vt: fix race in vt_waitactive()

pm_restore_console() is called from the suspend/resume path, and this
calls vt_move_to_console(), which calls vt_waitactive().

There's a race in this path which causes the process which requests the
suspend to sleep indefinitely waiting for an event which already
happened:

P1                                      P2
 vt_move_to_console()
  set_console()
    schedule_console_callback()
  vt_waitactive()
    check n == fg_console +1
                                       console_callback()
                                         switch_screen()
                                         vt_event_post() // no waiters

    vt_event_wait() // forever

Fix the race by ensuring we're registered for the event before we check
if it's already completed.

Signed-off-by: Rabin Vincent <rabin.vincent@stericsson.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 34 insertions, 13 deletions

diff --git a/drivers/tty/vt/vt_ioctl.c b/drivers/tty/vt/vt_ioctl.c
index 64618547be11..b841f56d2e66 100644
--- a/drivers/tty/vt/vt_ioctl.c
+++ b/drivers/tty/vt/vt_ioctl.c
@@ -110,16 +110,7 @@ void vt_event_post(unsigned int event, unsigned int old, unsigned int new)
                 wake_up_interruptible(&vt_event_waitqueue);
 }
 
-/**
- *      vt_event_wait           -       wait for an event
- *      @vw: our event
- *
- *      Waits for an event to occur which completes our vt_event_wait
- *      structure. On return the structure has wv->done set to 1 for success
- *      or 0 if some event such as a signal ended the wait.
- */
-
-static void vt_event_wait(struct vt_event_wait *vw)
+static void __vt_event_queue(struct vt_event_wait *vw)
 {
         unsigned long flags;
         /* Prepare the event */
@@ -129,8 +120,18 @@ static void vt_event_wait(struct vt_event_wait *vw)
         spin_lock_irqsave(&vt_event_lock, flags);
         list_add(&vw->list, &vt_events);
         spin_unlock_irqrestore(&vt_event_lock, flags);
+}
+
+static void __vt_event_wait(struct vt_event_wait *vw)
+{
         /* Wait for it to pass */
         wait_event_interruptible(vt_event_waitqueue, vw->done);
+}
+
+static void __vt_event_dequeue(struct vt_event_wait *vw)
+{
+        unsigned long flags;
+
         /* Dequeue it */
         spin_lock_irqsave(&vt_event_lock, flags);
         list_del(&vw->list);
@@ -138,6 +139,22 @@ static void vt_event_wait(struct vt_event_wait *vw)
 }
 
 /**
+ *      vt_event_wait           -       wait for an event
+ *      @vw: our event
+ *
+ *      Waits for an event to occur which completes our vt_event_wait
+ *      structure. On return the structure has wv->done set to 1 for success
+ *      or 0 if some event such as a signal ended the wait.
+ */
+
+static void vt_event_wait(struct vt_event_wait *vw)
+{
+        __vt_event_queue(vw);
+        __vt_event_wait(vw);
+        __vt_event_dequeue(vw);
+}
+
+/**
  *      vt_event_wait_ioctl     -       event ioctl handler
  *      @arg: argument to ioctl
  *
@@ -177,10 +194,14 @@ int vt_waitactive(int n)
 {
         struct vt_event_wait vw;
         do {
-                if (n == fg_console + 1)
-                        break;
                 vw.event.event = VT_EVENT_SWITCH;
-                vt_event_wait(&vw);
+                __vt_event_queue(&vw);
+                if (n == fg_console + 1) {
+                        __vt_event_dequeue(&vw);
+                        break;
+                }
+                __vt_event_wait(&vw);
+                __vt_event_dequeue(&vw);
                 if (vw.done == 0)
                         return -EINTR;
         } while (vw.event.newev != n);