x86/mm: fully honor PS bits in guest page table walks

In L4 entries it is currently unconditionally reserved (and hence
should, when set, always result in a reserved bit page fault), and is
reserved on hardware not supporting 1Gb pages (and hence should, when
set, similarly cause a reserved bit page fault on such hardware).

This is CVE-2016-4480 / XSA-176.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Tested-by: Andrew Cooper <andrew.cooper3@citrix.com>

--- a/xen/arch/x86/mm/guest_walk.c
+++ b/xen/arch/x86/mm/guest_walk.c
@@ -226,6 +226,11 @@ guest_walk_tables(struct vcpu *v, struct
         rc |= _PAGE_PRESENT;
         goto out;
     }
+    if ( gflags & _PAGE_PSE )
+    {
+        rc |= _PAGE_PSE | _PAGE_INVALID_BIT;
+        goto out;
+    }
     rc |= ((gflags & mflags) ^ mflags);
 
     /* Map the l3 table */
@@ -247,7 +252,7 @@ guest_walk_tables(struct vcpu *v, struct
     }
     rc |= ((gflags & mflags) ^ mflags);
     
-    pse1G = (gflags & _PAGE_PSE) && guest_supports_1G_superpages(v); 
+    pse1G = !!(gflags & _PAGE_PSE);
 
     if ( pse1G )
     {
@@ -267,6 +272,8 @@ guest_walk_tables(struct vcpu *v, struct
             /* _PAGE_PSE_PAT not set: remove _PAGE_PAT from flags. */
             flags &= ~_PAGE_PAT;
 
+        if ( !guest_supports_1G_superpages(v) )
+            rc |= _PAGE_PSE | _PAGE_INVALID_BIT;
         if ( gfn_x(start) & GUEST_L3_GFN_MASK & ~0x1 )
             rc |= _PAGE_INVALID_BITS;