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Advisory

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            Xen Security Advisory CVE-2023-46840 / XSA-450
                               version 2

           VT-d: Failure to quarantine devices in !HVM builds

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

Public release.

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

Incorrect placement of a preprocessor directive in source code results
in logic that doesn't operate as intended when support for HVM guests is
compiled out of Xen.

IMPACT
======

When a device is removed from a domain, it is not properly quarantined
and retains its access to the domain to which it was previously
assigned.

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

Xen 4.17 and onwards are vulnerable.  Xen 4.16 and older are not
vulnerable.

Only Xen running on x86 platforms with an Intel-compatible VT-d IOMMU is
vulnerable.  Platforms from other manufacturers, or platforms without a
VT-d IOMMU are not vulnerable.

Only systems where PCI devices are passed through to untrusted or
semi-trusted guests are vulnerable.  Systems which do not assign PCI
devices to untrusted guests are not vulnerable.

Xen is only vulnerable when CONFIG_HVM is disabled at build time.  Most
deployments of Xen are expected to have CONFIG_HVM enabled at build
time, and would therefore not be vulnerable.

MITIGATION
==========

There is no mitigation.

CREDITS
=======

This issue was discovered by Teddy Astie of Vates

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.

xsa450.patch           xen-unstable - Xen 4.17.x

$ sha256sum xsa450*
738c79b92ab5ea57f446df3daff6564727fea5feebf8fadeb32acd0cf06ff9fb  xsa450.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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