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            Xen Security Advisory CVE-2017-17044 / XSA-246
                              version 3

         x86: infinite loop due to missing PoD error checking

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

CVE assigned.

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

Failure to recognize errors being returned from low level functions in
Populate on Demand (PoD) code may result in higher level code entering
an infinite loop.

IMPACT
======

A malicious HVM guest can cause one pcpu to permanently hang.  This
normally cascades into the whole system freezing, resulting in a a
host Denial of Service (DoS).

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

Xen versions from 3.4.x onwards are affected.

Only x86 systems are vulnerable.  ARM is not vulnerable.

x86 PV VMs cannot leverage the vulnerability.

Only systems with 2MiB or 1GiB HAP pages enabled are vulnerable.

The vulnerability is largely restricted to HVM guests which have been
constructed in Populate-on-Demand mode (i.e. with memory < maxmem):

x86 HVM domains without PoD (i.e. started with memory == maxmem, or
without mentioning "maxmem" in the guest config file) also cannot
leverage the vulnerability, in recent enough Xen versions:
  4.8.x and later: all versions safe if PoD not configured
  4.7.x: 4.7.1 and later safe if PoD not configured
  4.6.x: 4.6.4 and later safe if PoD not configured
  4.5.x: 4.5.4 and later safe if PoD not configured
  4.4.x and earlier: all versions vulnerable even if PoD not configured

The commit required to prevent this vulnerability when PoD
not configured is 2a99aa99fc84a45f505f84802af56b006d14c52e
  xen/physmap: Do not permit a guest to populate PoD pages for itself
and the corresponding backports.

MITIGATION
==========

Running only PV guests will avoid this issue.

Running HVM guests only in non-PoD mode (maxmem == memory) will also
avoid this issue.  NOTE: In older releases of Xen, an HVM guest can
create PoD entries itself; so this mitigation will not be effective.

Specifying "hap_1gb=0 hap_2mb=0" on the hypervisor command line will
avoid the vulnerability.

Alternatively, running all x86 HVM guests in shadow mode will also
avoid this vulnerability.  (For example, by specifying "hap=0" in the
xl domain configuration file.)

CREDITS
=======

This issue was discovered by Julien Grall of Linaro.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

xsa246.patch           xen-unstable
xsa246-4.9.patch       Xen 4.9.x, Xen 4.8.x
xsa246-4.7.patch       Xen 4.7.x, Xen 4.6.x, Xen 4.5.x

$ sha256sum xsa246*
df08a3be419f2384b495dc52c3e6ebef1eb67d8b562afe85fb6fe6a723334472  xsa246.patch
b41550688e88a2a7a22349a07168f3a3ddf6fad8b3389fa27de44ae6731b6a8b  xsa246-4.7.patch
ea591542774c22db65dcb340120cebf58e759670b5a9fbde42ee93ed594650c8  xsa246-4.9.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, with ONE exception:

Removing the ability to boot in populate-on-demand mode is NOT
permitted during the embargo on public cloud systems.  This is because
doing so might alert attackers to the nature of the vulnerability.
Deployment of this mitigation is permitted only AFTER the embargo
ends.

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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