core-admin/qubes-rpc-policy/qubes.VMShell.policy

## Note that policy parsing stops at the first match,
## so adding anything below "$anyvm $anyvm action" line will have no effect

## Please use a single # to start your custom comments

$anyvm	$dispvm	allow
$anyvm	$anyvm	deny

# WARNING: The qubes.VMShell service is dangerous and there are really few
# cases when it could be safely used. Allowing one VM to execute qubes.VMShell
# over the other VM allows the former to TAKE FULL CONTROL over the later. In
# most cases this is not what we want!
#
# Instead we should be using task-specific qrexec services which provide
# assurance as to what program will be responding to the (untrusted) VM
# requests.
#
# It is, however, safe, in most cases, to allow ultimate control of the
# creating AppVM over the DisposableVM it creates as part of the qrexec service
# invocation. That's why by default we have "$anyvm  $dispvm allow" rule. Note
# that it does _not_ allow any AppVM to execute qubes.VMShell service over any
# DispVM created in the system -- that would obviously be wrong. It only allows
# qubes.VMShell service access to the AppVM which creates the DispVM as part of
# this very service invocation.
#
# See e.g. this thread for some discussion:
# https://groups.google.com/d/msg/qubes-users/xnAByaL_bjI/3PjYdiTDW-0J
#
#