Add policy provider that would filter extensions/apps allowed on the

extensions/common/api/_behavior_features.json

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // This features file defines switches used to control Extension behaviour,
 // typically whitelist configuration.
 // See chrome/common/extensions/api/_features.md to understand this file, as
 // well as feature.h, simple_feature.h, and base_feature_provider.h.
 
 {
@@ skipping 28 matching lines @@
     "channel": "stable",
     "extension_types": "all",
     "whitelist": [
       "CBCC42ABED43A4B58FE3810E62AFFA010EB0349F"   // https://crbug.com/538252
     ]
   },
   "allow_usb_devices_permission_interface_class": {
     "channel": "dev",
     "extension_types": ["platform_app"],
     "session_types": ["kiosk"] 
+  },
+  "signin_screen": [ {
+    "blacklist": [
+      "9E527CDA9D7C50844E8A5DB964A54A640AE48F98"   // Chrome remote desktop
+    ],
+    // Allow developers to test new features.
+    "channel": "dev",
+    "component_extensions_auto_granted": false,
+    "extension_types": ["platform_app"],
+    "location": "policy",
+    "platforms": ["chromeos"]
+  }, {
+    // Strict restrictions for stable channel.
+    // For now only whitelisted apps.
+    "channel": "stable",
+    "component_extensions_auto_granted": false,
+    "extension_types": ["platform_app"],
+    "location": "policy",
+    "platforms": ["chromeos"],
+    "whitelist": [
+      "EC3DE21E048B67319893889529354DFBFA96FD23"  // Smart Card Connector

[emaxx, 2016/11/10 14:44:32]

As you started adding real-world apps, please add the second (and the last) app
that was planned to be whitelisted in the first version:
"CSSI Smart Card Middleware" (ID "haeblkpifdemlfnkogkipmghfcbonief")

[Denis Kuznetsov (DE-MUC), 2016/11/11 15:50:17]

Done.

+    ]
+  }, {
+    // Explicitly whitelist extensions that should always be available

[emaxx, 2016/11/10 14:44:32]

Can the "location" key be also specified here?
These all, as far as I understand, are component extensions.

I'm not sure how easy could it be to make some third-party extension have the ID
hash from this list, but the additional level of defense shouldn't be excessive.

[Denis Kuznetsov (DE-MUC), 2016/11/11 15:50:17]

Done.

+    // on a sign-in screen.
+    "channel": "stable",
+    "component_extensions_auto_granted": false,
+    "platforms": ["chromeos"],
+    "whitelist": [
+      "E24F1786D842E91E74C27929B0B3715A4689A473",  // Gnubby component extension
+      "6F9E349A0561C78A0D3F41496FE521C5151C7F71",  // Gnubby app
+      "8EBDF73405D0B84CEABB8C7513C9B9FA9F1DC2CE",  // Genius app (help)
+      "3F50C3A83839D9C76334BCE81CDEC06174F266AF"   // Virtual Keyboard
+    ]
   }
+  ]
 }