[PATCH] x86/PVH: extend checking in hwdom_fixup_p2m()

[Jan Beulich <jbeulich@xxxxxxxx>]

We're generally striving to minimize behavioral differences between PV
and PVH Dom0. Using (just?) is_memory_hole() in the PVH case looks quite
a bit weaker to me, compared to the page ownership check done in the PV
case. Extend checking accordingly.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
The addition may actually be suitable to replace the use of
is_memory_hole() here. While dropping that would in particular extend
coverage to E820_RESERVED regions, those are identity-mapped anyway
(albeit oddly enough still by IOMMU code).

--- a/xen/arch/x86/hvm/emulate.c
+++ b/xen/arch/x86/hvm/emulate.c
@@ -184,6 +184,22 @@ static int hwdom_fixup_p2m(paddr_t addr)
          !is_memory_hole(_mfn(gfn), _mfn(gfn)) )
         return -EPERM;
 
+    /*
+     * Much like get_page_from_l1e() for PV Dom0 does, check that the page
+     * accessed is actually an MMIO one: Either its MFN is out of range, or
+     * it's owned by DOM_IO.
+     */
+    if ( mfn_valid(_mfn(gfn)) )
+    {
+        struct page_info *pg = mfn_to_page(_mfn(gfn));
+        const struct domain *owner = page_get_owner_and_reference(pg);
+
+        if ( owner )
+            put_page(pg);
+        if ( owner != dom_io )
+            return -EPERM;
+    }
+
     mfn = get_gfn(currd, gfn, &type);
     if ( !mfn_eq(mfn, INVALID_MFN) || !p2m_is_hole(type) )
         rc = mfn_eq(mfn, _mfn(gfn)) ? -EEXIST : -ENOTEMPTY;