Accept client certificates from an authn/authz plugin

(Plugin interface reference: https://kubernetes.io/docs/reference/access-authn-authz/authentication/#input-and-output-formats)

When handling the response from the authn/authz plugin, `token` will be used if provided, which maintains current behaviour. Newly added is handling `clientCertificateData`: if it is present, that certificate (and its key) will be used as provided by the plugin. (And any certificate/key pair provided via the `users` section of the configuration file will be ignored.)

config/kube_config.py

@@ -472,11 +472,31 @@ class KubeConfigLoader(object):
             return
         try:
             status = ExecProvider(self._user['exec']).run()
-            if 'token' not in status:
-                logging.error('exec: missing token field in plugin output')
-                return None
-            self.token = "Bearer %s" % status['token']
-            return True
+            if 'token' in status:
+                self.token = "Bearer %s" % status['token']
+                return True
+            if 'clientCertificateData' in status:
+                # https://kubernetes.io/docs/reference/access-authn-authz/authentication/#input-and-output-formats
+                # Plugin has provided certificates instead of a token.
+                if 'clientKeyData' not in status:
+                    logging.error('exec: missing clientKeyData field in '
+                                  'plugin output')
+                    return None
+                base_path = self._get_base_path(self._cluster.path)
+                self.cert_file = FileOrData(
+                    status, None,
+                    data_key_name='clientCertificateData',
+                    file_base_path=base_path,
+                    base64_file_content=False).as_file()
+                self.key_file = FileOrData(
+                    status, None,
+                    data_key_name='clientKeyData',
+                    file_base_path=base_path,
+                    base64_file_content=False).as_file()
+                return True
+            logging.error('exec: missing token or clientCertificateData field '
+                          'in plugin output')
+            return None
         except Exception as e:
             logging.error(str(e))
 
@@ -512,12 +532,16 @@ class KubeConfigLoader(object):
                 self.ssl_ca_cert = FileOrData(
                     self._cluster, 'certificate-authority',
                     file_base_path=base_path).as_file()
-                self.cert_file = FileOrData(
-                    self._user, 'client-certificate',
-                    file_base_path=base_path).as_file()
-                self.key_file = FileOrData(
-                    self._user, 'client-key',
-                    file_base_path=base_path).as_file()
+                if 'cert_file' not in self.__dict__:
+                    # cert_file could have been provided by
+                    # _load_from_exec_plugin; only load from the _user
+                    # section if we need it.
+                    self.cert_file = FileOrData(
+                        self._user, 'client-certificate',
+                        file_base_path=base_path).as_file()
+                    self.key_file = FileOrData(
+                        self._user, 'client-key',
+                        file_base_path=base_path).as_file()
         if 'insecure-skip-tls-verify' in self._cluster:
             self.verify_ssl = not self._cluster['insecure-skip-tls-verify']
 

config/kube_config_test.py

@@ -541,6 +541,13 @@ class TestKubeConfigLoader(BaseTestCase):
                     "user": "exec_cred_user"
                 }
             },
+            {
+                "name": "exec_cred_user_certificate",
+                "context": {
+                    "cluster": "ssl",
+                    "user": "exec_cred_user_certificate"
+                }
+            },
             {
                 "name": "contexttestcmdpath",
                 "context": {
@@ -865,6 +872,16 @@ class TestKubeConfigLoader(BaseTestCase):
                     }
                 }
             },
+            {
+                "name": "exec_cred_user_certificate",
+                "user": {
+                    "exec": {
+                        "apiVersion": "client.authentication.k8s.io/v1beta1",
+                        "command": "custom-certificate-authenticator",
+                        "args": []
+                    }
+                }
+            },
             {
                 "name": "usertestcmdpath",
                 "user": {
@@ -1295,6 +1312,24 @@ class TestKubeConfigLoader(BaseTestCase):
             active_context="exec_cred_user").load_and_set(actual)
         self.assertEqual(expected, actual)
 
+    @mock.patch('kubernetes.config.kube_config.ExecProvider.run')
+    def test_user_exec_auth_certificates(self, mock):
+        mock.return_value = {
+            "clientCertificateData": TEST_CLIENT_CERT,
+            "clientKeyData": TEST_CLIENT_KEY,
+        }
+        expected = FakeConfig(
+            host=TEST_SSL_HOST,
+            cert_file=self._create_temp_file(TEST_CLIENT_CERT),
+            key_file=self._create_temp_file(TEST_CLIENT_KEY),
+            ssl_ca_cert=self._create_temp_file(TEST_CERTIFICATE_AUTH),
+            verify_ssl=True)
+        actual = FakeConfig()
+        KubeConfigLoader(
+            config_dict=self.TEST_KUBE_CONFIG,
+            active_context="exec_cred_user_certificate").load_and_set(actual)
+        self.assertEqual(expected, actual)
+
     def test_user_cmd_path(self):
         A = namedtuple('A', ['token', 'expiry'])
         token = "dummy"