[gdm: 1/3] display: Exit with failure if loading existing users fails

[Ray Strode <halfline src gnome org>]

commit dc8235128c3a1fcd5da8f30ab6839d413d353f28
Author: Marco Trevisan (Treviño) <mail 3v1n0 net>
Date:   Tue Oct 27 15:14:27 2020 +0100

    display: Exit with failure if loading existing users fails
    
    Given not having users may make GDM to launch initial setup, that
    allows to create new users (potentially with sudo capabilities), it's
    better to make look_for_existing_users() to return its status and only
    if it didn't fail continue the gdm execution.
    
    GHSL-2020-202
    CVE-2020-16125
    
    Fixes #642

 daemon/gdm-display.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)
---
diff --git a/daemon/gdm-display.c b/daemon/gdm-display.c
index d1d24956f..687e7da4b 100644
--- a/daemon/gdm-display.c
+++ b/daemon/gdm-display.c
@@ -510,7 +510,7 @@ gdm_display_real_prepare (GdmDisplay *self)
         return TRUE;
 }
 
-static void
+static gboolean
 look_for_existing_users_sync (GdmDisplay *self)
 {
         GdmDisplayPrivate *priv;
@@ -528,7 +528,7 @@ look_for_existing_users_sync (GdmDisplay *self)
                                                              &error);
 
         if (!priv->accountsservice_proxy) {
-                g_warning ("Failed to contact accountsservice: %s", error->message);
+                g_critical ("Failed to contact accountsservice: %s", error->message);
                 goto out;
         }
 
@@ -541,7 +541,7 @@ look_for_existing_users_sync (GdmDisplay *self)
                                               &error);
 
         if (!call_result) {
-                g_warning ("Failed to list cached users: %s", error->message);
+                g_critical ("Failed to list cached users: %s", error->message);
                 goto out;
         }
 
@@ -551,6 +551,7 @@ look_for_existing_users_sync (GdmDisplay *self)
         g_variant_unref (call_result);
 out:
         g_clear_error (&error);
+        return priv->accountsservice_proxy != NULL && call_result != NULL;
 }
 
 gboolean
@@ -568,7 +569,9 @@ gdm_display_prepare (GdmDisplay *self)
         /* FIXME: we should probably do this in a more global place,
          * asynchronously
          */
-        look_for_existing_users_sync (self);
+        if (!look_for_existing_users_sync (self)) {
+                exit (EXIT_FAILURE);
+        }
 
         priv->doing_initial_setup = wants_initial_setup (self);