gnttab: add missing version check to GNTTABOP_swap_grant_ref handling

... avoiding NULL derefs when the version to use wasn't set yet (via
GNTTABOP_setup_table or GNTTABOP_set_version).

This is CVE-2015-4163 / XSA-134.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
master commit: 5d5c09d853
master date: 2015-06-11 14:44:12 +0200

xen/common/grant_table.c

@@ -2394,6 +2394,9 @@ __gnttab_swap_grant_ref(grant_ref_t ref_a, grant_ref_t ref_b)
 
     spin_lock(&gt->lock);
 
+    if ( gt->gt_version == 0 )
+        PIN_FAIL(out, GNTST_general_error, "grant table not yet set up\n");
+
     /* Bounds check on the grant refs */
     if ( unlikely(ref_a >= nr_grant_entries(d->grant_table)))
         PIN_FAIL(out, GNTST_bad_gntref, "Bad ref-a (%d).\n", ref_a);