KVM: MMU: Set nx bit correctly on shadow ptes

While the page table walker correctly generates a guest page fault
if a guest tries to execute a non-executable page, the shadow code does
not mark it non-executable.  This means that if a guest accesses an nx
page first with a read access, then subsequent code fetch accesses will
succeed.

Fix by setting the nx bit on shadow ptes.
Signed-off-by: Avi Kivity <avi@qumranet.com>

drivers/kvm/paging_tmpl.h

@@ -255,6 +255,8 @@ static void FNAME(set_pte)(struct kvm_vcpu *vcpu, pt_element_t gpte,
 	spte |= gpte & PT64_NX_MASK;
 	if (!dirty)
 		pte_access &= ~ACC_WRITE_MASK;
+	if (!(pte_access & ACC_EXEC_MASK))
+		spte |= PT64_NX_MASK;
 
 	page = gfn_to_page(vcpu->kvm, gfn);