[Xen-devel] Xen Security Advisory 256 - x86 PVH guest without LAPIC may DoS the host

[Xen.org security team <security@xxxxxxx>]

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                    Xen Security Advisory XSA-256
                              version 2

             x86 PVH guest without LAPIC may DoS the host

UPDATES IN VERSION 2
====================

Public release.

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

So far, x86 PVH guests can be configured with or without Local APICs.
Configurations with Local APICs are identical to x86 HVM guests, and
will use as much hardware acceleration support as possible.
Configurations without Local APICs try to turn off all hardware
acceleration, and disable all software emulation.

Multiple paths in Xen assume the presence of a Local APIC without
sufficient checks, and can fall over a NULL pointer.  On Intel hardware,
the logic to turn off hardware acceleration is incomplete and leaves the
guest with full control of the real Task Priority Register.

IMPACT
======

A malicious or buggy guest may cause a hypervisor crash, resulting in
a Denial of Service (DoS) affecting the entire host.

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

Xen version 4.8 and onwards are vulnerable.

Only x86 systems are vulnerable.  ARM systems are not vulnerable.

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

MITIGATION
==========

Running only PV or HVM guests avoids the vulnerability.

Running all PVH guests with "apic=1" in the guest configuration file
(or equivalent thereof) also avoids the vulnerability.

CREDITS
=======

This issue was discovered by Ian Jackson of Citrix.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

xsa256.patch           xen-unstable, Xen 4.10.x, Xen 4.9.x
xsa256-4.8.patch       Xen 4.8.x

$ sha256sum xsa256*
3e45cc3f2ea516e7470083592041e238c0dfe32324790b2fba0e47c9efe38865  xsa256.patch
c029fcb67ff7c3c9a2adcb8e6f5e245a0d347acc8a9b3530591a639cbf321349  
xsa256-4.8.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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Attachment: xsa256.patch
Description: Binary data

Attachment: xsa256-4.8.patch
Description: Binary data

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