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            Xen Security Advisory CVE-2016-7777 / XSA-190
                              version 5

        CR0.TS and CR0.EM not always honored for x86 HVM guests

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

Public release.

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

Instructions touching FPU, MMX, or XMM registers are required to raise
a Device Not Available Exception (#NM) when either CR0.EM or CR0.TS are
set.  (Their AVX or AVX-512 extensions would consider only CR0.TS.)
While during normal operation this is ensured by the hardware, if a
guest modifies instructions while the hypervisor is preparing to
emulate them, the #NM delivery could be missed.

Guest code in one task may thus (unintentionally or maliciously) read
or modify register state belonging to another task in the same VM.

IMPACT
======

A malicious unprivileged guest user may be able to obtain or corrupt
sensitive information (including cryptographic material) in other
programs in the same guest.

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

All versions of Xen expose the vulnerabilty to their x86 HVM guests.

In order to exploit the vulnerability, the attacker needs to be able to
trigger the Xen instruction emulator.

On Xen 4.7 the emulator can only be triggered: by user mode tasks which
have been given access to memory-mapped IO; in guests which have been
migrated between systems with CPUs from different vendors; or in guests
which have been configured with a CPU vendor different from the host's.

On Xen 4.6 and earlier, all HVM guests can trigger the emulator by
attempting to execute an invalid opcode, exposing the vulnerability.

The vulnerability is only exposed to x86 HVM guests.

The vulnerability is not exposed to x86 PV or ARM guests.

MITIGATION
==========

On Xen 4.7, not migrating across CPU vendors will avoid this
vulnerability.  (Unless the guest grants mmio access to unprivileged
tasks, or has been configured with a specific CPU vendor, eg using the
xl "cpuid" configuraton option.)

CREDITS
=======

This issue was discovered by Jan Beulich from SUSE.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

xsa190.patch           xen-unstable, Xen 4.7.x
xsa190-4.6.patch       Xen 4.6.x
xsa190-4.5.patch       Xen 4.5.x, Xen 4.4.x

$ sha256sum xsa190*
21e7b1d08874527ab2e4cd23d467e9945afcd753dd3390ab2aaf9d24d231916c  xsa190.patch
477d56c41cc2101432459ab79e4d5663aade779c36285f5c1d6d6ed4e34e1009  xsa190-4.5.patch
dbfc4b36132c841959847dfbb85a188ee6489ad3b8d7ecec43c55a303a43df21  xsa190-4.6.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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