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            Xen Security Advisory CVE-2016-10024 / XSA-202
                               version 3

             x86 PV guests may be able to mask interrupts

UPDATES IN VERSION 3
====================

Public release.

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.

CREDITS
=======

This issue was discovered by Jan Beulich of SUSE.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

xsa202.patch           xen-unstable, Xen 4.8.x, Xen 4.7.x
xsa202-4.6.patch       Xen 4.6.x, Xen 4.5.x
xsa202-4.4.patch       Xen 4.4.x

$ sha256sum xsa202*
057be742acfef200ba6f094a5dce486dd1c4e15013afe3efc963523ce2ec9cbb  xsa202.patch
cd53dc8b761dc7eb60998ea2419c98af926aa62b4317dbef15f597f5554f9015  xsa202-4.4.patch
e007187639f5392a9256979504d50eff0ae38309a61524ea42c4150fab38b6f4  xsa202-4.6.patch
$

DEPLOYMENT DURING EMBARGO
=========================

Deployment of the patches and/or mitigations described above (or
others which are substantially similar) is permitted during the
embargo, even on public-facing systems with untrusted guest users and
administrators.

But: Distribution of updated software is prohibited (except to other
members of the predisclosure list).

Predisclosure list members who wish to deploy significantly different
patches and/or mitigations, please contact the Xen Project Security
Team.

(Note: this during-embargo deployment notice is retained in
post-embargo publicly released Xen Project advisories, even though it
is then no longer applicable.  This is to enable the community to have
oversight of the Xen Project Security Team's decisionmaking.)

For more information about permissible uses of embargoed information,
consult the Xen Project community's agreed Security Policy:
  http://www.xenproject.org/security-policy.html
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