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Advisory

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            Xen Security Advisory CVE-2016-9382 / XSA-192
                              version 3

               x86 task switch to VM86 mode mis-handled

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

Public release.

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

LDTR, just like TR, is purely a protected mode facility.  Hence even
when switching to a VM86 mode task, LDTR loading needs to follow
protected mode semantics.  This was violated by the code.

IMPACT
======

On SVM (AMD hardware): a malicious unprivileged guest process can
escalate its privilege to that of the guest operating system.

On both SVM and VMX (Intel hardware): a malicious unprivileged guest
process can crash the guest.

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

Only 32-bit x86 HVM guests are vulnerable.  Furthermore, only guest
operating systems which actually make use of hardware task switching,
and allow a new task to start in VM86 mode, are vulnerable.  We are
not aware of any such operating systems.

The vulnerability is NOT exposed on any PV guests.
The vulnerability is NOT exposed on any 64-bit guests,

ARM systems are NOT vulnerable.

Xen versions from 4.0 onwards are affected.  Xen versions 3.4 and
earlier are not affected.

MITIGATION
==========

For guests which are affected, the vulnerability could possibly be
mitigated by disabling access to VM86 mode by unprivileged guest
programs.  Details would depend on the (so far hypothetical)
vulnerable guest kernel.

CREDITS
=======

This issue was discovered by Jan Beulich of SUSE.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

xsa192.patch           xen-unstable, Xen 4.7.x, Xen 4.6.x
xsa192-4.5.patch       Xen 4.5.x, Xen 4.4.x

$ sha256sum xsa192*
687b0216eefd5ecef8a3135cc6f542cb3d9ff35e8e9696a157703e84656c35e8  xsa192.patch
bb0c6622c6f5c5eb9a680020d865802069446830b4a170bcb82336f6c3b77f55  xsa192-4.5.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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