ISSUE DESCRIPTION
=================

When polling event channels, in general arbitrary port numbers can be
specified.  Specifically, there is no requirement that a polled event
channel ports has ever been created.  When the code was generalised
from an earlier implementation, introducing some intermediate
pointers, a check should have been made that these intermediate
pointers are non-NULL.  However, that check was omitted.

IMPACT
======

A malicious or buggy guest may cause the hypervisor to access
addresses it doesn't control, usually leading to a host crash (Denial
of Service).  Information leaks cannot be excluded.

VULNERABLE SYSTEMS
==================

Xen versions 4.4 and newer are vulnerable.  Xen versions 4.3 and
earlier are not affected.

Both x86 and ARM systems are vulnerable.

While all guest kinds can cause a Denial of Service, only x86 PV guests
may be able to leverage the possible information leaks.

MITIGATION
==========

There is no known mitigation.

External References:

http://xenbits.xen.org/xsa/advisory-221.html

Acknowledgements:

Name: the Xen project
Upstream: Ankur Arora (Oracle)