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            Xen Security Advisory CVE-2014-8866 / XSA-111
                              version 4

   Excessive checking in compatibility mode hypercall argument translation

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

Fix patch name.

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

The hypercall argument translation needed for 32-bit guests running on
64-bit hypervisors performs checks on the final register state.  These
checks cover all registers potentially holding hypercall arguments,
not just the ones actually doing so for the hypercall being processed,
since the code was originally intended for use only by PV guests.

While this is not a problem for PV guests (as they can't enter 64-bit
mode and hence can't alter the high halves of any of the registers),
the subsequent reuse of the same functionality for HVM guests exposed
those checks to values (specifically, unexpected values for the high
halves of registers not holding hypercall arguments) controlled by
guest software.

IMPACT
======

A buggy or malicious HVM guest can crash the host.

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

Xen 3.3 and onward are vulnerable.

Only x86 systems are vulnerable.  ARM systems are not vulnerable.

MITIGATION
==========

Running only PV guests will avoid this issue.

There is no mitigation available for HVM guests on any version of Xen
so far released by xenproject.org.

CREDITS
=======

This issue was discovered by Jan Beulich of SUSE.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

xsa111.patch                 xen-unstable, Xen 4.4.x
xsa111-4.3.patch             Xen 4.3.x
xsa111-4.2.patch             Xen 4.2.x

$ sha256sum xsa111*.patch
f6e1bf166ebed6235802e4e42853430d2f5b456c1837908a4f7ed6d4d150e4b4  xsa111-4.2.patch
e9b03a4443a40142cc5c21848dc9589770620dde8924344c4a00028c4dace9f2  xsa111-4.3.patch
3c418f065cd452c225af34c3cccf9bdbc37efb6c6a5fc5940fd83ad8620510d3  xsa111.patch
$
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