From aa3c3a77fe880f60d9688d00c1c5d29e49d64852 Mon Sep 17 00:00:00 2001
From: Ben Grande <ben@invisiblethingslab.com>
Date: Thu, 15 Jan 2026 14:37:44 +0100
Subject: [PATCH] Return empty string when device denied is None

Loading the XML interprets empty as None.

Fixes: https://github.com/QubesOS/qubes-issues/issues/10254
---
 qubes/tests/api_admin.py | 4 ++++
 qubes/vm/qubesvm.py      | 8 +++++---
 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/qubes/tests/api_admin.py b/qubes/tests/api_admin.py
index 7dee1cb2d..9c2aa53f4 100644
--- a/qubes/tests/api_admin.py
+++ b/qubes/tests/api_admin.py
@@ -4252,6 +4252,10 @@ def test_660_vm_device_denied_list_empty(self):
         self.assertEqual(actual, "")
         self.assertFalse(self.app.save.called)
 
+        self.vm.devices_denied = None
+        self.assertEqual(self.vm.devices_denied, "")
+        self.assertFalse(self.app.save.called)
+
     def test_661_vm_device_denied_list(self):
         self.vm.devices_denied = "b******p012345pff**2*"
         actual = self.call_mgmt_func(
diff --git a/qubes/vm/qubesvm.py b/qubes/vm/qubesvm.py
index c652ce298..9c8d67cca 100644
--- a/qubes/vm/qubesvm.py
+++ b/qubes/vm/qubesvm.py
@@ -137,10 +137,12 @@ def _setter_default_user(self, prop, value):
 
 def _setter_denied_list(self, prop, value):
     """Helper for setting denied list"""
-    value = str(value)
-    if len(value) == 0:
-        return value
+    # Although the property has type str, loading an empty value from XML leads
+    # to None.
+    if not value:
+        return ""
 
+    value = str(value)
     # remove duplicates
     value = "".join(
         sorted(map(repr, set(DeviceInterface.from_str_bulk(value))))