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Advisory

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            Xen Security Advisory CVE-2020-29483 / XSA-325
                               version 3

              Xenstore: guests can disturb domain cleanup

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

Public release.

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

Xenstored and guests communicate via a shared memory page using a
specific protocol. When a guest violates this protocol, xenstored will
drop the connection to that guest.

Unfortunately this is done by just removing the guest from xenstored's
internal management, resulting in the same actions as if the guest had
been destroyed, including sending an @releaseDomain event.

@releaseDomain events do not say guest has been removed.  All watchers
of this event must look at the states of all guests to find the guest
which has been removed.  When an @releaseDomain is generated due to
domain xenstored protocol violation, As the guest is still running, so
the watchers will not react.

Later, when the guest is actually destroyed, xenstored will no longer
have it stored in its internal data base, so no further @releaseDomain
event will be sent. This can lead to a zombie domain; memory mappings
of that guest's memory will not be removed, due to the missing
event. This zombie domain will be cleaned up only after another domain
is destroyed, as that will trigger another @releaseDomain event.

If the device model of the guest which violated the Xenstore protocol
is running in a stub-domain, a use-after-free case could happen in
xenstored, after having removed the guest from its internal data base,
possibly resulting in a crash of xenstored.

IMPACT
======

A malicious guest can block resources of the host for a period after
its own death.

Guests with a stub domain device model can eventually crash xenstored,
resulting in a more serious denial of service (the prevention of any
further domain management operations).

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

All versions of Xen are affected.

Only the C variant of Xenstore is affected, the Ocaml variant is not
affected.

Only HVM guests with a stubdom device model can cause a serious DoS.

MITIGATION
==========

Using the Ocaml variant of Xenstore (oxenstored) avoids the issue;
Running HVM domains with a dom0 device model rather than a stubdom
device model will avoid the more serious DoS.

However, given the other vulnerabilities in both versions of xenstored
being reported at this time, changing xenstored implementation, or
switching to dom0 xenstored, is not a recommended approach to
mitigation of individual issues.

CREDITS
=======

This issue was discovered by Pawel Wieczorkiewicz 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.

xsa325.patch           xen-unstable
xsa325-4.14.patch      Xen 4.14 - 4.10

$ sha256sum xsa325*
29a81606e9c0e036dcc39b2a7e6ec0b1ce7d658972a368907b02d56f2aae3dc2  xsa325.meta
56e09d92fa3d623b2896fd6e6a08805514b2ff9b1cde526968be3925fda28705  xsa325.patch
702f0f4c20e685d2e23a9c1a31c0e0fda1824c9209bd8affca9dd3489dfbd23d  xsa325-4.14.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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