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Advisory

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            Xen Security Advisory CVE-2020-25604 / XSA-336
                               version 3

           race when migrating timers between x86 HVM vCPU-s

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

Public release.

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

When migrating timers of x86 HVM guests between its vCPU-s, the locking
model used allows for a second vCPU of the same guest also operating on
the timers to release a lock that it didn't acquire.

IMPACT
======

The most likely effect of the issue is a hang or crash of the
hypervisor, i.e. a Denial of Service (DoS).

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

All versions of Xen are affected.

Only x86 systems are vulnerable.  Arm systems are not vulnerable.

Only x86 HVM guests can leverage the vulnerability.  x86 PV and PVH
cannot leverage the vulnerability.

Only guests with more than one vCPU can exploit the vulnerability.

MITIGATION
==========

Running only PV and PVH guests will avoid the vulnerability.

CREDITS
=======

This issue was discovered by Igor Druzhinin of Citrix.

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.

xsa336.patch           Xen 4.12 - xen-unstable
xsa336-4.11.patch      Xen 4.10 - 4.11

$ sha256sum xsa336*
6cb13a54c2b0fcb6948a1c4045095da4e43aad262a1dd8993ea2a3bd90d4c72d  xsa336.meta
ecb59876fb92cfe0916ed5f3227a30efe038224c1f6ec36bc3706c4e2214552c  xsa336.patch
c0c7983bfd70eb54277af9fddfcc3cc95bbd745d92d9ffb71d5b32281c437510  xsa336-4.11.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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