[patch 21/33] vt: fix potential race in VT_WAITACTIVE handler

[Greg KH]

-stable review patch.  If anyone has any objections, please let us know.

------------------
From: Michal Januszewski <spock@xxxxxxxxxx>

[PATCH] vt: fix potential race in VT_WAITACTIVE handler

On a multiprocessor machine the VT_WAITACTIVE ioctl call may return 0 if
fg_console has already been updated in redraw_screen() but the console
switch itself hasn't been completed.  Fix this by checking fg_console in
vt_waitactive() with the console sem held.

Signed-off-by: Michal Januszewski <spock@xxxxxxxxxx>
Acked-by: Antonino Daplas <adaplas@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Chuck Ebbert <cebbert@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>

---
 drivers/char/vt_ioctl.c |   16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

--- a/drivers/char/vt_ioctl.c
+++ b/drivers/char/vt_ioctl.c
@@ -1038,10 +1038,22 @@ int vt_waitactive(int vt)
 
 	add_wait_queue(&vt_activate_queue, &wait);
 	for (;;) {
-		set_current_state(TASK_INTERRUPTIBLE);
 		retval = 0;
-		if (vt == fg_console)
+
+		/*
+		 * Synchronize with redraw_screen(). By acquiring the console
+		 * semaphore we make sure that the console switch is completed
+		 * before we return. If we didn't wait for the semaphore, we
+		 * could return at a point where fg_console has already been
+		 * updated, but the console switch hasn't been completed.
+		 */
+		acquire_console_sem();
+		set_current_state(TASK_INTERRUPTIBLE);
+		if (vt == fg_console) {
+			release_console_sem();
 			break;
+		}
+		release_console_sem();
 		retval = -EINTR;
 		if (signal_pending(current))
 			break;

-- 
-
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