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            Xen Security Advisory CVE-2016-7092 / XSA-185
                              version 3

        x86: Disallow L3 recursive pagetable for 32-bit PV guests

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

Public release.

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

On real hardware, a 32-bit PAE guest must leave the USER and RW bit
clear in L3 pagetable entries, but the pagetable walk behaves as if
they were set.  (The L3 entries are cached in processor registers, and
don't actually form part of the pagewalk.)

When running a 32-bit PV guest on a 64-bit Xen, Xen must always OR in
the USER and RW bits for L3 updates for the guest to observe
architectural behaviour.  This is unsafe in combination with recursive
pagetables.

As there is no way to construct an L3 recursive pagetable in native
32-bit PAE mode, disallow this option in 32-bit PV guests.

IMPACT
======

A malicious 32-bit PV guest administrator can escalate their privilege
to that of the host.

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

All versions of Xen are vulnerable.

Only 64-bit builds of the hypervisor are vulnerable.  For Xen 4.3 and
earlier, 32-bit builds of the hypervisor are not vulnerable.

The vulnerability is only exposed to 32-bit PV guests on x86 hardware.

The vulnerability is not exposed to 64-bit PV guests, x86 HVM guests,
or ARM guests.

MITIGATION
==========

Running only 64-bit PV or HVM guests will avoid this vulnerability.

CREDITS
=======

This issue was found in parallel by multiple discoverers, who each
disclosed it to the Xen Project Security Team.

The first report to us was made by Jérémie Boutoille of Quarkslab.
The second report, one working day later, by Shangcong Luan of Alibaba
Cloud.

RESOLUTION
==========

Applying the attached patch resolves this issue.

xsa185.patch           xen-unstable - Xen 4.4

$ sha256sum xsa185*
3328a1953ecdf4de35462ea8396b0927171d718e95f73a87a7f651427bd8f8b4  xsa185.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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