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Advisory

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            Xen Security Advisory CVE-2023-46835 / XSA-445
                               version 3

        x86/AMD: mismatch in IOMMU quarantine page table levels

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

Public release.

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

The current setup of the quarantine page tables assumes that the
quarantine domain (dom_io) has been initialized with an address width
of DEFAULT_DOMAIN_ADDRESS_WIDTH (48) and hence 4 page table levels.

However dom_io being a PV domain gets the AMD-Vi IOMMU page tables
levels based on the maximum (hot pluggable) RAM address, and hence on
systems with no RAM above the 512GB mark only 3 page-table levels are
configured in the IOMMU.

On systems without RAM above the 512GB boundary
amd_iommu_quarantine_init() will setup page tables for the scratch
page with 4 levels, while the IOMMU will be configured to use 3 levels
only, resulting in the last page table directory (PDE) effectively
becoming a page table entry (PTE), and hence a device in quarantine
mode gaining write access to the page destined to be a PDE.

Due to this page table level mismatch, the sink page the device gets
read/write access to is no longer cleared between device assignment,
possibly leading to data leaks.

IMPACT
======

A device in quarantine mode can access data from previous quarantine
page table usages, possibly leaking data used by previous domains that
also had the device assigned.

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

All Xen versions supporting PCI passthrough are affected.

Only x86 AMD systems with IOMMU hardware are vulnerable.

Only x86 guests which have physical devices passed through to them can
leverage the vulnerability.

MITIGATION
==========

Not passing through physical devices to guests will avoid the
vulnerability.

Not using quarantine scratch-page mode will avoid the vulnerability,
but could result in other issues.

CREDITS
=======

This issue was discovered by Roger Pau Monné of XenServer.

RESOLUTION
==========

Applying the appropriate 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.

xsa445.patch           xen-unstable
xsa445-4.17.patch      Xen 4.17.x
xsa445-4.16.patch      Xen 4.16.x
xsa445-4.15.patch      Xen 4.15.x

$ sha256sum xsa445*
751892f1a603dbee7ecb82d046aee6d87bf10398f365d3880a7f7d32eb3d73c1  xsa445.patch
9ae729410504961578e679ba19931646802b213d026b6587fb1abb43b2629186  xsa445-4.15.patch
55fe5925741b650fe2583a1e9855ea66c4fe0212de4fe93535fd592188fa64d4  xsa445-4.16.patch
7c4478d348dad0d9c71685a8c402df78d74c6b4d3c3e1627115b91967e54d94a  xsa445-4.17.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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