Add "paranoid restore" mode

Having Admin API, it is possible to do this properly now:
 - create DisposableVM
 - assign it proper permissions to create VMs and control those created
   VMs
 - run restore process inside
 - cleanup DisposableVM afterwards

Since the RestoreInDisposableVM class contains de facto reverse parser
for qvm-backup-restore command line, add a test that will spot when it
gets out of sync.

This feature depends on modifications in various other components,
including:
 - linux-utils and core-agent-linux for update qfile-unpacker
 - core-admin for qrexec policy modification

QubesOS/qubes-issues#5310

doc/manpages/qvm-backup-restore.rst

@@ -92,6 +92,14 @@ Options
    Provided backup location is a qrexec service name (optionally with an
    argument, separated by ``+``), instead of file path or a command.
 
+.. option:: --paranoid-mode, --plan-b
+
+  Isolate restore process in a DisposableVM, defend against potentially
+  compromised backup. In this mode some parts of the backup are skipped,
+  specifically:
+
+    - dom0 home directory (desktop environment settings)
+    - PCI devices assignments
 
 Authors
 =======

qubesadmin/backup/dispvm.py

@@ -0,0 +1,275 @@
+#
+# The Qubes OS Project, http://www.qubes-os.org
+#
+# Copyright (C) 2019 Marek Marczykowski-Górecki
+#                               <marmarek@invisiblethingslab.com>
+#
+# This program 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 program 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 program; if not, write to the Free Software Foundation, Inc.,
+# 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+
+"""Handle backup extraction using DisposableVM"""
+import datetime
+import logging
+import string
+
+import subprocess
+#import typing
+import qubesadmin
+import qubesadmin.exc
+import qubesadmin.utils
+import qubesadmin.vm
+
+LOCKFILE = '/var/run/qubes/backup-paranoid-restore.lock'
+
+Option = collections.namedtuple('Option', ('opts', 'handler'))
+
+# Convenient functions for 'handler' value of Option object
+#  (see RestoreInDisposableVM.arguments):
+
+def handle_store_true(option, value):
+    """Handle argument enabling an option (action="store_true")"""
+    if value:
+        return [option.opts[0]]
+    return []
+
+
+def handle_store_false(option, value):
+    """Handle argument disabling an option (action="false")"""
+    if not value:
+        return [option.opts[0]]
+    return []
+
+def handle_verbose(option, value):
+    """Handle argument --quiet / --verbose options (action="count")"""
+    if option.opts[0] == '--verbose':
+        value -= 1  # verbose defaults to 1
+    return [option.opts[0]] * value
+
+
+def handle_store(option, value):
+    """Handle argument with arbitrary string value (action="store")"""
+    if value:
+        return [option.opts[0], str(value)]
+    return []
+
+
+def handle_append(option, value):
+    """Handle argument with a list of values (action="append")"""
+    return itertools.chain(*([option.opts[0], v] for v in value))
+
+
+def skip(_option, _value):
+    """Skip argument"""
+    return []
+
+
+def handle_unsupported(option, value):
+    """Reject argument as unsupported"""
+    if value:
+        raise NotImplementedError(
+            '{} option is not supported with --paranoid-mode'.format(
+                option.opts[0]))
+    return []
+
+class RestoreInDisposableVM:
+    """Perform backup restore with actual archive extraction isolated
+    within DisposableVM"""
+    #dispvm: typing.Optional[qubesadmin.vm.QubesVM]
+
+    #: map of args attr -> original option
+    arguments = {
+        'quiet': Option(('--quiet', '-q'), handle_verbose),
+        'verbose': Option(('--verbose', '-v'), handle_verbose),
+        'verify_only': Option(('--verify-only',), handle_store_true),
+        'skip_broken': Option(('--skip-broken',), handle_store_true),
+        'ignore_missing': Option(('--ignore-missing',), handle_store_true),
+        'skip_conflicting': Option(('--skip-conflicting',), handle_store_true),
+        'rename_conflicting': Option(('--rename-conflicting',),
+            handle_store_true),
+        'exclude': Option(('--exclude', '-x'), handle_append),
+        'dom0_home': Option(('--skip-dom0-home',), handle_store_false),
+        'ignore_username_mismatch': Option(('--ignore-username-mismatch',),
+            handle_store_true),
+        'ignore_size_limit': Option(('--ignore-size-limit',),
+            handle_store_true),
+        'compression': Option(('--compression-filter', '-Z'), handle_store),
+        'appvm': Option(('--dest-vm', '-d'), handle_store),
+        'pass_file': Option(('--passphrase-file', '-p'), handle_unsupported),
+        'location_is_service': Option(('--location-is-service',),
+            handle_store_true),
+        'paranoid_mode': Option(('--paranoid-mode', '--plan-b',), skip),
+        # make the verification easier, those don't really matter
+        'help': Option(('--help', '-h'), skip),
+        'force_root': Option(('--force-root',), skip),
+    }
+
+    def __init__(self, app, args):
+        """
+
+        :param app: Qubes() instance
+        :param args: namespace instance as with qvm-backup-restore arguments
+        parsed. See :py:module:`qubesadmin.tools.qvm_backup_restore`.
+        """
+        self.app = app
+        self.args = args
+
+        # only one backup restore is allowed at the time, use constant names
+        #: name of DisposableVM using to extract the backup
+        self.dispvm_name = 'disp-backup-restore'
+        #: tag given to this DisposableVM - qrexec policy is configured for it
+        self.dispvm_tag = 'backup-restore-mgmt'
+        #: tag automatically added to restored VMs
+        self.restored_tag = 'backup-restore-in-progress'
+        #: tag added to a VM storing the backup archive
+        self.storage_tag = 'backup-restore-storage'
+
+        self.terminal_app = ('xterm', '-hold', '-title', 'Backup restore', '-e')
+
+        self.dispvm = None
+
+        if args.appvm:
+            self.backup_storage_vm = self.app.domains[args.appvm]
+        else:
+            self.backup_storage_vm = self.app.domains['dom0']
+
+        self.storage_access_proc = None
+        self.storage_access_id = None
+        self.log = logging.getLogger('qubesadmin.backup.dispvm')
+
+    def clear_old_tags(self):
+        """Remove tags from old restore operation"""
+        for domain in self.app.domains:
+            domain.tags.discard(self.restored_tag)
+            domain.tags.discard(self.dispvm_tag)
+            domain.tags.discard(self.storage_tag)
+
+    def create_dispvm(self):
+        """Create DisposableVM used to restore"""
+        self.dispvm = self.app.add_new_vm('DispVM', self.dispvm_name, 'red',
+                                          template=self.app.management_dispvm)
+        self.dispvm.auto_cleanup = True
+        self.dispvm.features['tag-created-vm-with'] = self.restored_tag
+
+    def register_backup_source(self):
+        """Tell backup archive holding VM we want this content.
+
+        This function registers a backup source, receives a token needed to
+        access it (stored in *storage_access_id* attribute). The access is
+        revoked when connection referenced in *storage_access_proc* attribute
+        is closed.
+        """
+        self.storage_access_proc = self.backup_storage_vm.run_service(
+            'qubes.RegisterBackupLocation', stdin=subprocess.PIPE,
+            stdout=subprocess.PIPE)
+        self.storage_access_proc.stdin.write(
+            (self.args.backup_location.
+             replace("\r", "").replace("\n", "") + "\n").encode())
+        self.storage_access_proc.stdin.flush()
+        storage_access_id = self.storage_access_proc.stdout.readline().strip()
+        allowed_chars = (string.ascii_letters + string.digits).encode()
+        if not storage_access_id or \
+                not all(c in allowed_chars for c in storage_access_id):
+            if self.storage_access_proc.returncode == 127:
+                raise qubesadmin.exc.QubesException(
+                    'Backup source registration failed - qubes-core-agent '
+                    'package too old?')
+            raise qubesadmin.exc.QubesException(
+                'Backup source registration failed - got invalid id')
+        self.storage_access_id = storage_access_id.decode('ascii')
+        # keep connection open, closing it invalidates the access
+
+        self.backup_storage_vm.tags.add(self.storage_tag)
+
+    def invalidate_backup_access(self):
+        """Revoke access to backup archive"""
+        self.backup_storage_vm.tags.discard(self.storage_tag)
+        self.storage_access_proc.stdin.close()
+        self.storage_access_proc.wait()
+
+    def prepare_inner_args(self):
+        """Prepare arguments for inner (in-DispVM) qvm-backup-restore command"""
+        new_options = []
+        new_positional_args = []
+
+        for attr, opt in self.arguments.items():
+            if not hasattr(self.args, attr):
+                continue
+            new_options.extend(opt.handler(opt, getattr(self.args, attr)))
+
+        new_options.append('--location-is-service')
+
+        # backup location, replace by qrexec service to be called
+        new_positional_args.append(
+            'qubes.RestoreById+' + self.storage_access_id)
+        if self.args.vms:
+            new_positional_args.extend(self.args.vms)
+
+        return new_options + new_positional_args
+
+    def finalize_tags(self):
+        """Make sure all the restored VMs are marked with
+        restored-from-backup-xxx tag, then remove backup-restore-in-progress
+        tag"""
+        self.app.domains.clear_cache()
+        for domain in self.app.domains:
+            if 'backup-restore-in-progress' not in domain.tags:
+                continue
+            if not any(t.startswith('restored-from-backup-')
+                       for t in domain.tags):
+                self.log.warning('Restored domain %s was not tagged with '
+                                 'restored-from-backup-* tag',
+                                 domain.name)
+                # add fallback tag
+                domain.tags.add('restored-from-backup-at-{}'.format(
+                    datetime.date.strftime(datetime.date.today(), '%F')))
+            domain.tags.discard('backup-restore-in-progress')
+
+    def run(self):
+        """Run the backup restore operation"""
+        lock = qubesadmin.utils.LockFile(LOCKFILE, True)
+        lock.acquire()
+        try:
+            self.create_dispvm()
+            self.clear_old_tags()
+            self.register_backup_source()
+            args = self.prepare_inner_args()
+            self.dispvm.start()
+            self.dispvm.run_service_for_stdio('qubes.WaitForSession')
+            self.dispvm.tags.add(self.dispvm_tag)
+            self.dispvm.run_with_args(*self.terminal_app,
+                                      'qvm-backup-restore', *args,
+                                      stdout=subprocess.DEVNULL,
+                                      stderr=subprocess.DEVNULL)
+        except subprocess.CalledProcessError as e:
+            if e.returncode == 127:
+                raise qubesadmin.exc.QubesException(
+                    'qvm-backup-restore tool or {} '
+                    'missing in {} template, install qubes-core-admin-client '
+                    'package there'.format(self.terminal_app[0],
+                                           self.dispvm.template.template.name)
+                )
+            raise qubesadmin.exc.QubesException(
+                'qvm-backup-restore failed with {}'.format(e.returncode))
+        finally:
+            if self.dispvm is not None:
+                # first revoke permission, then cleanup
+                self.dispvm.tags.discard(self.dispvm_tag)
+                # autocleanup removes the VM
+                try:
+                    self.dispvm.kill()
+                except qubesadmin.exc.QubesVMNotStartedError:
+                    # delete it manually
+                    del self.app.domains[self.dispvm]
+            self.finalize_tags()
+            lock.release()

qubesadmin/tests/tools/qvm_backup_restore.py

@@ -17,12 +17,15 @@
 #
 # You should have received a copy of the GNU Lesser General Public License along
 # with this program; if not, see <http://www.gnu.org/licenses/>.
+import itertools
+
 import qubesadmin.tests
 import qubesadmin.tests.tools
 import qubesadmin.tools.qvm_backup_restore
 from unittest import mock
 from qubesadmin.backup import BackupVM
 from qubesadmin.backup.restore import BackupRestore
+from qubesadmin.backup.dispvm import RestoreInDisposableVM
 
 
 class TC_00_qvm_backup_restore(qubesadmin.tests.QubesTestCase):
@@ -231,3 +234,14 @@ class TC_00_qvm_backup_restore(qubesadmin.tests.QubesTestCase):
                 qubesadmin.tools.qvm_backup_restore.handle_broken(
                     self.app, mock_args, mock_restore_info)
             self.assertAppropriateLogging('NetVM', 'error')
+
+    def test_100_restore_in_dispvm_parser(self):
+        """Verify if qvm-backup-restore tool options matches un-parser
+        for paranoid restore mode"""
+        parser = qubesadmin.tools.qvm_backup_restore.parser
+        actions = parser._get_optional_actions()
+        options_tool = set(itertools.chain(*(a.option_strings for a in actions)))
+
+        options_parser = set(itertools.chain(
+            *(o.opts for o in RestoreInDisposableVM.arguments.values())))
+        self.assertEqual(options_tool, options_parser)

qubesadmin/tools/qvm_backup_restore.py

@@ -24,12 +24,17 @@ import getpass
 import sys
 
 from qubesadmin.backup.restore import BackupRestore
+from qubesadmin.backup.dispvm import RestoreInDisposableVM
 import qubesadmin.exc
 import qubesadmin.tools
 import qubesadmin.utils
 
 parser = qubesadmin.tools.QubesArgumentParser()
 
+# WARNING:
+# When adding options, update/verify also
+# qubeadmin.restore.dispvm.RestoreInDisposableVM.arguments
+#
 parser.add_argument("--verify-only", action="store_true",
     dest="verify_only", default=False,
     help="Verify backup integrity without restoring any "
@@ -88,6 +93,10 @@ parser.add_argument("--location-is-service", action="store_true",
     help="Interpret backup location as a qrexec service name,"
          "possibly with an argument separated by +.Requires -d option.")
 
+parser.add_argument('--paranoid-mode', '--plan-b', action="store_true",
+    help="Isolate restore process in a DispVM, defend against untrusted backup;"
+         "implies --skip-dom0-home")
+
 parser.add_argument('backup_location', action='store',
     help="Backup directory name, or command to pipe from")
 
@@ -212,6 +221,19 @@ def main(args=None, app=None):
     if args.location_is_service and not args.appvm:
         parser.error('--location-is-service option requires -d')
 
+    if args.paranoid_mode:
+        args.dom0_home = False
+        args.app.log.info("Starting restore process in a DisposableVM...")
+        args.app.log.info("When operation completes, close its window "
+                          "manually.")
+        restore_in_dispvm = RestoreInDisposableVM(args.app, args)
+        try:
+            restore_in_dispvm.run()
+        except qubesadmin.exc.QubesException as e:
+            parser.error_runtime(str(e))
+            return 1
+        return
+
     if args.pass_file is not None:
         pass_f = open(args.pass_file) if args.pass_file != "-" else sys.stdin
         passphrase = pass_f.readline().rstrip()