From 75b65846ca77bd2d42e25365b4b7242a406330cf Mon Sep 17 00:00:00 2001
From: Ray Strode <rstrode@redhat.com>
Date: Tue, 7 Apr 2020 14:37:41 -0400
Subject: [PATCH 44/51] session-worker: ensure initial vt is never picked for
 !is_initial displays

Normally, a !is_initial display would never "get" tty1, since the system
boots to tty1.  But if, for some reason, the user booted to runlevel 3,
then switched to runlevel 5, the login screen could get started when
tty1 is free.

That means, e.g., an autologin user can end up getting allocated tty1,
which is bad, since we assume tty1 is used for the login screen.

This commit opens up /dev/tty1 when querying for available VTs, so that
it never gets returned by the kernel as available.
---
 daemon/gdm-session-worker.c | 39 +++++++++++++++++++++++++------------
 1 file changed, 27 insertions(+), 12 deletions(-)

diff --git a/daemon/gdm-session-worker.c b/daemon/gdm-session-worker.c
index 0bd78cfaf..42c415837 100644
--- a/daemon/gdm-session-worker.c
+++ b/daemon/gdm-session-worker.c
@@ -2167,105 +2167,120 @@ gdm_session_worker_start_session (GdmSessionWorker  *worker,
 
         /* If we end up execing again, make sure we don't use the executable context set up
          * by pam_selinux durin pam_open_session
          */
 #ifdef HAVE_SELINUX
         setexeccon (NULL);
 #endif
 
         worker->priv->child_pid = session_pid;
 
         g_debug ("GdmSessionWorker: session opened creating reply...");
         g_assert (sizeof (GPid) <= sizeof (int));
 
         g_debug ("GdmSessionWorker: state SESSION_STARTED");
         gdm_session_worker_set_state (worker, GDM_SESSION_WORKER_STATE_SESSION_STARTED);
 
         gdm_session_worker_watch_child (worker);
 
  out:
         if (error_code != PAM_SUCCESS) {
                 gdm_session_worker_uninitialize_pam (worker, error_code);
                 return FALSE;
         }
 
         return TRUE;
 }
 
 static gboolean
 set_up_for_new_vt (GdmSessionWorker *worker)
 {
-        int fd;
+        int initial_vt_fd;
         char vt_string[256], tty_string[256];
         int session_vt = 0;
 
-        fd = open ("/dev/tty0", O_RDWR | O_NOCTTY);
-
-        if (fd < 0) {
-                g_debug ("GdmSessionWorker: couldn't open VT master: %m");
+        /* open the initial vt.  We need it for two scenarios:
+         *
+         * 1) display_is_initial is TRUE.  We need it directly.
+         * 2) display_is_initial is FALSE. We need it to mark
+         * the initial VT as "in use" so it doesn't get returned
+         * by VT_OPENQRY
+         * */
+        g_snprintf (tty_string, sizeof (tty_string), "/dev/tty%d", GDM_INITIAL_VT);
+        initial_vt_fd = open (tty_string, O_RDWR | O_NOCTTY);
+
+        if (initial_vt_fd < 0) {
+                g_debug ("GdmSessionWorker: couldn't open console of initial fd: %m");
                 return FALSE;
         }
 
         if (worker->priv->display_is_initial) {
                 session_vt = GDM_INITIAL_VT;
         } else {
-                if (ioctl(fd, VT_OPENQRY, &session_vt) < 0) {
+
+                /* Typically VT_OPENQRY is called on /dev/tty0, but we already
+                 * have /dev/tty1 open above, so might as well use it.
+                 */
+                if (ioctl (initial_vt_fd, VT_OPENQRY, &session_vt) < 0) {
                         g_debug ("GdmSessionWorker: couldn't open new VT: %m");
                         goto fail;
                 }
         }
 
         worker->priv->session_vt = session_vt;
 
-        close (fd);
-        fd = -1;
-
         g_assert (session_vt > 0);
 
         g_snprintf (vt_string, sizeof (vt_string), "%d", session_vt);
 
         /* Set the VTNR. This is used by logind to configure a session in
          * the logind-managed case, but it doesn't hurt to set it always.
          * When logind gains support for XDG_VTNR=auto, we can make the
          * OPENQRY and this whole path only used by the new VT code. */
         gdm_session_worker_set_environment_variable (worker,
                                                      "XDG_VTNR",
                                                      vt_string);
 
-        g_snprintf (tty_string, 256, "/dev/tty%d", session_vt);
-        worker->priv->session_tty_fd = open (tty_string, O_RDWR | O_NOCTTY);
+        if (worker->priv->display_is_initial) {
+             worker->priv->session_tty_fd = initial_vt_fd;
+        } else {
+             g_snprintf (tty_string, sizeof (tty_string), "/dev/tty%d", session_vt);
+             worker->priv->session_tty_fd = open (tty_string, O_RDWR | O_NOCTTY);
+             close (initial_vt_fd);
+        }
+
         pam_set_item (worker->priv->pam_handle, PAM_TTY, tty_string);
 
         return TRUE;
 
 fail:
-        close (fd);
+        close (initial_vt_fd);
         return FALSE;
 }
 
 static gboolean
 set_xdg_vtnr_to_current_vt (GdmSessionWorker *worker)
 {
         int fd;
         char vt_string[256];
         struct vt_stat vt_state = { 0 };
 
         fd = open ("/dev/tty0", O_RDWR | O_NOCTTY);
 
         if (fd < 0) {
                 g_debug ("GdmSessionWorker: couldn't open VT master: %m");
                 return FALSE;
         }
 
         if (ioctl (fd, VT_GETSTATE, &vt_state) < 0) {
                 g_debug ("GdmSessionWorker: couldn't get current VT: %m");
                 goto fail;
         }
 
         close (fd);
         fd = -1;
 
         g_snprintf (vt_string, sizeof (vt_string), "%d", vt_state.v_active);
 
         gdm_session_worker_set_environment_variable (worker,
                                                      "XDG_VTNR",
                                                      vt_string);
-- 
2.27.0