Index: sys/kern/kern_event.c
===================================================================
RCS file: /cvsroot/src/sys/kern/kern_event.c,v
retrieving revision 1.97
diff -p -u -r1.97 kern_event.c
--- sys/kern/kern_event.c	7 Nov 2017 18:35:57 -0000	1.97
+++ sys/kern/kern_event.c	11 Nov 2017 02:34:56 -0000
@@ -982,6 +982,10 @@ kqueue_register(struct kqueue *kq, struc
 		}
 		mutex_enter(&fdp->fd_lock);
 		ff = fdp->fd_dt->dt_ff[fd];
+		if (ff->ff_refcnt & FR_CLOSING) {
+			error = EBADF;
+			goto done;
+		}
 		if (fd <= fdp->fd_lastkqfile) {
 			SLIST_FOREACH(kn, &ff->ff_knlist, kn_link) {
 				if (kq == kn->kn_kq &&