Re: Sandbox all the WebKit!

Good questions! Under flatpak, WebKit will actually use flatpak-spawn 
to create a flatpak subsandbox, instead of using its own bubblewrap 
sandbox. So yes, WebKit's bubblewrap sandbox does not get used, but 
there is a flatpak "subsandbox" instead. It effectively does:

$ flatpak-spawn --sandbox --no-network --sandbox-flag=share-gpu 
--sandbox-flag=share-display --sandbox-flag=share-sound 
--sandbox-flag=allow-a11y --sandbox-flag=allow-dbus

So network access is blocked. We have an new experimental GPU process 
that should allow blocking GPU as well in the (distant) future. D-Bus 
access is restricted to whatever flatpak allows (portals, plus services 
namespaced by the app ID). The other permissions are probably not 
removable.

Regardless of whether WebKit is using its bubblewrap sandbox or 
flatpak-spawn, limited host access is provided via xdg-desktop-portal 
and xdg-desktop-portal-gtk, so distros should be sure to add an 
appropriate runtime Requires: xdg-desktop-portal-gtk (which I had 
forgotten to do in Fedora until... yesterday).

Thank Patrick (TingPing) for putting this all together. I think Alex 
helped too with some changes in flatpak-sandbox? I'm very impressed at 
how well it worked out. :)

Michael