As part of CVE-2013-4288, the D-Bus clients were allowed (and
encouraged) to submit the UID of the subject of authorization checks
to avoid races against UID changes (notably using executables
set-UID to root).

However, that also allowed any client to submit an arbitrary UID, and
that could be used to bypass "can only ask about / affect the same UID"
checks in CheckAuthorization / RegisterAuthenticationAgent /
UnregisterAuthenticationAgent.  This allowed an attacker:

- With CheckAuthorization, to cause the registered authentication
  agent in victim's session to pop up a dialog, or to determine whether
  the victim currently has a temporary authorization to perform an
  operation.

  (In principle, the attacker can also determine whether JavaScript
  rules allow the victim process to perform an operation; however,
  usually rules base their decisions on information determined from
  the supplied UID, so the attacker usually won't learn anything new.)

- With RegisterAuthenticationAgent, to prevent the victim's
  authentication agent to work (for a specific victim process),
  or to learn about which operations requiring authorization
  the victim is attempting.

To fix this, expose internal _polkit_unix_process_get_owner() /
obsolete polkit_unix_process_get_owner() as a private
polkit_unix_process_get_racy_uid__() (being more explicit about the
dangers on relying on it), and use it in
polkit_backend_session_monitor_get_user_for_subject() to return
a boolean indicating whether the subject UID may be caller-chosen.

Then, in the permission checks that require the subject to be
equal to the caller, fail on caller-chosen UIDs (and continue
through the pre-existing code paths which allow root, or root-designated
server processes, to ask about arbitrary subjects.)

Signed-off-by: Miloslav Trmač <mitr@redhat.com>