Bug 1335442 (CVE-2016-4963, xsa178)

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

libxl's device-handling code freely uses and trusts information from
the backend directories in xenstore.

The backend domain (driver domain) can store bogus data in the
backend, causing libxl's enquiry functions to fail, confusing
management tools.

A driver domain can also remove its backend directory from xenstore
entirely, preventing the device from showing up in device listings and
preventing it from being removed and replaced.

A driver domain can cause libxl to generate disk eject events for
disks for which the driver domain is not responsible.

IMPACT
======

A malicious driver domain can deny service to management tools.

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

This vulnerability is only applicable to systems which are using
driver domains, and then only where the driver domain is not intended
to be fully trusted with respect to the host.

Such Xen systems using libxl based toolstacks (for example xl or
libvirt with the libxl driver) are vulnerable.

Note that even with this vulnerability a driver domain based system is
better from a security point of view, than a system where devices are
provided directly by dom0.  Users and vendors of systems using driver
domains should not change their configuration.

All versions of libxl which support the relevant driver domains are
vulnerable.

MITIGATION
==========

No mitigation is available.

External References:

http://xenbits.xen.org/xsa/advisory-178.html

Acknowledgements:

Name: the Xen project