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Advisory

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            Xen Security Advisory CVE-2019-18420 / XSA-296
                               version 5

                         VCPUOP_initialise DoS

UPDATES IN VERSION 5
====================

Include recipe for versions 4.12 to 4.9 in xsa296.meta

Canonicalize version text.

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

hypercall_create_continuation() is a variadic function which uses a
printf-like format string to interpret its parameters.  Error handling
for a bad format character was done using BUG(), which crashes Xen.

One path, via the VCPUOP_initialise hypercall, has a bad format
character.  The BUG() can be hit if VCPUOP_initialise executes for a
sufficiently long period of time for a continuation to be created.

IMPACT
======

Malicious guests may cause a hypervisor crash, resulting in a Denial of
Service (Dos).

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

Xen versions 4.6 and newer are vulnerable.  Xen versions 4.5 and earlier
are not vulnerable.

Only x86 PV guests can exploit the vulnerability.  HVM and PVH guests,
and guests on ARM systems, cannot exploit the vulnerability.

MITIGATION
==========

There are no mitigations.

CREDITS
=======

This issue was discovered by Andrew Cooper of Citrix.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

xsa296.patch           Xen 4.9 - xen-unstable
xsa296-4.8.patch       Xen 4.7 - 4.8

$ sha256sum xsa296*
a02e2748870c129691ad2721f916b9cf77b095b6fba292e607eba558ced8dab1  xsa296.meta
ccfd81b162b8535d952f56b1f87dfdd960e71bf07c1cf8388976e78e2e86cde5  xsa296.patch
b283be3df6789402553172b7fd582bfffb4fa72a6b33543439bd2fb1b87bfbd4  xsa296-4.8.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is then no longer applicable.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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