Xen Security Advisory 375 v2 (CVE-2021-0089) - Speculative Code Store Bypass

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

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            Xen Security Advisory CVE-2021-0089 / XSA-375
                              version 2

                    Speculative Code Store Bypass

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

New 4.12 backport (also targeting 4.11), addressing a build issue.

Discuss the need for SPECULATIVE_HARDEN_BRANCH in Resolution.

Provide Arm information links.

Public release.

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

Modern superscalar processors may employ sophisticated decoding and
caching of the instruction stream to improve performance.  However, a
consequence is that self-modifying code updates may not take effect
instantly.

Whatever the architectural guarantees, some CPUs have microarchitectural
behaviour whereby the stale instruction stream may be speculatively
decoded and executed.

Speculation of this form can suffer from type confusion in registers,
and potentially leak data.

For more details, see:
  https://www.vusec.net/projects/fpvi-scsb
  https://www.amd.com/en/corporate-product-security-bulletin-amd-sb-1003
  
https://software.intel.com/content/www/us/en/develop/articles/software-security-guidance/advisory-guidance/speculative-code-store-bypass.html
  
https://software.intel.com/content/www/us/en/develop/articles/software-security-guidance/advisory-guidance/floating-point-value-injection.html
  
https://developer.arm.com/support/arm-security-updates/speculative-processor-vulnerability/frequently-asked-questions#scsb
  
https://developer.arm.com/support/arm-security-updates/speculative-processor-vulnerability/frequently-asked-questions#fvpi

IMPACT
======

In attacker might be able to infer the contents of arbitrary host
memory, including memory assigned to other guests.

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

Systems running all versions of Xen are affected.

Whether a CPU is potentially vulnerable depends on its
microarchitecture.  Consult your hardware vendor.

Xen running on ARM does not have runtime self-modying code, so is
believed to be not vulnerable, irrespective of any hardware
susceptibility.

Xen running on x86 does have runtime self-modying code as part of
emulation, and is believed to be potentially vulnerable.

Xen is not vulnerable if retpoline or lfence mitigations for Spectre v2
protection are active.  Protections depend on compiler support (as
indicated by INDIRECT_THUNK), and a runtime setting (BTI-Thunk):

  # xl dmesg | grep -e INDIRECT_THUNK -e BTI-Thunk
  (XEN)   Compiled-in support: INDIRECT_THUNK SHADOW_PAGING
  (XEN)   Xen settings: BTI-Thunk RETPOLINE, SPEC_CTRL: IBRS+ SSBD-, Other: 
SRB_LOCK+ IBPB L1D_FLUSH VERW BRANCH_HARDEN

BTI-Thunk as either RETPOLINE or LFENCE prevents the vulnerability.

MITIGATION
==========

If Spectre v2 support is compiled in, but JMP is used by default,
RETPOLINE or LFENCE can be selected with `spec-ctrl=bti-thunk=retpoline`
or `spec-ctrl=bti-thunk=lfence`.

CREDITS
=======

This issue was discovered by Enrico Barberis, Hany Ragab, Herbert Bos,
and Cristiano Giuffrida from the VUSec group at VU Amsterdam.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.  Note that
in 4.13 and newer the patch will only take effect when the
SPECULATIVE_HARDEN_BRANCH hypervisor config option is enabled.  4.12 and
older do not have such an option, and the change will take effect
unconditionally.

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.

xsa375.patch           xen-unstable - 4.14.x
xsa375-4.13.patch      Xen 4.13.x
xsa375-4.12.patch      Xen 4.12.x - 4.11.x

$ sha256sum xsa375*
367d5bb97c942b9f744a57645df87148772c0879de6f351f36f88147f3958e83  xsa375.meta
301ef80da837bc2af36a0958f35f42f4d267b20ec6e91ae5faf2616167ef49f8  xsa375.patch
dc024daf17242b6477a16a349754a94b2b25cbbfd8c14475741b778710a44c93  
xsa375-4.12.patch
f70511d843c6617b932da11ffe857e2e3aa3834ccff07d4d0beba90d63a3dae2  
xsa375-4.13.patch
$

NOTE CONCERNING CVE-2021-0086
=============================

Floating Point Value Injection (FPVI) was discovered and disclosed in
the same research as SCSB.  Xen on x86 does in some cases emulate
floating point operations with guest provided inputs, but does not have
subsequent control flow dependent on results, transient or otherwise, of
the operation.

Therefore, we believe Xen is not vulnerable to FPVI, irrespective of any
hardware susceptibility.

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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Attachment: xsa375.meta
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Attachment: xsa375.patch
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Attachment: xsa375-4.12.patch
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Attachment: xsa375-4.13.patch
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