[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@gentoo.org>
Acked-by: Antonino Daplas <adaplas@pol.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/char/vt_ioctl.c

@@ -1039,10 +1039,22 @@ 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;