparisons strip the monotonic clock value
 // to ensure an accurate result, even after laptop sleep.
-func validClientCert(cn instance.ConnName, l debug.Logger, c *tls.Config) bool {
-	// The following conditions should be impossible (no certs, nil leaf), but
-	// just in case there's an unknown edge case, check assumptions before
-	// proceeding.
-	if len(c.Certificates) == 0 {
-		return false
-	}
-	if c.Certificates[0].Leaf == nil {
-		return false
-	}
+func validClientCert(cn instance.ConnName, l debug.Logger, expiration time.Time) bool {
 	// Use UTC() to strip monotonic clock value to guard against inaccurate
 	// comparisons, especially after laptop sleep.
 	// See the comments on the monotonic clock in the Go documentation for
 	// details: https://pkg.go.dev/time#hdr-Monotonic_Clocks
 	now := time.Now().UTC()
-	expiration := c.Certificates[0].Leaf.NotAfter.UTC()
-	valid := expiration.After(now)
+	valid := expiration.UTC().After(now)
 	l.Debugf(
 		"[%v] Now = %v, Current cert expiration = %v",
 		cn.String(),
 		now.Format(time.RFC3339),
-		expiration.Format(time.RFC3339),
+		expiration.UTC().Format(time.RFC3339),
 	)
 	l.Debugf("[%v] Cert is valid = %v", cn.String(), valid)
 	return valid

dialer_test.go

@@ -16,8 +16,6 @@ package cloudsqlconn
 
 import (
 	"context"
-	"crypto/tls"
-	"crypto/x509"
 	"errors"
 	"fmt"
 	"io"
@@ -630,8 +628,8 @@ func TestDialerRemovesInvalidInstancesFromCache(t *testing.T) {
 	badCN, _ := instance.ParseConnName(badInstanceConnectionName)
 	spy := &spyConnectionInfoCache{
 		connectInfoCalls: []struct {
-			tls *tls.Config
-			err error
+			info cloudsql.ConnectionInfo
+			err  error
 		}{{
 			err: errors.New("connect info failed"),
 		}},
@@ -671,18 +669,14 @@ func TestDialRefreshesExpiredCertificates(t *testing.T) {
 	cn, _ := instance.ParseConnName(icn)
 	spy := &spyConnectionInfoCache{
 		connectInfoCalls: []struct {
-			tls *tls.Config
-			err error
+			info cloudsql.ConnectionInfo
+			err  error
 		}{
 			// First call returns expired certificate
 			{
-				tls: &tls.Config{
-					Certificates: []tls.Certificate{{
-						Leaf: &x509.Certificate{
-							// Certificate expired 10 hours ago.
-							NotAfter: time.Now().Add(-10 * time.Hour),
-						},
-					}},
+				// Certificate expired 10 hours ago.
+				info: cloudsql.ConnectionInfo{
+					Expiration: time.Now().Add(-10 * time.Hour),
 				},
 			},
 			// Second call errors to validate error path
@@ -723,8 +717,8 @@ type spyConnectionInfoCache struct {
 	mu               sync.Mutex
 	connectInfoIndex int
 	connectInfoCalls []struct {
-		tls *tls.Config
-		err error
+		info cloudsql.ConnectionInfo
+		err  error
 	}
 	closeWasCalled        bool
 	forceRefreshWasCalled bool
@@ -739,7 +733,7 @@ func (s *spyConnectionInfoCache) ConnectionInfo(
 	defer s.mu.Unlock()
 	res := s.connectInfoCalls[s.connectInfoIndex]
 	s.connectInfoIndex++
-	return cloudsql.ConnectionInfo{Conf: res.tls}, res.err
+	return res.info, res.err
 }
 
 func (s *spyConnectionInfoCache) ForceRefresh() {

internal/cloudsql/instance.go

@@ -76,7 +76,7 @@ func (r *refreshOperation) isValid() bool {
 	default:
 		return false
 	case <-r.ready:
-		if r.err != nil || time.Now().After(r.result.Expiry.Round(0)) {
+		if r.err != nil || time.Now().After(r.result.Expiration.Round(0)) {
 			return false
 		}
 		return true
@@ -175,11 +175,13 @@ func (i *RefreshAheadCache) Close() error {
 // ConnectionInfo contains all necessary information to connect securely to the
 // server-side Proxy running on a Cloud SQL instance.
 type ConnectionInfo struct {
-	addrs        map[string]string
-	ServerCaCert *x509.Certificate
-	DBVersion    string
-	Conf         *tls.Config
-	Expiry       time.Time
+	ConnectionName    instance.ConnName
+	ClientCertificate tls.Certificate
+	ServerCaCert      *x509.Certificate
+	DBVersion         string
+	Expiration        time.Time
+
+	addrs map[string]string
 }
 
 // Addr returns the IP address or DNS name for the given IP type.
@@ -202,14 +204,79 @@ func (c ConnectionInfo) Addr(ipType string) (string, error) {
 	if !ok {
 		err := errtype.NewConfigError(
 			fmt.Sprintf("instance does not have IP of type %q", ipType),
-			// i.connName.String(),
-			"TODO",
+			c.ConnectionName.String(),
 		)
 		return "", err
 	}
 	return addr, nil
 }
 
+// TLSConfig constructs a TLS configuration for the given connection info.
+func (c ConnectionInfo) TLSConfig() *tls.Config {
+	pool := x509.NewCertPool()
+	pool.AddCert(c.ServerCaCert)
+	return &tls.Config{
+		ServerName:   c.ConnectionName.String(),
+		Certificates: []tls.Certificate{c.ClientCertificate},
+		RootCAs:      pool,
+		// We need to set InsecureSkipVerify to true due to
+		// https://github.com/GoogleCloudPlatform/cloudsql-proxy/issues/194
+		// https://tip.golang.org/doc/go1.11#crypto/x509
+		//
+		// Since we have a secure channel to the Cloud SQL API which we use to
+		// retrieve the certificates, we instead need to implement our own
+		// VerifyPeerCertificate function that will verify that the certificate
+		// is OK.
+		InsecureSkipVerify:    true,
+		VerifyPeerCertificate: verifyPeerCertificateFunc(c.ConnectionName, pool),
+		MinVersion:            tls.VersionTLS13,
+	}
+}
+
+// verifyPeerCertificateFunc creates a VerifyPeerCertificate func that
+// verifies that the peer certificate is in the cert pool. We need to define
+// our own because CloudSQL instances use the instance name (e.g.,
+// my-project:my-instance) instead of a valid domain name for the certificate's
+// Common Name.
+func verifyPeerCertificateFunc(
+	cn instance.ConnName, pool *x509.CertPool,
+) func(rawCerts [][]byte, _ [][]*x509.Certificate) error {
+	return func(rawCerts [][]byte, _ [][]*x509.Certificate) error {
+		if len(rawCerts) == 0 {
+			return errtype.NewDialError(
+				"no certificate to verify", cn.String(), nil,
+			)
+		}
+
+		cert, err := x509.ParseCertificate(rawCerts[0])
+		if err != nil {
+			return errtype.NewDialError(
+				"failed to parse X.509 certificate", cn.String(), err,
+			)
+		}
+
+		opts := x509.VerifyOptions{Roots: pool}
+		if _, err = cert.Verify(opts); err != nil {
+			return errtype.NewDialError(
+				"failed to verify certificate", cn.String(), err,
+			)
+		}
+
+		certInstanceName := fmt.Sprintf("%s:%s", cn.Project(), cn.Name())
+		if cert.Subject.CommonName != certInstanceName {
+			return errtype.NewDialError(
+				fmt.Sprintf(
+					"certificate had CN %q, expected %q",
+					cert.Subject.CommonName, certInstanceName,
+				),
+				cn.String(),
+				nil,
+			)
+		}
+		return nil
+	}
+}
+
 // ConnectionInfo returns an IP address specified by ipType (i.e., public or
 // private) and a TLS config that can be used to connect to a Cloud SQL
 // instance.
@@ -353,7 +420,7 @@ func (i *RefreshAheadCache) scheduleRefresh(d time.Duration) *refreshOperation {
 			i.logger.Debugf(
 				"[%v] Current certificate expiration = %v",
 				i.connName.String(),
-				r.result.Expiry.UTC().Format(time.RFC3339),
+				r.result.Expiration.UTC().Format(time.RFC3339),
 			)
 		default:
 			i.logger.Debugf(
@@ -392,7 +459,7 @@ func (i *RefreshAheadCache) scheduleRefresh(d time.Duration) *refreshOperation {
 		// Update the current results, and schedule the next refresh in
 		// the future
 		i.cur = r
-		t := refreshDuration(time.Now(), i.cur.result.Expiry)
+		t := refreshDuration(time.Now(), i.cur.result.Expiration)
 		i.logger.Debugf(
 			"[%v] Connection info refresh operation scheduled at %v (now + %v)",
 			i.connName.String(),