From 15a19ac7856c539aa9cfbf76997d18b0275aae35 Mon Sep 17 00:00:00 2001
From: Iain Lane <iainl@gnome.org>
Date: Mon, 4 Feb 2019 15:12:38 +0000
Subject: [PATCH 4/4] GdmManager: Don't perform timed login if session gets
 started

At the moment it's possible for the login screen to initiate
a timed login operation shortly after a user successfully starts
their session.

GDM won't complete the timed login operation, since a session is
already running, but will erroneously overwrite the username
associated with the session, misattributing the users session
to the timed login user.

Later, attempts to log in as the timed user will instead unlock the
session for the other user, since that session is now associated
with the timed login user.

This commit refuses timed login requests on sessions that are
already running, so the username doesn't get corrupted.

CVE-2019-3825

Closes https://gitlab.gnome.org/GNOME/gdm/issues/460
---
 daemon/gdm-manager.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/daemon/gdm-manager.c b/daemon/gdm-manager.c
index 0cc06a978..056560b20 100644
--- a/daemon/gdm-manager.c
+++ b/daemon/gdm-manager.c
@@ -2116,60 +2116,68 @@ on_session_client_ready_for_session_to_start (GdmSession      *session,
         } else {
                 g_debug ("GdmManager: Will start session when ready and told");
         }
 
         waiting_to_start_user_session = GPOINTER_TO_INT (g_object_get_data (G_OBJECT (session),
                                                                        "waiting-to-start"));
 
         g_object_set_data (G_OBJECT (session),
                            "start-when-ready",
                            GINT_TO_POINTER (client_is_ready));
 
         if (client_is_ready && waiting_to_start_user_session) {
                 start_user_session_if_ready (manager, session, service_name);
         }
 }
 
 static void
 on_session_client_connected (GdmSession      *session,
                              GCredentials    *credentials,
                              GPid             pid_of_client,
                              GdmManager      *manager)
 {
         GdmDisplay *display;
         char    *username;
         int      delay;
         gboolean enabled;
         gboolean allow_timed_login = FALSE;
 
         g_debug ("GdmManager: client with pid %d connected", (int) pid_of_client);
 
+        if (gdm_session_is_running (session)) {
+                const char *session_username;
+                session_username = gdm_session_get_username (session);
+                g_debug ("GdmManager: ignoring connection, since session already running (for user %s)",
+                         session_username);
+                return;
+        }
+
         display = get_display_for_user_session (session);
 
         if (display == NULL) {
                 return;
         }
 
         if (!display_is_on_seat0 (display)) {
                 return;
         }
 
 #ifdef WITH_PLYMOUTH
         if (manager->priv->plymouth_is_running) {
                 plymouth_quit_with_transition ();
                 manager->priv->plymouth_is_running = FALSE;
         }
 #endif
 
         g_object_get (G_OBJECT (display), "allow-timed-login", &allow_timed_login, NULL);
 
         if (!allow_timed_login) {
                 return;
         }
 
         enabled = get_timed_login_details (manager, &username, &delay);
 
         if (! enabled) {
                 return;
         }
 
         gdm_session_set_timed_login_details (session, username, delay);
-- 
2.21.0