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Advisory

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            Xen Security Advisory CVE-2016-2271 / XSA-170
                              version 3

      VMX: guest user mode may crash guest with non-canonical RIP

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

Public release.

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

VMX refuses attempts to enter a guest with an instruction pointer which
doesn't satisfy certain requirements.  In particular, the instruction
pointer needs to be canonical when entering a guest currently in 64-bit
mode.  This is the case even if the VM entry information specifies an
exception to be injected immediately (in which case the bad instruction
pointer would possibly never get used for other than pushing onto the
exception handler's stack).  Provided the guest OS allows user mode to
map the virtual memory space immediately below the canonical/non-
canonical address boundary, a non-canonical instruction pointer can
result even from normal user mode execution. VM entry failure, however,
is fatal to the guest.

IMPACT
======

Malicious HVM guest user mode code may be able to crash the guest.

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

All Xen versions are affected.

Only systems using Intel or Cyrix CPUs are affected. ARM and AMD
systems are unaffected.

Only HVM guests are affected.

MITIGATION
==========

Running only PV guests will avoid this vulnerability.

Running HVM guests on only AMD hardware will also avoid this
vulnerability.

CREDITS
=======

This issue was discovered by Ling Liu of Qihoo 360 Inc.

RESOLUTION
==========

Applying the appropriate attached patch works around this issue.  Note
that it does so in a way which isn't architecturally correct, but no
better solution has been found (nor suggested by Intel).

xsa170.patch           xen-unstable, Xen 4.6.x
xsa170-4.5.patch       Xen 4.5.x, Xen 4.4.x
xsa170-4.3.patch       Xen 4.3.x

$ sha256sum xsa170*
77b4b14b2c93da5f68e724cf74e1616f7df2e78305f66d164b3de2d980221a9a  xsa170.patch
b35679bf7a35615d827efafff8d13c35ceec1184212e3c8ba110722b9ae8426f  xsa170-4.3.patch
1df068fb439c7edc1e86dfa9ea3b9ae99b58cdc3ac874b96cdf63b26ef9a6b98  xsa170-4.5.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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