Bug 1402758 (CVE-2016-10024, xsa202)

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

Certain PV guest kernel operations (page table writes in particular)
need emulation, and use Xen's general x86 instruction emulator.  This
allows a malicious guest kernel which asynchronously modifies its
instruction stream to effect the clearing of EFLAGS.IF from the state
used to return to guest context.

IMPACT
======

A malicious guest kernel administrator can cause a host hang or
crash, resulting in a Denial of Service.

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

All Xen versions are vulnerable.

Only x86 PV guests can exploit the vulnerability.

Neither ARM guests nor x86 HVM guests can exploit the vulnerability.

MITIGATION
==========

Running only HVM guests will avoid the vulnerability.

For PV guests the vulnerability can be avoided if the guest kernel is
controlled by the host rather than guest administrator, provided that
further steps are taken to prevent the guest administrator from loading
code into the kernel (e.g. by disabling loadable modules etc) or from
using other mechanisms which allow them to run code at kernel privilege.

External References:

http://xenbits.xen.org/xsa/advisory-202.html

Acknowledgements:

Name: the Xen project