Index: vm_mmap.c
===================================================================
--- vm_mmap.c
+++ vm_mmap.c
@@ -257,6 +257,9 @@
 
 	/* Adjust size for rounding (on both ends). */
 	size += pageoff;			/* low end... */
+	/* Check for rounding up to zero. */
+	if ((vm_size_t) round_page(size) < size)
+		return (EINVAL);
 	size = (vm_size_t) round_page(size);	/* hi end */
 
 	/* Ensure alignment is at least a page and fits in a pointer. */