How it should work:

When a D-Bus message activates a service, LSMs (SELinux or AppArmor) check
whether the message can be delivered after the service has been activated. The
service is considered activated when its well-known name is requested with
org.freedesktop.DBus.RequestName. When the message delivery is denied, the
service stays activated but should not receive the activating message (the
message which triggered the activation). dbus-daemon is supposed to drop the
activating message and reply to the sender with a D-Bus error message.

However, it does not work as expected:

1. The error message is delivered to the service instead of being delivered to
   the sender. As an example, the error message could be something like:

     An SELinux policy prevents this sender from sending this
     message to this recipient, [...] member="MaliciousMethod"

   If the sender and the service are malicious confederates and agree on a
...