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

Single memory accesses in source code can be translated to multiple
ones in machine code by the compiler, requiring special caution when
accessing shared memory.  Such precaution was missing from the
hypervisor code inspecting the state of I/O requests sent to the device
model for assistance.

Due to the offending field being a bitfield, it is however believed
that there is no issue in practice, since compilers, at least when
optimizing (which is always the case for non-debug builds), should find
it more expensive to extract the bit field value twice than to keep the
calculated value in a register.

IMPACT
======

This vulnerability is exposed to malicious device models.  In
conventional Xen systems this means the qemu which service an HVM
domain.  On such systems this vulnerability can only be exploited if
the attacker has gained control of the device model qemu via another
vulnerability.

Privilege escalation, host crash (Denial of Service), and leaked
information all cannot be excluded.

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

All Xen versions are affected.

Only x86 variants of Xen are susceptible.  ARM variants are not
affected.

Only HVM guests expose this vulnerability.

MITIGATION
==========

Running only PV guests will avoid this issue.

External References:

http://xenbits.xen.org/xsa/advisory-166.html

Acknowledgements:

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