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            Xen Security Advisory CVE-2019-19578 / XSA-309
                               version 3

                Linear pagetable use / entry miscounts

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

Public release.

Updated metadata to add 4.13, update StableRef's

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

"Linear pagetables" is a technique which involves either pointing a
pagetable at itself, or to another pagetable of the same or higher
level.  Xen has limited support for linear pagetables: A page may
either point to itself, or point to another pagetable of the same
level (i.e., L2 to L2, L3 to L3, and so on).

XSA-240 introduced an additional restriction that limited the "depth"
of such chains by allowing pages to either *point to* other pages of
the same level, or *be pointed to* by other pages of the same level,
but not both.  To implement this, we keep track of the number of
outstanding times a page points to or is pointed to another page
table, to prevent both from happening at the same time.

Unfortunately, the original commit introducing this reset this count
when resuming validation of a partially-validated pagetable,
incorrectly dropping some "linear_pt_entry" counts.

If an attacker could engineer such a situation to occur, they might be
able to make loops or other arbitrary chains of linear pagetables, as
described in XSA-240.

IMPACT
======

A malicious or buggy PV guest may cause the hypervisor to crash,
resulting in Denial of Service (DoS) affecting the entire host.
Privilege escalation and information leaks cannot be excluded.

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

All versions of Xen are vulnerable.

Only x86 systems are affected.  Arm systems are not affected.

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

Only systems which have enabled linear pagetables are vulnerable.
Systems which have disabled linear pagetables, either by selecting
CONFIG_PV_LINEAR_PT=n when building the hypervisor, or adding
pv-linear-pt=false on the command-line, are not vulnerable.

MITIGATION
==========

If you don't have any guests which need linear pagetables, you can
disable the feature by adding pv-linear-pt=false to your Xen
command-line.  NetBSD is known to use linear pagetables; Linux and
MiniOS are known not to use linear pagetables.

CREDITS
=======

This issue was discovered by Manuel Bouyer and diagnosed as a security
issue by Jan Beulich of SUSE.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

xsa309.patch           xen-unstable, Xen 4.13 - Xen 4.8

$ sha256sum xsa309*
ddd00dfbc85bada4e4cee8a51b989e3138cc47c58992657054246bc95c8ae34d  xsa309.meta
0e4b75f4416624de698f3ed619c28418917ab0a5c9663c1641804e1d0a0dec1b  xsa309.patch
$

DEPLOYMENT DURING EMBARGO
=========================

Deployment of the patches 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).

Deployment of the `pv-linear-pt=false` mitigation is NOT permitted
(except where all the affected systems and VMs are administered and
used only by organisations which are members of the Xen Project
Security Issues Predisclosure List).  Specifically, deployment on
public cloud systems is NOT permitted.

This is because someone may notice the feature going away, and armed
with the knowledge of where the issue is, re-discover it.

Deployment of the mitigation is permitted only AFTER the embargo ends.
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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