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

Device model code dealing with guest PCI MSI-X interrupt management activities logs messages on certain (supposedly) invalid guest operations.

IMPACT
======

A buggy or malicious guest repeatedly invoking such operations may result in the host disk to fill up, possibly leading to a Denial of Service.

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

Xen versions 3.3 and onwards are vulnerable due to supporting PCI pass-through.

Only x86 systems are vulnerable. ARM systems are not vulnerable.

Only HVM guests with their device model run in Dom0 can take advantage of this vulnerability.

Only HVM guests which have been granted access to physical PCI devices (`PCI passthrough') can take advantage of this vulnerability.

Furthermore, the vulnerability is only applicable when the passed-through PCI devices are MSI-X capable. (Many modern devices are.)

MITIGATION
==========

This issue can be avoided by not assigning MSI-X capable PCI devices to untrusted HVM guests.

This issue can also be avoided by only using PV guests.

It can also be avoided by configuring HVM guests with their device model run in a separate (stub) domain. (When using xl, this can be requested with "device_model_stubdomain_override=1" in the domain configuration file.)


Acknowledgements:

Red Hat would like to thank the Xen project for reporting this issue.