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            Xen Security Advisory CVE-2020-27675 / XSA-331
                              version 3

         Race condition in Linux event handler may crash dom0

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

CVE assigned.

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

The Linux kernel event channel handling code doesn't defend the
handling of an event against the same event channel being removed in
parallel.

This can result in accesses to already freed memory areas or NULL
pointer dereferences in the event handling code, leading to
misbehaviour of the system or even crashes.

IMPACT
======

A misbehaving guest can trigger a dom0 crash by sending events for a
paravirtualized device while simultaneously reconfiguring it.

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

All systems with a Linux dom0 are vulnerable.

All Linux kernel versions are vulnerable.

MITIGATION
==========

There is no known mitigation.

CREDITS
=======

This issue was discovered by Jinoh Kang of Theori.

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.

xsa331-linux.patch     Linux

$ sha256sum xsa331*
8583392c0c573f7baa85e41c9afbdf74dcb04aea1be992d78991f0787230a193  xsa331-linux.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is then no longer applicable.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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