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Advisory

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            Xen Security Advisory CVE-2022-42324 / XSA-420
                               version 2

            Oxenstored 32->31 bit integer truncation issues

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

Public release.

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

Integers in Ocaml are 63 or 31 bits of signed precision.

The Ocaml Xenbus library takes a C uint32_t out of the ring and casts it
directly to an Ocaml integer.  In 64-bit Ocaml builds this is fine, but
in 32-bit builds, it truncates off the most significant bit, and then
creates unsigned/signed confusion in the remainder.

This in turn can feed a negative value into logic not expecting a
negative value, resulting in unexpected exceptions being thrown.

The unexpected exception is not handled suitably, creating a busy-loop
trying (and failing) to take the bad packet out of the xenstore ring.

IMPACT
======

A malicious or buggy guest can write a packet into the xenstore ring
which causes 32-bit builds of oxenstored to busy loop.

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

All versions of Xen are affected.

Systems running a 32-bit build of oxenstored are affected.

Systems running a 64-bit build of oxenstored, or systems running (C)
xenstored are not affected.

MITIGATION
==========

Running xenstored instead of oxenstored will avoid the vulnerability.

CREDITS
=======

This issue was discovered by Jürgen Groß of SUSE.

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.

xsa420.patch           xen-unstable - Xen 4.13.x

$ sha256sum xsa420*
565b332d325fd0fdeb5fee890c0cd9b53c4478c46c6b7ec7b24fd3444d2dc812  xsa420.meta
bfa83ca1e78ef81f93c3d94cb1522d1cffed8b9989c5639e8ec663fad0a71027  xsa420.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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