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Advisory

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            Xen Security Advisory CVE-2016-7093 / XSA-186
                              version 4

      x86: Mishandling of instruction pointer truncation during emulation

UPDATES IN VERSION 4
====================

Public release.

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

When emulating HVM instructions, Xen uses a small i-cache for fetches
from guest memory.  The code that handles cache misses does not check
if the address from which it fetched lies within the cache before
blindly writing to it.  As such it is possible for the guest to
overwrite hypervisor memory.

It is currently believed that the only way to trigger this bug is to
use the way that Xen currently incorrectly wraps CS:IP in 16 bit
modes.  The included patch prevents such wrapping.

IMPACT
======

A malicious HVM guest administrator can escalate their privilege to that
of the host.

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

Xen versions 4.7.0 and later are vulnerable.
Xen releases 4.6.3 and 4.5.3 are vulnerable.

Xen releases 4.6.0 to 4.6.2 inclusive are NOT vulnerable.
Xen releases 4.5.2 and earlier are NOT vulnerable.

The vulnerability is only exposed to HVM guests on x86 hardware.

The vulnerability is not exposed to x86 PV guests, or ARM guests.

MITIGATION
==========

Running only PV guests will avoid this vulnerability.

CREDITS
=======

This issue was discovered by Brian Marcotte.

RESOLUTION
==========

Applying the first patch will resolve the issue.

Users wishing to independently verify the correctness of the fix may
find the second patch helpful.  The second patch makes it easier to
use the "fep" (Force Emulation Prefix) feature to reproduce the
erroneous condition in a test environment.  The "fep" feature requires
explicit enablement on the hypervisor command line, and is unsuitable
for production systems.  Accordingly, applying the second patch does
not affect production systems and does not improve security.

Xen version     First patch               Second patch
 xen-unstable:   xsa186-0001-*.patch       xsa186-0002-*.patch
 Xen 4.7.x:      xsa186-0001-*.patch       xsa186-4.7-0002-*.patch
 Xen 4.6.3:      xsa186-0001-*.patch       xsa186-4.6-0002-*.patch
 Xen 4.5.3:      xsa186-0001-*.patch       xsa186-4.6-0002-*.patch

$ sha256sum xsa186*
f2082a36d968a47e477bb5082d0e0aaa58e6cb3dc20b26389f043a9b7b595fa6  xsa186-0001-x86-emulate-Correct-boundary-interactions-of-emulate.patch
412fa58edcbd1c7fdbfec7e28898cf98585593e6a24ccfb088dc0b84715286a5  xsa186-0002-hvm-fep-Allow-testing-of-instructions-crossing-the-1.patch
7482a823c3443e26dee1111c4904162845eaa9f826aa7bf8348007406d91bddd  xsa186-4.6-0002-hvm-fep-Allow-testing-of-instructions-crossing-the.patch
5a826a32763d82ac83c924f8c89d12aae5f069a4cbc7d5193aa8413a02b6dc05  xsa186-4.7-0002-hvm-fep-Allow-testing-of-instructions-crossing-the.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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