Index: sys/amd64/amd64/pmap.c
===================================================================
--- sys/amd64/amd64/pmap.c
+++ sys/amd64/amd64/pmap.c
@@ -2575,6 +2575,12 @@
 		pml4 = &pmap->pm_pml4[pml4index];
 		*pml4 = VM_PAGE_TO_PHYS(m) | PG_U | PG_RW | PG_V | PG_A | PG_M;
 		if (pmap->pm_pml4u != NULL && pml4index < NUPML4E) {
+			/*
+			 * Safety belt: ensure that usermode traps if
+			 * kernel mode mappings leak into user mode.
+			 */
+			*pml4 |= pg_nx;
+
 			pml4u = &pmap->pm_pml4u[pml4index];
 			*pml4u = VM_PAGE_TO_PHYS(m) | PG_U | PG_RW | PG_V |
 			    PG_A | PG_M;