Xen Security Advisory 407 v1 (CVE-2022-23816,CVE-2022-23825,CVE-2022-29900) - Retbleed - arbitrary speculative code execution with return instructions

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

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 Xen Security Advisory CVE-2022-23816,CVE-2022-23825,CVE-2022-29900 / XSA-407

   Retbleed - arbitrary speculative code execution with return instructions

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

Researchers at ETH Zurich have discovered Retbleed, allowing for
arbitrary speculative execution in a victim context.

For more details, see:
  https://comsec.ethz.ch/retbleed

ETH Zurich have allocated CVE-2022-29900 for AMD and CVE-2022-29901 for
Intel.

Despite the similar preconditions, these are very different
microarchitectural behaviours between vendors.

On AMD CPUs, Retbleed is one specific instance of a more general
microarchitectural behaviour called Branch Type Confusion.  AMD have
assigned CVE-2022-23816 (Retbleed) and CVE-2022-23825 (Branch Type
Confusion).

For more details, see:
  https://www.amd.com/en/corporate/product-security/bulletin/amd-sb-1037

On Intel CPUs, Retbleed is not a new vulnerability; it is only
applicable to software which did not follow Intel's original Spectre-v2
guidance.  Intel are using the ETH Zurich allocated CVE-2022-29901.

For more details, see:
  
https://www.intel.com/content/www/us/en/security-center/advisory/intel-sa-00702.html
  
https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/advisory-guidance/return-stack-buffer-underflow.html

ARM have indicated existing guidance on Spectre-v2 is sufficient.

IMPACT
======

An 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.

For ARM and Intel CPUs, Xen implemented the vendor-recommended defaults
in XSA-254 and follow-on fixes.  Therefore, the Xen Security Team
believes there are no further changes necessary on these CPUs.
Administrators who deviated from the default mitigations are potentially
affected and should re-evaluate their threat model.

For AMD, CPUs from the Zen2 microarchitecture and earlier are
potentially vulnerable.  Zen3 and later CPUs are not believed to be
vulnerable.

The patches for Xen implement the IBPB-at-entry mitigation.  This
depends on the IBPB microcode distributed by AMD in 2018 as part of the
original Spectre/Meltdown work.  Consult your dom0 OS vendor.

In addition to IBPB, "cross thread" safety is necessary.  On Zen2 CPUs,
Xen uses STIBP by default.  On Zen1 CPUs, SMT needs disabling either in
the firmware, or by passing `smt=0` on Xen's command line.  On Fam15h
CPUs, Cluster Multi-Threading needs disabling in firmware.

Due to performance concerns, dom0 is excluded from IBPB-on-entry
protections by default.  This is because PV dom0 is trusted in most
deployments.  If your threat model model doesn't allow for dom0 to be
treated specially, boot with `spec-ctrl=ibpb-entry` which will cause
IBPB-on-entry protections to be applied to dom0 too.

MITIGATION
==========

There are no mitigations.

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.

For the 4.15 and 4.16 branches in particular, these patches depend on:

 - x86/spec-ctrl: Only adjust MSR_SPEC_CTRL for idle with legacy IBRS
 - x86/spec-ctrl: Knobs for STIBP and PSFD, and follow hardware STIBP hint
 - xen/cmdline: Extend parse_boolean() to signal a name match
 - x86/spec-ctrl: Add fine-grained cmdline suboptions for primitives

which have been recently backported.

xsa407/xsa407-?.patch           xen-unstable
xsa407/xsa407-4.16-*.patch      Xen 4.16.x
xsa407/xsa407-4.15-*.patch      Xen 4.15.x
xsa407/xsa407-4.14-*.patch      Xen 4.14.x
xsa407/xsa407-4.13-*.patch      Xen 4.13.x

$ sha256sum xsa407* xsa407*/*
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xsa407/xsa407-1.patch
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xsa407/xsa407-2.patch
1f901df3a382a547dda6ef4ef088a5cf60a2d2b0382be451b148bf166cf43013  
xsa407/xsa407-3.patch
8f75a9eee8ee2a563bc90e493ba1e4ac29335f677a3d2049ec27e138b7e3021e  
xsa407/xsa407-4.13-01.patch
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xsa407/xsa407-4.13-02.patch
9a392bca751a6d6b9489b536ffd7e14722f22e44d266631898ec024b1b258e27  
xsa407/xsa407-4.13-03.patch
1eae8ece87fc06ca883fc7510b80ced195c3ec44a589fcf464ead076de4d1afd  
xsa407/xsa407-4.13-04.patch
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xsa407/xsa407-4.13-05.patch
cc3435e7bf2331a61c6e6731d8c0c4edd10ac49c85c9702e3d790309e1bb494a  
xsa407/xsa407-4.13-06.patch
46f7b3d8a4ae39fa325dda2d77091b0768367a3d2cf6a341996042e511e46b93  
xsa407/xsa407-4.13-07.patch
cb31c3104890c83fad719c8a2c7b0ae242625132a1e9d6afbf6310af10a8c14a  
xsa407/xsa407-4.13-08.patch
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xsa407/xsa407-4.13-09.patch
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xsa407/xsa407-4.13-10.patch
aa0ed6a1126c4d9d5fc94c00a51ddf27f4357c4a1cb258f72f0c17ac4ce0d191  
xsa407/xsa407-4.13-11.patch
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xsa407/xsa407-4.13-12.patch
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xsa407/xsa407-4.13-13.patch
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xsa407/xsa407-4.13-14.patch
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xsa407/xsa407-4.13-15.patch
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xsa407/xsa407-4.13-16.patch
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xsa407/xsa407-4.13-17.patch
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xsa407/xsa407-4.13-18.patch
a02fd749eb761b93fe7b2e5977a9aa493af13044165b71de9e7625c0237c2fde  
xsa407/xsa407-4.13-19.patch
cf127677913b8127c9a71b1c9b3badf9f2c2064d1ed2602d236ba610f7335c8a  
xsa407/xsa407-4.13-20.patch
0289eb4a9098ab806f5b847e5f55652817b9bb8c9ebc98e28fe8ac626c77f77c  
xsa407/xsa407-4.13-21.patch
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xsa407/xsa407-4.14-01.patch
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xsa407/xsa407-4.14-02.patch
69938e6c1293040aff921f2cd6bf2ad850caa682745f0d6be8bc2aabb3802edf  
xsa407/xsa407-4.14-03.patch
47d67a565d3077688a43937d7cb6cc79d43a8d5e8563711a1476924c696a9759  
xsa407/xsa407-4.14-04.patch
14d15b20e053c7dec2e9dd9cbd108284b0ac2069dac2e5c4e76ab4c78637fbe8  
xsa407/xsa407-4.14-05.patch
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xsa407/xsa407-4.14-06.patch
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xsa407/xsa407-4.14-07.patch
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xsa407/xsa407-4.14-08.patch
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xsa407/xsa407-4.14-09.patch
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xsa407/xsa407-4.14-10.patch
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xsa407/xsa407-4.14-11.patch
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xsa407/xsa407-4.14-12.patch
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xsa407/xsa407-4.15-1.patch
cd38bb072a8e99760a80464482d645aba1531fdb4f4d04eabd7c48e2db00c8e1  
xsa407/xsa407-4.15-2.patch
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xsa407/xsa407-4.15-3.patch
ae8e8f220a708401a68535e88a3092b35c3db0a20bd3e3a27cdcc7e88d1ff600  
xsa407/xsa407-4.15-4.patch
aba615483add2199ad2912557e0b9024d6efd6573fa8009590502d483a78e63a  
xsa407/xsa407-4.15-5.patch
55e58ce88ff7126c314c7e24f75a700a3263388137ecd725d2e459e21c018f64  
xsa407/xsa407-4.15-6.patch
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xsa407/xsa407-4.15-7.patch
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xsa407/xsa407-4.15-8.patch
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xsa407/xsa407-4.16-1.patch
b936a9a36c336d1dcf05923f9a07728522f6a6d1474006ec179981a4787a4522  
xsa407/xsa407-4.16-2.patch
825a683f37964186ab669468c517c342dc55b1e86898a75c86f8ff0de47e1b76  
xsa407/xsa407-4.16-3.patch
402795d0cb418503c3e90b65f3bf546493a7411d14208ac718ba8639f67d1860  
xsa407/xsa407-4.16-4.patch
ec6009b2ddaa74099725844bf4343efb8510015fb851d3ccc26913f877db0bdf  
xsa407/xsa407-4.16-5.patch
4fa9c65ee0bdf8650b0cd483c205a305352d918408b91d4adf83c84d1b269b2e  
xsa407/xsa407-4.16-6.patch
1c01c1508103de49cc1895a60babe9d33feaa27da8d2bd89c6895c0173e280d7  
xsa407/xsa407-4.16-7.patch
d2a4e06959dff5a9772b13d921332804fbaf81f012c0b9cf85f8b9dd008c61de  
xsa407/xsa407-4.16-8.patch
c178e43d3f569086aee66ffaf28f22156bdb22144bdff7ffe4f7c20242abe73c  
xsa407/xsa407-4.patch
eb9985afa38b1d2bffd6a48772a429fc0f88375cd3fc0b977f9a8a0981ab87b4  
xsa407/xsa407-5.patch
aea9fb436a1f3dc38a874b8b3e4d0f1a82fb14c5e50c579a978aee1a83bfdb72  
xsa407/xsa407-6.patch
cf1e9796dbaaedf1e3ba7efb830fe99dea8f09125d7ec7bd2a16b11cfc131aa6  
xsa407/xsa407-7.patch
cc46f1da318dfa72b87bbc069bf448eed3d1b264281e3a7d9a6bad8f6519e8c3  
xsa407/xsa407-8.patch
$

NOTE CONCERNING LACK OF EMBARGO
===============================

The disclosers did not authorise us to predisclose.
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