Bug 1355983 (CVE-2016-6259, xsa183)

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

Supervisor Mode Access Prevention is a hardware feature designed to make
an Operating System more robust, by raising a pagefault rather than
accidentally following a pointer into userspace.  However, legitimate
accesses into userspace require whitelisting, and the exception delivery
mechanism for 32bit PV guests wasn't whitelisted.

IMPACT
======

A malicious 32-bit PV guest kernel can trigger a safety check, crashing
the hypervisor and causing a denial of service to other VMs on the host.

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

Xen version 4.5 and newer are vulnerable.  Versions 4.4 and older are
not, due to not having software support for SMAP.

The vulnerability is only exposed on x86 hardware supporting the SMAP
feature (Intel Broadwell and later CPUs).  The vulnerability is not
exposed on ARM hardware, or x86 hardware which do not support SMAP.

The vulnerability is only exposed to x86 32bit PV guests.  The
vulnerability is not exposed to 64bit PV guests or HVM guests.

MITIGATION
==========

Running only HVM guests or 64-bit PV guests, avoids the vulnerability.

Disabling SMAP in the hypervisor by booting Xen with "smap=0" on the
command line will avoid this vulnerability.  (Depending on the
circumstances this workaround may pose a small risk of increasing the
impact of other, possibly unknown, vulnerabilities.)

External References:

http://xenbits.xen.org/xsa/advisory-183.html

Acknowledgements:

Name: the Xen project