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

Grant copying code made an implication that any grant pin would be
accompanied by a suitable page reference.  Other portions of code,
however, did not match up with that assumption.  When such a grant
copy operation is being done on a grant of a dying domain, the
assumption turns out wrong.

IMPACT
======

A malicious guest administrator can cause can cause hypervisor memory
corruption, most likely resulting in host crash and a Denial of
Service.  Privilege escalation and information leaks cannot be ruled
out.

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

Xen versions from 4.2 onwards are vulnerable.  Xen versions 4.1 and
earlier are not vulnerable.

Both x86 and ARM are vulnerable, and on x86 both PV and HVM guests can
trigger the vulnerability.

MITIGATION
==========

Running only guests without para-virtual drivers, and known not to
issue grant table operations can avoid the vulnerability.

External References:

http://xenbits.xen.org/xsa/advisory-236.html