[PATCH v2 for-4.19 1/3] x86/EPT: correct special page checking in epte_get_entry_emt()

[Jan Beulich <jbeulich@xxxxxxxx>]

mfn_valid() granularity is (currently) 256Mb. Therefore the start of a
1Gb page passing the test doesn't necessarily mean all parts of such a
range would also pass. Yet using the result of mfn_to_page() on an MFN
which doesn't pass mfn_valid() checking is liable to result in a crash
(the invocation of mfn_to_page() alone is presumably "just" UB in such a
case).

Fixes: ca24b2ffdbd9 ("x86/hvm: set 'ipat' in EPT for special pages")
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
Of course we could leverage mfn_valid() granularity here to do an
increment by more than 1 if mfn_valid() returned false. Yet doing so
likely would want a suitable helper to be introduced first, rather than
open-coding such logic here.
---
v2: New.

--- a/xen/arch/x86/mm/p2m-ept.c
+++ b/xen/arch/x86/mm/p2m-ept.c
@@ -519,8 +519,12 @@ int epte_get_entry_emt(struct domain *d,
     }
 
     for ( special_pgs = i = 0; i < (1ul << order); i++ )
-        if ( is_special_page(mfn_to_page(mfn_add(mfn, i))) )
+    {
+        mfn_t cur = mfn_add(mfn, i);
+
+        if ( mfn_valid(cur) && is_special_page(mfn_to_page(cur)) )
             special_pgs++;
+    }
 
     if ( special_pgs )
     {