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Advisory

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            Xen Security Advisory CVE-2022-42335 / XSA-430
                               version 2

             x86 shadow paging arbitrary pointer dereference

UPDATES IN VERSION 2
====================

Public release.

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

In environments where host assisted address translation is necessary
but Hardware Assisted Paging (HAP) is unavailable, Xen will run guests
in so called shadow mode.  Due to too lax a check in one of the hypervisor
routines used for shadow page handling it is possible for a guest with a PCI
device passed through to cause the hypervisor to access an arbitrary pointer
partially under guest control.

IMPACT
======

Guests running in shadow mode and having a PCI device passed through may be
able to cause Denial of Service and other problems, escalation of privilege
cannot be ruled out.

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

Only Xen version 4.17 is vulnerable.

Only x86 systems are vulnerable.  The vulnerability can be leveraged only
by HVM guests running with shadow paging and having a PCI device passed
through.

MITIGATION
==========

Not passing through PCI devices to HVM guests will avoid the vulnerability.

Running HVM guests only in HAP (Hardware Assisted Paging) mode will also
avoid the vulnerability.

CREDITS
=======

This issue was discovered by Roger Pau Monné of XenServer.

RESOLUTION
==========

Applying the attached patch resolves this issue.

Note that patches for released versions are generally prepared to
apply to the stable branches, and may not apply cleanly to the most
recent release tarball.  Downstreams are encouraged to update to the
tip of the stable branch before applying these patches.

xsa430.patch           xen-unstable - Xen 4.17.x

$ sha256sum xsa430*
c861cabdf546ec7583f2193f9c4f8a62579047315e5fe9eca3e9e944b67ca852  xsa430.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.
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