Disclaimer: Community trackers are created by Red Hat Product Security team on a best effort basis. Package maintainers are required to ascertain if the flaw indeed affects their package, before starting the update process.

Accesses to the CMOS memory contents are done using an indirect IO port
pair.  Therefore Xen needs to cache the guest chosen index, and one of
the usages of the index didn't take the necessary locking to avoid
concurrent changes.  As a result, a guest could change the index after
it being checked, causing a subsequent out-of-bound read access to the
contents of an array.