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Advisory

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            Xen Security Advisory CVE-2020-29484 / XSA-324
                               version 3

            Xenstore: guests can crash xenstored via watchs

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

Public release.

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

When a Xenstore watch fires, the xenstore client which registered the
watch will receive a Xenstore message containing the path of the
modified Xenstore entry which triggered the watch, and the tag which
was specified when registering the watch.

Any communication with xenstored is done via Xenstore messages,
consisting of a message header and the payload. The payload length is
limited to 4096 bytes. Any request to xenstored resulting in a
response with a payload longer than 4096 bytes will result in an
error.

When registering a watch the payload length limit applies to the
combined length of the watched path and the specified tag. As watches
for a specific path are also triggered for all nodes below that path,
the payload of a watch event message can be longer than the payload
needed to register the watch.

A malicious guest which registers a watch using a very large tag (ie
with a registration operation payload length close to the 4096 byte
limit) can cause the generation of watch events with a payload length
larger than 4096 bytes, by writing to Xenstore entries below the
watched path.

This will result in an error condition in xenstored.  This error can
result in a NULL pointer dereference leading to a crash of xenstored.

IMPACT
======

A malicious guest administrator can cause xenstored to crash, leading
to a denial of service.  Following a xenstored crash, domains may
continue to run, but management operations will be impossible.

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

All Xen versions are affected.

Only C xenstored is affected, oxenstored is not affected.

MITIGATION
==========

There are no mitigations.

Changing to use of Ocaml xenstored would avoid this vulnerability.
However, given the other vulnerabilities in both versions of xenstored
being reported at this time, changing xenstored implementation is not a
recommended approach to mitigation of individual issues.

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.

xsa324.patch           xen-unstable - 4.10

$ sha256sum xsa324*
78932f0a83b479902553b1acdf601f7625b383497c03c6e834a0a2b847f1a72e  xsa324.meta
8dba79842fa913290c7043d065a50abb0efe27fa5a173e421c21c544cc1e264c  xsa324.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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