It is now actually working :)

config/exec_provider.py

@@ -51,23 +51,25 @@ def run(self, previous_response=None):
         self.env['KUBERNETES_EXEC_INFO'] = json.dumps(kubernetes_exec_info)
         process = subprocess.Popen(
             self.args,
-            stdin=subprocess.PIPE,
+            stdout=subprocess.PIPE,
             stderr=subprocess.PIPE,
-            env=self.env)
-        process.wait()
-        if process.returncode != 0:
-            msg = 'exec: process returned %d' % process.returncode
-            stderr = process.stderr.read().strip()
+            env=self.env,
+            universal_newlines=True)
+        (stdout, stderr) = process.communicate()
+        exit_code = process.wait()
+        if exit_code != 0:
+            msg = 'exec: process returned %d' % exit_code
+            stderr = stderr.strip()
             if stderr:
                 msg += '. %s' % stderr
             raise ConfigException(msg)
-        stdout = process.stdout.read()
+        stdout = stdout
         try:
             data = json.loads(stdout)
         except json.decoder.JSONDecodeError as de:
             raise ConfigException(
                 'exec: failed to decode process output: %s' % de)
-        if all(k in data for k in ('apiVersion', 'kind', 'status')):
+        if not all(k in data for k in ('apiVersion', 'kind', 'status')):
             raise ConfigException(
                 'exec: malformed response. plugin returned: %s' % stdout)
         if data['apiVersion'] != self.api_version: