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

VMIDs are a finite hardware resource, and allocated as part of domain
creation.  If no free VMIDs are available when trying to create a new domain,
a bug in the error path causes a NULL pointer to be used, resulting in a Data
Abort and host crash.

IMPACT
======

Attempting to create too many concurrent domains causes a host crash rather
than a graceful error.  A malicious device driver domain can hold references
to domains, preventing its VMID being released.

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

Xen versions 4.4 and later are affected.  Older Xen versions are unaffected.

x86 systems are not affected.

Only arm systems with less-privileged device driver domains can expose this
vulnerability.

MITIGATION
==========

There is no mitigation.  Not using driver domains reclassifies the problem,
but does not fix it.

NOTE REGARDING LACK OF EMBARGO
==============================

The crash was discussed publicly on xen-devel, before it was appreciated
that there was a security problem.

CREDITS
=======

This issue was discovered by Aaron Cornelius of DornerWorks.

External References:

http://xenbits.xen.org/xsa/advisory-181.html

Acknowledgements:

Name: the Xen project
Upstream: Aaron Cornelius (DornerWorks)