Add an extension preventing starting a VM while it's being restored

Do not allow starting a VM while the restoring management VM has still
control over it. Specifically, that restoring VM will not be able to
start just restored VM.

QubesOS/qubes-issues#5310

qubes/ext/backup_restore.py

@@ -0,0 +1,39 @@
+# -*- encoding: utf8 -*-
+#
+# The Qubes OS Project, http://www.qubes-os.org
+#
+# Copyright (C) 2019 Marek Marczykowski-Górecki
+#   <marmarek@invisiblethingslab.com>
+#
+# This library is free software; you can redistribute it and/or
+# modify it under the terms of the GNU Lesser General Public
+# License as published by the Free Software Foundation; either
+# version 2.1 of the License, or (at your option) any later version.
+#
+# This library is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+# Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public
+# License along with this library; if not, see <https://www.gnu.org/licenses/>.
+
+"""
+Backup restore related functionality. Specifically:
+ - prevent starting a domain currently being restored
+"""
+
+import qubes.api
+import qubes.ext
+import qubes.vm.adminvm
+
+
+class BackupRestoreExtension(qubes.ext.Extension):
+    # pylint: disable=too-few-public-methods
+    @qubes.ext.handler('domain-pre-start')
+    def on_domain_pre_start(self, vm, event, **kwargs):
+        """Prevent starting a VM during restore"""
+        # pylint: disable=unused-argument,no-self-use
+        if 'backup-restore-in-progress' in vm.tags:
+            raise qubes.exc.QubesVMError(
+                vm, 'Restore of this domain in progress, cannot start')

rpm_spec/core-dom0.spec.in

@@ -416,6 +416,7 @@ done
 %{python3_sitelib}/qubes/ext/__pycache__/*
 %{python3_sitelib}/qubes/ext/__init__.py
 %{python3_sitelib}/qubes/ext/admin.py
+%{python3_sitelib}/qubes/ext/backup_restore.py
 %{python3_sitelib}/qubes/ext/block.py
 %{python3_sitelib}/qubes/ext/core_features.py
 %{python3_sitelib}/qubes/ext/gui.py

setup.py

@@ -61,6 +61,8 @@ if __name__ == '__main__':
             ],
             'qubes.ext': [
                 'qubes.ext.admin = qubes.ext.admin:AdminExtension',
+                'qubes.ext.backup_restore = '
+                'qubes.ext.backup_restore:BackupRestoreExtension',
                 'qubes.ext.core_features = qubes.ext.core_features:CoreFeatures',
                 'qubes.ext.gui = qubes.ext.gui:GUI',
                 'qubes.ext.audio = qubes.ext.audio:AUDIO',