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Advisory

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            Xen Security Advisory CVE-2022-23035 / XSA-395
                               version 2

          Insufficient cleanup of passed-through device IRQs

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

Adjust patch subject.

Public release.

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

The management of IRQs associated with physical devices exposed to x86
HVM guests involves an iterative operation in particular when cleaning
up after the guest's use of the device.  In the case where an interrupt
is not quiescent yet at the time this cleanup gets invoked, the cleanup
attempt may be scheduled to be retried.  When multiple interrupts are
involved, this scheduling of a retry may get erroneously skipped.  At
the same time pointers may get cleared (resulting in a de-reference of
NULL) and freed (resulting in a use-after-free), while other code would
continue to assume them to be valid.

IMPACT
======

The precise impact is system specific, but would typically be a Denial
of Service (DoS) affecting the entire host.  Privilege escalation and
information leaks cannot be ruled out.

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

Xen versions 4.6 and later are vulnerable.  Xen versions 4.5 and earlier
are not vulnerable.

Only x86 HVM guests with one or more passed-through physical devices
using (together) multiple physical interupts can leverage the
vulnerability.  x86 PV guests cannot leverage the vulnerability.  x86
HVM guests without passed-through devices or with a passed-through
device using just a single physical interrupt also cannot leverage the
vulnerability.  Device pass-through is unsupported for x86 PVH guests
and all Arm guests.

MITIGATION
==========

There is no mitigation (other than not passing through to x86 HVM guests
PCI devices with, overall, more than a single physical interrupt).

CREDITS
=======

This issue was discovered by Julien Grall of Amazon.

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.

xsa395.patch           xen-unstable - Xen 4.15.x
xsa395-4.14.patch      Xen 4.14.x - Xen 4.12.x

$ sha256sum xsa395*
f460be598b936bb5cfb9276787f2f21d90b029d1fe10dabd572ae50f84a1124d  xsa395.meta
295b876c52cf5efe19150757275da3d154beb72ac2d7be267e16c9262e410de3  xsa395.patch
5697f3137e0a202744f31b1c6cbcfa459d8fa9b4b68be59561b78c40fe1233c5  xsa395-4.14.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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