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            Xen Security Advisory CVE-2021-28690 / XSA-377
                               version 2

        x86: TSX Async Abort protections not restored after S3

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

Public release.

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

This issue relates to the TSX Async Abort speculative security vulnerability.
Please see https://xenbits.xen.org/xsa/advisory-305.html for details.

Mitigating TAA by disabling TSX (the default and preferred option) requires
selecting a non-default setting in MSR_TSX_CTRL.  This setting isn't restored
after S3 suspend.

IMPACT
======

After using S3 suspend at least once, CPU0 remains vulnerable to TAA.

This is an information leak.  For full details of the impact, see
XSA-305.

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

See XSA-305 for details of susceptibility to TAA.

Only systems which are susceptible to TAA and have the XSA-305 fix are
vulnerable.  Only systems which support S3 suspend/resume are vulnerable.

The vulnerability is only exposed if S3 suspend/resume is used.

MITIGATION
==========

Not using S3 suspend/resume avoids the vulnerability.

CREDITS
=======

This issue was discovered by Andrew Cooper of Citrix.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

Note that patches for released versions are generally prepared to
apply to the stable branches, and may not apply cleanly to the most
recent release tarball.  Downstreams are encouraged to update to the
tip of the stable branch before applying these patches.

xsa377.patch           xen-unstable - Xen 4.13.x
xsa377-4.12.patch      Xen 4.12.x
xsa377-4.11.patch      Xen 4.11.x

$ sha256sum xsa377*
532cb030f97d72e8e534ad97182cd5e3aa0efeef405e255bb49649b4f0dd9947  xsa377.meta
21a30dbf80f6e78057cc7e785c8fda475d5a8a0b6b9442af3bd8ca31dd69becf  xsa377.patch
3279317d56e7b8d0a2b0152b64b4c577381b8b01fa0a1a21ec6f855bb964278a  xsa377-4.11.patch
65f61f1cb7bb0e068fd32e40755b9a9aae464d15ccd42c94dae68e495c5a45e0  xsa377-4.12.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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