From: Andrew Cooper <andrew.cooper3@citrix.com>
Subject: grant_table: fix GNTTABOP_cache_flush handling

Don't fall over a NULL grant_table pointer when the owner of the domain
is a system domain (DOMID_{XEN,IO} etc).

This is XSA-232.

Reported-by: Matthew Daley <mattd@bugfuzz.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -3053,7 +3053,7 @@ static int cache_flush(gnttab_cache_flus
 
     page = mfn_to_page(mfn);
     owner = page_get_owner_and_reference(page);
-    if ( !owner )
+    if ( !owner || !owner->grant_table )
     {
         rcu_unlock_domain(d);
         return -EPERM;