userfaultfd: non-cooperative: robustness check

Similar to the handle_userfault() case, also make sure to never attempt
to send any event past the PF_EXITING point of no return.

This is purely a robustness check.

Link: http://lkml.kernel.org/r/20170224181957.19736-3-aarcange@redhat.com
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Acked-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Pavel Emelyanov <xemul@parallels.com>
Cc: Hillf Danton <hillf.zj@alibaba-inc.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/userfaultfd.c

@@ -530,8 +530,13 @@ out:
 static int userfaultfd_event_wait_completion(struct userfaultfd_ctx *ctx,
 					     struct userfaultfd_wait_queue *ewq)
 {
-	int ret = 0;
+	int ret;
+
+	ret = -1;
+	if (WARN_ON_ONCE(current->flags & PF_EXITING))
+		goto out;
 
+	ret = 0;
 	ewq->ctx = ctx;
 	init_waitqueue_entry(&ewq->wq, current);
 
@@ -566,7 +571,7 @@ static int userfaultfd_event_wait_completion(struct userfaultfd_ctx *ctx,
 	 * ctx may go away after this if the userfault pseudo fd is
 	 * already released.
 	 */
-
+out:
 	userfaultfd_ctx_put(ctx);
 	return ret;
 }