[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>

1 files changed, 14 insertions, 2 deletions

diff --git a/drivers/char/vt_ioctl.c b/drivers/char/vt_ioctl.c
index 1fa2da8f4fbe..c9f2dd620e87 100644
--- a/drivers/char/vt_ioctl.c
+++ b/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;