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

The compiler can emit optimizations in the PV backend drivers which
can lead to double fetch vulnerabilities. Specifically the shared
memory between the frontend and backend can be fetched twice (during
which time the frontend can alter the contents) possibly leading to
arbitrary code execution in backend.

IMPACT
======

Malicious guest administrators can cause denial of service.  If driver
domains are not in use, the impact can be a host crash, or privilege escalation.

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

Systems running PV or HVM guests are vulnerable.

ARM and x86 systems are vulnerable.

All OSes providing PV backends are susceptible, this includes
Linux and NetBSD. By default the Linux distributions compile kernels
with optimizations.

External References:

http://xenbits.xen.org/xsa/advisory-155.html

Acknowledgements:

Red Hat would like to thank the Xen project for reporting this issue. Upstream acknowledges Felix Wilhelm (ERNW Research, KIT / Operating Systems Group) as the original reporter.