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            Xen Security Advisory CVE-2018-12891 / XSA-264
                               version 3

           preemption checks bypassed in x86 PV MM handling

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

Public release.

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

Certain PV MMU operations may take a long time to process.  For that
reason Xen explicitly checks for the need to preempt the current vCPU at
certain points.  A few rarely taken code paths did bypass such checks.
By suitably enforcing the conditions through its own page table
contents, a malicious guest may cause such bypasses to be used for an
unbounded number of iterations.

IMPACT
======

A malicious or buggy PV guest may cause a Denial of Service (DoS)
affecting the entire host.  Specifically, it may prevent use of a
physical CPU for an indeterminate period of time.

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

All Xen versions from 3.4 onwards are vulnerable.  Xen versions 3.3 and
earlier are vulnerable to an even wider class of attacks, due to them
lacking preemption checks altogether in the affected code paths.

Only x86 systems are affected.  ARM systems are not affected.

Only multi-vCPU x86 PV guests can leverage the vulnerability.  x86 HVM
or PVH guests as well as x86 single-vCPU PV ones cannot leverage the
vulnerability.

MITIGATION
==========

Running only HVM, PVH, or single-vCPU PV guests will avoid this
vulnerability.

For PV guests, the vulnerability can be avoided if the guest kernel is
controlled by the host rather than guest administrator, provided that
further steps are taken to prevent the guest administrator from loading
code into the kernel (e.g. by disabling loadable modules etc) or from
using other mechanisms which allow them to run code at kernel privilege.

CREDITS
=======

This issue was discovered by Jan Beulich of SUSE.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

xsa264.patch           xen-unstable
xsa264-4.10.patch      Xen 4.10.x ... 4.6.x

$ sha256sum xsa264*
a7d2edf219af3375ac0d49bff9e64628c70e704fcf131ea21684694517aa9210  xsa264.patch
66aca234b168abc01f28fe131b7e07645a73fd5d0f1d141d68343f31914d96cc  xsa264-4.10.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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