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

When the libxl toolstack launches qemu for HVM guests, it pipes the
output of stderr to a file in /var/log/xen.  This output is not
rate-limited in any way.  The guest can easily cause qemu to print
messages to stderr, causing this file to become arbitrarily large.

IMPACT
======

The disk containing the logfile can be exausted, possibly causing a
denial-of-service (DoS).

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

All versions of Xen are affected.

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

Only systems running HVM guests are affected; systems running only PV
guests are not affected.

Both qemu-upstream and qemu-traditional are affected.

MITIGATION
==========

Running only PV guests will avoid this vulnerability.

References:

http://seclists.org/oss-sec/2016/q2/390