fix console change race exposed by CFS

The new behaviour of CFS exposes a race which occurs if a switch is
requested when vt_mode.mode is VT_PROCESS.

The process with vc->vt_pid is signaled before vc->vt_newvt is set.
This causes the switch to fail when triggered by the monitoing process
because the target is still -1.

[ If the signal sending fails, the subsequent "reset_vc(vc)" will then
  reset vt_newvt to -1, so this works for that case too.   - Linus ]

Signed-off-by: Jan Lübbe <jluebbe@lasnet.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 5 insertions, 2 deletions

diff --git a/drivers/char/vt_ioctl.c b/drivers/char/vt_ioctl.c
index c6f6f4209739..045e6888d15b 100644
--- a/drivers/char/vt_ioctl.c
+++ b/drivers/char/vt_ioctl.c
@@ -1208,15 +1208,18 @@ void change_console(struct vc_data *new_vc)
                 /*
                  * Send the signal as privileged - kill_pid() will
                  * tell us if the process has gone or something else
-                 * is awry
+                 * is awry.
+                 *
+                 * We need to set vt_newvt *before* sending the signal or we
+                 * have a race.
                  */
+                vc->vt_newvt = new_vc->vc_num;
                 if (kill_pid(vc->vt_pid, vc->vt_mode.relsig, 1) == 0) {
                         /*
                          * It worked. Mark the vt to switch to and
                          * return. The process needs to send us a
                          * VT_RELDISP ioctl to complete the switch.
                          */
-                        vc->vt_newvt = new_vc->vc_num;
                         return;
                 }