Prevent backing up dom0 it itself (in home dir)

resolves: QubesOS/qubes-issues#10127

Author: alimirjamali

qubes/backup.py

@@ -28,6 +28,7 @@
 import itertools
 import logging
 import os
+from pathlib import Path
 import pwd
 import shutil
 import stat
@@ -452,6 +453,15 @@ def get_files_to_backup(self):
         if 0 in [vm.qid for vm in self.vms_for_backup]:
             local_user = grp.getgrnam("qubes").gr_mem[0]
             home_dir = pwd.getpwnam(local_user).pw_dir
+
+            # Checking if target is not user home directory in dom0
+            if self.target_dir == home_dir \
+                    or Path(home_dir) in Path(self.target_dir).parents \
+                    or not self.target_dir:
+                raise qubes.exc.QubesException(
+                    "Can not backup dom0 home directory to itself!"
+                )
+
             # Home dir should have only user-owned files, so fix it now
             # to prevent permissions problems - some root-owned files can
             # left after 'sudo bash' and similar commands