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


(cherry picked from commit dc8235128c3a1fcd5da8f30ab6839d413d353f28)

1 files changed, 7 insertions, 4 deletions

diff --git a/daemon/gdm-display.c b/daemon/gdm-display.c
index d1d24956..687e7da4 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);