Report if video capturing meets output protection requirement.

content/common/media/media_stream_messages.h

Index: content/common/media/media_stream_messages.h
diff --git a/content/common/media/media_stream_messages.h b/content/common/media/media_stream_messages.h
index d4847c214644679e3894dc00a2c978873920220c..70b5f0304b0960e219f178929d638ea4b9427025 100644
--- a/content/common/media/media_stream_messages.h
+++ b/content/common/media/media_stream_messages.h
@@ -152,3 +152,10 @@ IPC_MESSAGE_CONTROL2(MediaStreamHostMsg_SubscribeToDeviceChangeNotifications,
 // Cancel notifications about changes in the set of media devices.
 IPC_MESSAGE_CONTROL1(MediaStreamHostMsg_CancelDeviceChangeNotifications,
                      int /* render frame id */)
+
+// Tell the browser process if the video capture is secure (i.e., all
+// connected video sinks meet the requirement of output protection.).
+IPC_MESSAGE_CONTROL3(MediaStreamHostMsg_SetCapturingLinkSecured,
+                     int,                      /* session_id */
+                     content::MediaStreamType, /* type */
+                     bool /* is_secure */)

[nasko, 2016/05/09 21:33:29]

This seems incorrect from threat modelling perspective. We should not be
trusting the renderer process to tell us secure vs non-secure state. Is there no
way for the browser process to know this information?

[miu, 2016/05/09 23:09:31]

In the general case, you'd be absolutely correct. However, this is being used
for a more specialized purpose: At a high level, we are trying to determine
whether protected video content is being screen-captured (e.g., via the
chrome.tabCapture API) and then the resulting video stream routed by an
extension to insecure links/destinations.

The browser process only trusts the "is_secure" value in this IPC message if
it's coming from a trusted, whitelisted extension. Extensions run in separate
render processes. So it shouldn't be possible, for example, for a user's visit
to a malicious web page to compromise a render process running a trusted
extension to make it report falsehood in this IPC message.

[xjz, 2016/05/10 00:28:25]

Yuri: Thanks for clarifying this!