--- sys/amd64/vmm/amd/svm.c.orig
+++ sys/amd64/vmm/amd/svm.c
@@ -2198,8 +2198,11 @@
 		return (svm_modify_intr_shadow(svm_sc, vcpu, val));
 	}
 
-	if (vmcb_write(svm_sc, vcpu, ident, val) == 0) {
-		return (0);
+	/* Do not permit user write access to VMCB fields by offset. */
+	if (!VMCB_ACCESS_OK(ident)) {
+		if (vmcb_write(svm_sc, vcpu, ident, val) == 0) {
+			return (0);
+		}
 	}
 
 	reg = swctx_regptr(svm_get_guest_regctx(svm_sc, vcpu), ident);
--- sys/amd64/vmm/intel/vmx.c.orig
+++ sys/amd64/vmm/intel/vmx.c
@@ -3341,6 +3341,10 @@
 	if (vmxctx_setreg(&vmx->ctx[vcpu], reg, val) == 0)
 		return (0);
 
+	/* Do not permit user write access to VMCS fields by offset. */
+	if (reg < 0)
+		return (EINVAL);
+
 	error = vmcs_setreg(&vmx->vmcs[vcpu], running, reg, val);
 
 	if (error == 0) {