Add TLS support to exec authenticator plugin

https://github.com/kubernetes/community/blob/master/contributors/design-proposals/auth/kubectl-exec-plugins.md#tls-client-certificate-support

Allows exec plugin to return raw TLS key/cert data. This data populates
transport.Config.TLS fields.
transport.Config.TLS propagates custom credentials using
tls.Config.GetClientCertificate callback.
On key/cert rotation, all connections using old credentials are
closed

Kubernetes-commit: cd89f9473faa60c15b8e9d223e5c4f9dab53627a

plugin/pkg/client/auth/exec/exec.go

@@ -18,11 +18,15 @@ package exec
 
 import (
 	"bytes"
+	"context"
+	"crypto/tls"
 	"fmt"
 	"io"
+	"net"
 	"net/http"
 	"os"
 	"os/exec"
+	"reflect"
 	"sync"
 	"time"
 
@@ -35,6 +39,8 @@ import (
 	"k8s.io/client-go/pkg/apis/clientauthentication"
 	"k8s.io/client-go/pkg/apis/clientauthentication/v1alpha1"
 	"k8s.io/client-go/tools/clientcmd/api"
+	"k8s.io/client-go/transport"
+	"k8s.io/client-go/util/connrotation"
 )
 
 const execInfoEnv = "KUBERNETES_EXEC_INFO"
@@ -147,14 +153,55 @@ type Authenticator struct {
 	// The mutex also guards calling the plugin. Since the plugin could be
 	// interactive we want to make sure it's only called once.
 	mu          sync.Mutex
-	cachedToken string
+	cachedCreds *credentials
 	exp         time.Time
+
+	onRotate func()
 }
 
-// WrapTransport instruments an existing http.RoundTripper with credentials returned
-// by the plugin.
-func (a *Authenticator) WrapTransport(rt http.RoundTripper) http.RoundTripper {
-	return &roundTripper{a, rt}
+type credentials struct {
+	token string
+	cert  *tls.Certificate
+}
+
+// UpdateTransportConfig updates the transport.Config to use credentials
+// returned by the plugin.
+func (a *Authenticator) UpdateTransportConfig(c *transport.Config) error {
+	wt := c.WrapTransport
+	c.WrapTransport = func(rt http.RoundTripper) http.RoundTripper {
+		if wt != nil {
+			rt = wt(rt)
+		}
+		return &roundTripper{a, rt}
+	}
+
+	getCert := c.TLS.GetCert
+	c.TLS.GetCert = func() (*tls.Certificate, error) {
+		// If previous GetCert is present and returns a valid non-nil
+		// certificate, use that. Otherwise use cert from exec plugin.
+		if getCert != nil {
+			cert, err := getCert()
+			if err != nil {
+				return nil, err
+			}
+			if cert != nil {
+				return cert, nil
+			}
+		}
+		return a.cert()
+	}
+
+	var dial func(ctx context.Context, network, addr string) (net.Conn, error)
+	if c.Dial != nil {
+		dial = c.Dial
+	} else {
+		dial = (&net.Dialer{Timeout: 30 * time.Second, KeepAlive: 30 * time.Second}).DialContext
+	}
+	d := connrotation.NewDialer(dial)
+	a.onRotate = d.CloseAll
+	c.Dial = d.DialContext
+
+	return nil
 }
 
 type roundTripper struct {
@@ -169,11 +216,13 @@ func (r *roundTripper) RoundTrip(req *http.Request) (*http.Response, error) {
 		return r.base.RoundTrip(req)
 	}
 
-	token, err := r.a.token()
+	creds, err := r.a.getCreds()
 	if err != nil {
-		return nil, fmt.Errorf("getting token: %v", err)
+		return nil, fmt.Errorf("getting credentials: %v", err)
+	}
+	if creds.token != "" {
+		req.Header.Set("Authorization", "Bearer "+creds.token)
 	}
-	req.Header.Set("Authorization", "Bearer "+token)
 
 	res, err := r.base.RoundTrip(req)
 	if err != nil {
@@ -184,47 +233,60 @@ func (r *roundTripper) RoundTrip(req *http.Request) (*http.Response, error) {
 			Header: res.Header,
 			Code:   int32(res.StatusCode),
 		}
-		if err := r.a.refresh(token, resp); err != nil {
-			glog.Errorf("refreshing token: %v", err)
+		if err := r.a.maybeRefreshCreds(creds, resp); err != nil {
+			glog.Errorf("refreshing credentials: %v", err)
 		}
 	}
 	return res, nil
 }
 
-func (a *Authenticator) tokenExpired() bool {
+func (a *Authenticator) credsExpired() bool {
 	if a.exp.IsZero() {
 		return false
 	}
 	return a.now().After(a.exp)
 }
 
-func (a *Authenticator) token() (string, error) {
-	a.mu.Lock()
-	defer a.mu.Unlock()
-	if a.cachedToken != "" && !a.tokenExpired() {
-		return a.cachedToken, nil
+func (a *Authenticator) cert() (*tls.Certificate, error) {
+	creds, err := a.getCreds()
+	if err != nil {
+		return nil, err
 	}
-
-	return a.getToken(nil)
+	return creds.cert, nil
 }
 
-// refresh executes the plugin to force a rotation of the token.
-func (a *Authenticator) refresh(token string, r *clientauthentication.Response) error {
+func (a *Authenticator) getCreds() (*credentials, error) {
+	a.mu.Lock()
+	defer a.mu.Unlock()
+	if a.cachedCreds != nil && !a.credsExpired() {
+		return a.cachedCreds, nil
+	}
+
+	if err := a.refreshCredsLocked(nil); err != nil {
+		return nil, err
+	}
+	return a.cachedCreds, nil
+}
+
+// maybeRefreshCreds executes the plugin to force a rotation of the
+// credentials, unless they were rotated already.
+func (a *Authenticator) maybeRefreshCreds(creds *credentials, r *clientauthentication.Response) error {
 	a.mu.Lock()
 	defer a.mu.Unlock()
 
-	if token != a.cachedToken {
-		// Token already rotated.
+	// Since we're not making a new pointer to a.cachedCreds in getCreds, no
+	// need to do deep comparison.
+	if creds != a.cachedCreds {
+		// Credentials already rotated.
 		return nil
 	}
 
-	_, err := a.getToken(r)
-	return err
+	return a.refreshCredsLocked(r)
 }
 
-// getToken executes the plugin and reads the credentials from stdout. It must be
-// called while holding the Authenticator's mutex.
-func (a *Authenticator) getToken(r *clientauthentication.Response) (string, error) {
+// refreshCredsLocked executes the plugin and reads the credentials from
+// stdout. It must be called while holding the Authenticator's mutex.
+func (a *Authenticator) refreshCredsLocked(r *clientauthentication.Response) error {
 	cred := &clientauthentication.ExecCredential{
 		Spec: clientauthentication.ExecCredentialSpec{
 			Response:    r,
@@ -234,7 +296,7 @@ func (a *Authenticator) getToken(r *clientauthentication.Response) (string, erro
 
 	data, err := runtime.Encode(codecs.LegacyCodec(a.group), cred)
 	if err != nil {
-		return "", fmt.Errorf("encode ExecCredentials: %v", err)
+		return fmt.Errorf("encode ExecCredentials: %v", err)
 	}
 
 	env := append(a.environ(), a.env...)
@@ -250,23 +312,26 @@ func (a *Authenticator) getToken(r *clientauthentication.Response) (string, erro
 	}
 
 	if err := cmd.Run(); err != nil {
-		return "", fmt.Errorf("exec: %v", err)
+		return fmt.Errorf("exec: %v", err)
 	}
 
 	_, gvk, err := codecs.UniversalDecoder(a.group).Decode(stdout.Bytes(), nil, cred)
 	if err != nil {
-		return "", fmt.Errorf("decode stdout: %v", err)
+		return fmt.Errorf("decoding stdout: %v", err)
 	}
 	if gvk.Group != a.group.Group || gvk.Version != a.group.Version {
-		return "", fmt.Errorf("exec plugin is configured to use API version %s, plugin returned version %s",
+		return fmt.Errorf("exec plugin is configured to use API version %s, plugin returned version %s",
 			a.group, schema.GroupVersion{Group: gvk.Group, Version: gvk.Version})
 	}
 
 	if cred.Status == nil {
-		return "", fmt.Errorf("exec plugin didn't return a status field")
+		return fmt.Errorf("exec plugin didn't return a status field")
 	}
-	if cred.Status.Token == "" {
-		return "", fmt.Errorf("exec plugin didn't return a token")
+	if cred.Status.Token == "" && cred.Status.ClientCertificateData == "" && cred.Status.ClientKeyData == "" {
+		return fmt.Errorf("exec plugin didn't return a token or cert/key pair")
+	}
+	if (cred.Status.ClientCertificateData == "") != (cred.Status.ClientKeyData == "") {
+		return fmt.Errorf("exec plugin returned only certificate or key, not both")
 	}
 
 	if cred.Status.ExpirationTimestamp != nil {
@@ -274,7 +339,24 @@ func (a *Authenticator) getToken(r *clientauthentication.Response) (string, erro
 	} else {
 		a.exp = time.Time{}
 	}
-	a.cachedToken = cred.Status.Token
 
-	return a.cachedToken, nil
+	newCreds := &credentials{
+		token: cred.Status.Token,
+	}
+	if cred.Status.ClientKeyData != "" && cred.Status.ClientCertificateData != "" {
+		cert, err := tls.X509KeyPair([]byte(cred.Status.ClientCertificateData), []byte(cred.Status.ClientKeyData))
+		if err != nil {
+			return fmt.Errorf("failed parsing client key/certificate: %v", err)
+		}
+		newCreds.cert = &cert
+	}
+
+	oldCreds := a.cachedCreds
+	a.cachedCreds = newCreds
+	// Only close all connections when TLS cert rotates. Token rotation doesn't
+	// need the extra noise.
+	if a.onRotate != nil && oldCreds != nil && !reflect.DeepEqual(oldCreds.cert, a.cachedCreds.cert) {
+		a.onRotate()
+	}
+	return nil
 }