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            Xen Security Advisory CVE-2020-28368 / XSA-351
                               version 3

                 Information leak via power sidechannel

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

Fix some version tags.

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

Researchers have demonstrated using software power/energy monitoring
interfaces to create covert channels, and infer the operations/data used
by other contexts within the system.

Access to these interfaces should be restricted to privileged software,
but it was found that Xen doesn't restrict access suitably, and the
interfaces are accessible to all guests.

For more information, see:
  https://platypusattack.com
  https://www.intel.com/content/www/us/en/security-center/advisory/intel-sa-00389.html

IMPACT
======

An unprivileged guest administrator can sample platform power/energy
data.  This may be used to infer the operations/data used by other
contexts within the system.

The research demonstrates using this sidechannel to leak the AES keys
used elsewhere in the system.

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

Power/energy monitoring interfaces are platform and architecture
specific.  Consult your hardware vendor to ascertain what power feedback
interfaces are available.

For ARM systems, all versions of Xen are vulnerable.  The fix restricts
access to the AMU (Activity Monitors Unit) interface, introduced in
Armv8.4.

For x86 systems, Xen 4.14 and earlier are vulnerable - master is not
vulnerable, as these issues have been addressed in a more general
fashion.

The x86 fixes restrict access to:
 * Intel RAPL interface, introduced in SandyBridge CPUs.
 * Intel platform energy interface.
 * Intel perf_ctl interface, introduced in Pentium 4 CPUs and also
   implemented by other vendors.
 * AMD RAPL interface, introduced in Ryzen/EPYC CPUs.
 * AMD compute unit energy interface, present in Fam15/16 CPUs.

MITIGATION
==========

There are no mitigations available.

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.

xsa351-arm.patch             Xen unstable - 4.12.x [ARM]
xsa351-arm-4.11.patch        Xen 4.11.x - 4.10.x   [ARM]
xsa351-x86-4.14-?.patch      Xen 4.14.x            [x86]
xsa351-x86-4.13-?.patch      Xen 4.13.x            [x86]
xsa351-x86-4.12-?.patch      Xen 4.12.x            [x86]
xsa351-x86-4.11-?.patch      Xen 4.11.x - 4.10.x   [x86]

$ sha256sum xsa351*
cad287981a870f13484834fa2364ffee68178517e906f55d2889304a4a9eae06  xsa351.meta
70ebd0e93af240af2680374dcfd8ff4a5dd3eefccf670f1cb9b546d763d6a554  xsa351-arm.patch
49b52a1366912a29e184e3014a9f1f579e8a0dd8a36f01d38d995d2c8ed81928  xsa351-arm-4.11.patch
2e7b7c2b98625d70c8b10047a9f668372f3ccede167344dedb712312606acbca  xsa351-x86-4.11-1.patch
ab9e2cb7d5e3e0c3a916f006c697495f4f01146e09df60ece59ce0a8f7aa5ed0  xsa351-x86-4.11-2.patch
bb68f6e6905bc1566156cafab058cbaf02a17c197385c33a83b7f73885913c1c  xsa351-x86-4.12-1.patch
53f464269f59498f8a9a614f10a47cfb1d81c666f0d684346e28005015de962c  xsa351-x86-4.12-2.patch
67a29d66230faafd9a8047ac80ec18130b5659e80a38c3a412cb2be6d3288a8f  xsa351-x86-4.13-1.patch
f7d8717dec33ee7484b36490402d113f1e7e168e7541bcf193fef620df299f08  xsa351-x86-4.13-2.patch
7d4fbe11a766226d7f1b93c5bf34664d8855deee09d1feebc76f11e49f2aa9c9  xsa351-x86-4.14-1.patch
41df825deafe3ef28e8594ec956033689af69f84a4a6dd92f97d1071e925203d  xsa351-x86-4.14-2.patch
$

NOTE REGARDING LACK OF EMBARGO
==============================

Despite an attempt to organise predisclosure, the discoverers ultimately
did not authorise a predisclosure.
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