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Advisory

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	     Xen Security Advisory CVE-2013-2076 / XSA-52
                            version 3

           Information leak on XSAVE/XRSTOR capable AMD CPUs

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

Public release.

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

On AMD processors supporting XSAVE/XRSTOR (family 15h and up), when an
exception is pending, these instructions save/restore only the FOP,
FIP, and FDP x87 registers in FXSAVE/FXRSTOR.  This allows one domain
to determine portions of the state of floating point instructions of
other domains.

NOTE: This is the documented behavior of AMD64 processors, but it is
inconsistent with Intel processors in a security-relevant fashion that
was not addressed by the original implementation of XSAVE support on
Xen.

This vulnerability is similar to CVE-2006-1056, concerning
FXSAVE/FXRSTOR on AMD processors.

IMPACT
======

A malicious domain may be able to leverage this to obtain sensitive
information such as cryptographic keys from another domain.

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

Xen 4.0 and onwards are vulnerable when run on systems with AMD
processors supporting XSAVE.  Any kind of guest can exploit the
vulnerability.

In Xen 4.0.2 through 4.0.4 as well as in Xen 4.1.x XSAVE support is
disabled by default; therefore systems running these versions are not
vulnerable unless support is explicitly enabled using the "xsave"
hypervisor command line option.

Systems not using AMD processors, or using AMD processors not
supporting XSAVE (i.e. families prior to 15h), are not vulnerable.

Xen 3.x and earlier are not vulnerable.

MITIGATION
==========

Turning off XSAVE support via the "no-xsave" hypervisor command line
option will avoid the vulnerability.

RESOLUTION
==========

Applying the attached patch resolves this issue.

xsa52-4.1.patch             Xen 4.1.x
xsa52-4.2-unstable.patch    Xen 4.2.x, xen-unstable

$ sha256sum xsa52-*.patch
058741aae8881774cfe8f8d193fee9b92da62e61459b1e9617798ccee2ce8d75  xsa52-4.1.patch
5b8582185bf90386729e81db1f7780c69a891b074a87d9a619a90d6f639bea13  xsa52-4.2-unstable.patch
$
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