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            Xen Security Advisory CVE-2016-6259 / XSA-183
                              version 6

    x86: Missing SMAP whitelisting in 32-bit exception / event delivery

UPDATES IN VERSION 6
====================

Fix patch name.

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.)

CREDITS
=======

This issue was discovered by Andrew Cooper of Citrix.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

xsa183-unstable.patch  xen-unstable, 4.7.x
xsa183-4.6.patch       Xen 4.6.x, 4.5.x

$ sha256sum xsa183*
ea0ea4b294332814330f222e6d78eea3b19c394eac8ae22feb4a5bd21e90331f  xsa183-unstable.patch
0fee41f21a3eb4af1487590098047f4625688bcef7419572a8f418f9fb728468  xsa183-4.6.patch
$

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

Deployment of the patches 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: Deployment of the "smap=0" mitigation is NOT permitted (except
where all the affected systems and VMs are administered and used only
by organisations which are members of the Xen Project Security Issues
Predisclosure List).  Specifically, deployment on public cloud systems
is NOT permitted.  This is because this produces a guest-visible
change which could lead to rediscovery of the vulnerability.

And: 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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