Xen Security Advisory 279 v2 - x86: DoS from attempting to use INVPCID with a non-canonical addresses

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Xen Security Advisory 279 v2 - x86: DoS from attempting to use INVPCID with a non-canonical addresses

Xen.org security team
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                    Xen Security Advisory XSA-279
                              version 2

 x86: DoS from attempting to use INVPCID with a non-canonical addresses

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

Public release.

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

The INVPCID instruction raises #GP[0] if an attempt is made to
invalidate a non-canonical address.  Older flushing mechanisms such as
INVLPG tolerate this without error, and perform no action.

There is one guest accessible path in Xen where a non-canonical
address was passed into the TLB flushing code.  This previously had no
ill effect, but became vulnerable with the introduction of PCID to
reduce the performance hit from the Meltdown mitigations.

IMPACT
======

A buggy or malicious PV guest can crash the host.

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

Only hardware which supports the INVPCID instruction is vulnerable.  This is
available on Intel Haswell processors and later.  AMD x86 processors are not
known to support this instruction, and ARM processors are entirely unaffected.

Only versions of Xen with PCID support are vulnerable.  Support first appeared
in Xen 4.11 but was backported to the stable trees as part of the Meltdown
(XSA-254 / CVE-2017-5754) fixes.  Xen 4.10.2, 4.9.3, 4.8.4 as well as the
stable-4.7 and 4.6 branches are vulnerable.

The vulnerability is only exposed to 64-bit PV guests.  32-bit PV guests, as
well as HVM/PVH guests cannot exploit the vulnerability.

MITIGATION
==========

Booting Xen with `pcid=0` or `invpcid=0` on the command line will work around
the issue.  Alternatively, running untrusted 64bit PV guests inside xen-shim
will work around the issue.

CREDITS
=======

This issue was discovered by Matthew Daley.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

xsa279.patch             xen-unstable, Xen 4.11.x, Xen 4.10.x
xsa279-4.9.patch         Xen 4.9.x ... 4.7.x

$ sha256sum xsa279*
40319fcf33348176eb14d7fc7c68c255cc7291013242ea444de6d00602024a11  xsa279.meta
0c1d50effe6645051a15dd83af57088dd4a055e26a23b1fa9e6c3722a7973f5d  xsa279.patch
fd34f29bc7e53359585135408cbbd12e12a003f59b135e81cc44186c5cddd40d  xsa279-4.9.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.

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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xsa279.meta (2K) Download Attachment
xsa279.patch (1K) Download Attachment
xsa279-4.9.patch (1K) Download Attachment