x86: tighten page table owner checking in do_mmu_update()

MMU_MACHPHYS_UPDATE, not manipulating page tables, shouldn't ignore
a bad page table domain being specified.

Also pt_owner can't be NULL when reaching the "out" label, so the
respective check can be dropped.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Acked-by: Tim Deegan <tim@xxxxxxx>

--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -3618,6 +3618,11 @@ long do_mmu_update(
         break;
 
         case MMU_MACHPHYS_UPDATE:
+            if ( unlikely(d != pt_owner) )
+            {
+                rc = -EPERM;
+                break;
+            }
 
             mfn = req.ptr >> PAGE_SHIFT;
             gpfn = req.val;
@@ -3694,7 +3699,7 @@ long do_mmu_update(
     perfc_add(num_page_updates, i);
 
  out:
-    if ( pt_owner && (pt_owner != d) )
+    if ( pt_owner != d )
         rcu_unlock_domain(pt_owner);
 
     /* Add incremental work we have done to the @done output parameter. */