qubes-doc/user/how-to-guides/how-to-enter-fullscreen-mode.rst

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2024-05-21 14:59:46 -04:00
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How to enter fullscreen mode
============================
What is fullscreen mode?
------------------------
Normally, the Qubes GUI virtualization daemon restricts the VM from
“owning” the full screen, ensuring that there are always clearly marked
decorations drawn by the trusted Window Manager around each of the VMs
window. This allows the user to easily realize to which domain a
specific window belongs. See the
:doc:`screenshots </introduction/screenshots>` page for examples.
Why is fullscreen mode potentially dangerous?
---------------------------------------------
If one allowed one of the VMs to “own” the full screen, e.g. to show a
movie on a full screen, it might not be possible for the user to know if
the applications/VM really “released” the full screen, or if it has
started emulating the whole desktop and is pretending to be the trusted
Window Manager, drawing shapes on the screen that look e.g. like other
windows, belonging to other domains (e.g. to trick the user into
entering a secret passphrase into a window that looks like belonging to
some trusted domain).
Secure use of fullscreen mode
-----------------------------
However, it is possible to deal with fullscreen mode in a secure way
assuming there are mechanisms that can be used at any time to switch
between windows or show the full desktop and that cannot be intercepted
by the VM. The simplest example is the use of Alt+Tab for switching
between windows, which is a shortcut handled by dom0.
Other examples such mechanisms are the KDE “Present Windows” and
“Desktop Grid” effects, which are similar to Macs “Expose” effect, and
which can be used to immediately detect potential “GUI forgery”, as they
cannot be intercepted by any of the VM (as the GUID never passes down
the key combinations that got consumed by KDE Window Manager), and so
the VM cannot emulate those. Those effects are enabled by default in KDE
once Compositing gets enabled in KDE (System Settings -> Desktop ->
Enable Desktop Effects), which is recommended anyway. By default, they
are triggered by Ctrl-F8 and Ctrl-F9 key combinations, but can also be
reassigned to other shortcuts.
Enabling fullscreen mode for select VMs
---------------------------------------
You can always put a window into fullscreen mode in Xfce4 using the
trusted window manager by right-clicking on a windows title bar and
selecting “Fullscreen” or pressing ``alt`` + ``f11``. This functionality
should still be considered safe, since a VM window still cant
voluntarily enter fullscreen mode. The user must select this option from
the trusted window manager in dom0. To exit fullscreen mode from here,
press ``alt`` + ``space`` to bring up the title bar menu again, then
select “Leave Fullscreen” or simply press ``alt`` + ``f11``. For
StandaloneHVMs, you should set the screen resolution in the qube to that
of the host, (or larger), *before* setting fullscreen mode in Xfce4.
As an alternative to the Xfce4 method, you can enable fullscreen mode
for select VMs by creating the following entry in the
``/etc/qubes/guid.conf`` file in dom0:
.. code:: bash
VM: {
personal: {
allow_fullscreen = true;
};
};
The string personal above is an example only and should be replaced by
the actual name of the VM for which you want to enable this
functionality.
**Note:** There should be only one ``VM: {}`` block in the file (or you
will `get into problems <https://groups.google.com/d/msg/qubes-users/-Yf9yNvTsVI/xXsEm8y2lrYJ>`__).
One can also enable this functionality for all the VMs globally in the
same file, by modifying the global section:
.. code:: bash
global: {
# default values
allow_fullscreen = true;
#allow_utf8_titles = false;
#secure_copy_sequence = "Ctrl-Shift-c";
#secure_paste_sequence = "Ctrl-Shift-v";
#windows_count_limit = 500;
};
Be sure to restart the VM(s) after modifying this file, for the changes
to take effect.