x86: refine address validity checks before accessing page tables

In commit 40d66baa ("x86: correct LDT checks") and d06a0d71 ("x86: add
address validity check to guest_map_l1e()") I didn't really pay
attention to the fact that these checks would better be done before the
paging_mode_translate() ones, as there's also no equivalent check down
the shadow code paths involved here (at least not up to the first use
of the address), and such generic checks shouldn't really be done by
particular backend functions anyway.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/include/asm-x86/paging.h
+++ b/xen/include/asm-x86/paging.h
@@ -356,12 +356,14 @@ guest_map_l1e(struct vcpu *v, unsigned l
 {
     l2_pgentry_t l2e;
 
+    if ( unlikely(!__addr_ok(addr)) )
+        return NULL;
+
     if ( unlikely(paging_mode_translate(v->domain)) )
         return paging_get_hostmode(v)->guest_map_l1e(v, addr, gl1mfn);
 
     /* Find this l1e and its enclosing l1mfn in the linear map */
-    if ( !__addr_ok(addr) ||
-         __copy_from_user(&l2e,
+    if ( __copy_from_user(&l2e,
                           &__linear_l2_table[l2_linear_offset(addr)],
                           sizeof(l2_pgentry_t)) != 0 )
         return NULL;
@@ -382,16 +384,21 @@ guest_unmap_l1e(struct vcpu *v, void *p)
 
 /* Read the guest's l1e that maps this address. */
 static inline void
-guest_get_eff_l1e(struct vcpu *v, unsigned long addr, void *eff_l1e)
+guest_get_eff_l1e(struct vcpu *v, unsigned long addr, l1_pgentry_t *eff_l1e)
 {
+    if ( unlikely(!__addr_ok(addr)) )
+    {
+        *eff_l1e = l1e_empty();
+        return;
+    }
+
     if ( likely(!paging_mode_translate(v->domain)) )
     {
         ASSERT(!paging_mode_external(v->domain));
-        if ( !__addr_ok(addr) ||
-             __copy_from_user(eff_l1e,
+        if ( __copy_from_user(eff_l1e,
                               &__linear_l1_table[l1_linear_offset(addr)],
                               sizeof(l1_pgentry_t)) != 0 )
-            *(l1_pgentry_t *)eff_l1e = l1e_empty();
+            *eff_l1e = l1e_empty();
         return;
     }