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            Xen Security Advisory CVE-2022-42319 / XSA-416
                               version 2

   Xenstore: Guests can cause Xenstore to not free temporary memory

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

Public release.

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

When working on a request of a guest, xenstored might need to allocate
quite large amounts of memory temporarily. This memory is freed only
after the request has been finished completely.

A request is regarded to be finished only after the guest has read the
response message of the request from the ring page. Thus a guest not
reading the response can cause xenstored to not free the temporary
memory.

This can result in memory shortages causing Denial of Service (DoS) of
xenstored.

IMPACT
======

A malicious guest can cause DoS of xenstored, resulting in the inability
to create new guests or to change the configuration of already running
guests.

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

Xen systems with version 4.9 and newer running the C variant of Xenstore
(xenstored or xenstore-stubdom) are vulnerable.

Systems using the Ocaml variant of Xenstore (oxenstored) are not vulnerable.

MITIGATION
==========

Using oxenstored instead of xenstored will avoid the vulnerability.

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.

xsa416.patch           xen-unstable
xsa416-4.16.patch      Xen 4.16.x
xsa416-4.15.patch      Xen 4.15.x
xsa416-4.14.patch      Xen 4.14.x
xsa416-4.13.patch      Xen 4.13.x

$ sha256sum xsa416*
9f5f459b0c5b71ba25503d176c59732baf2ed174688b59e51010f22f8eecd218  xsa416.meta
6a28884392997baaea046ac8f1df86ffaec64febdabf17a8753e451ef3cab0f7  xsa416.patch
5838de576be5a71be46ab2e58287ea7d9c774f68f588c7d1f472a190b5923446  xsa416-4.13.patch
cb06c57ff0158899e61b26901e06d931838f68958d610fbeff1c76edb9c8b4e1  xsa416-4.14.patch
68b7128b0f1e1d6de345dbc47dfdd583beff840c96e625fcf6e6a82679ff3732  xsa416-4.15.patch
c4bb88546b272925d85d75d62f552d91698a10faaac1a0ce6cac3c54eeeda6a2  xsa416-4.16.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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