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

The page type system of Xen requires cleanup when the last reference
for a given page is being dropped.  In order to exclude simultaneous
updates to a given page by multiple parties, pages which are updated
are locked beforehand.  This locking includes temporarily increasing
the type reference count by one.  When the page is later unlocked, the
context precludes cleanup, so the reference that is then dropped must
not be the last one.  This was not properly enforced.

IMPACT
======

A malicious or buggy PV guest may cause a memory leak upon shutdown
of the guest, ultimately perhaps resulting in Denial of Service (DoS)
affecting the entire host.

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

All Xen versions from 3.4 onwards are vulnerable.  Xen versions 3.3 and
earlier are not vulnerable.

Only x86 systems are affected.  ARM systems are not affected.

Only x86 PV guests can leverage the vulnerability.  x86 HVM guests
cannot leverage the vulnerability.

MITIGATION
==========

Running only HVM guests will avoid this vulnerability.

For PV guests, the vulnerability can be avoided if the guest kernel is
controlled by the host rather than guest administrator, provided that
further steps are taken to prevent the guest administrator from loading
code into the kernel (e.g. by disabling loadable modules etc) or from
using other mechanisms which allow them to run code at kernel privilege.

External References:

http://xenbits.xen.org/xsa/advisory-242.html