[PATCH] argo: don't leak stack contents when returning ring info

[Jan Beulich <jbeulich@xxxxxxxx>]

The max_message_size field of the output gets filled only when the flags
field is non-zero. Don't copy back uninitialized data to guest context.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/common/argo.c
+++ b/xen/common/argo.c
@@ -1405,7 +1405,8 @@ fill_ring_data(const struct domain *curr
         rcu_unlock_domain(dst_d);
 
     if ( !ret && (__copy_field_to_guest(data_ent_hnd, &ent, flags) ||
-                  __copy_field_to_guest(data_ent_hnd, &ent, max_message_size)) 
)
+                  (ent.flags &&
+                   __copy_field_to_guest(data_ent_hnd, &ent, 
max_message_size))) )
         return -EFAULT;
 
     return ret;