Xen Security Advisory 433 v1 - x86/AMD: Zenbleed

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

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                    Xen Security Advisory XSA-433

                          x86/AMD: Zenbleed

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

Researchers at Google have discovered Zenbleed, a hardware bug causing
corruption of the vector registers.

When a VZEROUPPER instruction is discarded as part of a bad transient
execution path, its effect on internal tracking are not unwound
correctly.  This manifests as the wrong micro-architectural state
becoming architectural, and corrupting the vector registers.

Note: While this malfunction is related to speculative execution, this
      is not a speculative sidechannel vulnerability.

The corruption is not random.  It happens to be stale values from the
physical vector register file, a structure competitively shared between
sibling threads.  Therefore, an attacker can directly access data from
the sibling thread, or from a more privileged context.

For more details, see:
  https://www.amd.com/en/resources/product-security/bulletin/amd-sb-7008.html
  
https://github.com/google/security-research/security/advisories/GHSA-v6wh-rxpg-cmm8

IMPACT
======

With very low probability, corruption of the vector registers can occur.
This data corruption causes mis-calculations in subsequent logic.

An attacker can exploit this bug to read data from different contexts on
the same core.  Examples of such data includes key material, cypher and
plaintext from the AES-NI instructions, or the contents of REP-MOVS
instructions, commonly used to implement memcpy().

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

Systems running all versions of Xen are affected.

This bug is specific to the AMD Zen2 microarchitecture.  AMD do not
believe that other microarchitectures are affected.

MITIGATION
==========

This issue can be mitigated by disabling AVX, either by booting Xen with
`cpuid=no-avx` on the command line, or by specifying `cpuid="host:avx=0"` in
the vm.cfg file of all untrusted VMs.  However, this will come with a
significant impact on the system and is not recommended for anyone able to
deploy the microcode or patch described below.

RESOLUTION
==========

AMD are producing microcode updates to address the bug.  Consult your
dom0 OS vendor.  This microcode is effective when late-loaded, which can
be performed on a live system without reboot.

In cases where microcode is not available, the appropriate attached
patch updates Xen to use a control register to avoid the 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.

xsa433.patch           xen-unstable
xsa433-4.17.patch      Xen 4.17.x
xsa433-4.16.patch      Xen 4.16.x
xsa433-4.15.patch      Xen 4.15.x
xsa433-4.14.patch      Xen 4.14.x

$ sha256sum xsa433*
a9331733b63e3e566f1436a48e9bd9e8b86eb48da6a8ced72ff4affb7859e027  xsa433.patch
6f1db2a2078b0152631f819f8ddee21720dabe185ec49dc9806d4a9d3478adfd  
xsa433-4.14.patch
ca3a92605195307ae9b6ff87240beb52a097c125a760c919d7b9a0aff6e557c0  
xsa433-4.15.patch
e5e94b3de68842a1c8d222802fb204d64acd118e3293c8e909dfaf3ada23d912  
xsa433-4.16.patch
41d12104869b7e8307cd93af1af12b4fd75a669aeff15d31b234dc72981ae407  
xsa433-4.17.patch
$

NOTE CONCERNING TIMELINE
========================

This issue is subject to coordinated disclosure on August 8th.  The
discoverer chose to publish details ahead of this timeline.
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Attachment: xsa433.patch
Description: Binary data

Attachment: xsa433-4.14.patch
Description: Binary data

Attachment: xsa433-4.15.patch
Description: Binary data

Attachment: xsa433-4.16.patch
Description: Binary data

Attachment: xsa433-4.17.patch
Description: Binary data