[CVE-2026-64172][LOW] KVM: SVM: Disable AVIC IPI virtualization on Hygon Family 18h (erratum #1235)

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-64172
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: KVM: SVM: Disable AVIC IPI virtualization on Hygon Family 18h (erratum #1235)
Commit: 94ade38f317ea086a181db0e6b69c574b3b70a5e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=94ade38f317ea086a181db0e6b69c574b3b70a5e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64172
Analysis date: Sun, 19 Jul 2026 14:27:41 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
KVM SVM on Hygon Family 18h CPUs may miss AVIC IPI failure VM-Exits due to erratum 1235, causing missed vCPU wakeups and guest execution hangs, with impact limited to denial of service on affected hardware.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64172 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64172

The original announcement does not normally provide a security severity
estimate, CVSS assessment, or enough information to determine whether the
reported kernel bug represents a practically relevant security issue.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin. It combines
an autonomous classifier with LLM-assisted technical analysis and a
separate ActionableScore mechanism.

The purpose of this report is to prioritize Linux kernel CVEs before
manual review, identify cases that require prompt investigation, and
support automatic closure of issues that are unlikely to have meaningful
security impact.

Published priority for this report: LOW
Manual review required: NO

A detailed explanation of the methodology and priority rules is included
at the end of this message.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-64172	LOW	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-693;CWE-755;CWE-662;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'KVM SVM AVIC IPI virtualization on Hygon Family 18h CPUs can miss an expected VM-Exit when hardware observes a stale IsRunning bit during ICR write handling. This can make KVM miss the wakeup of a blocking target vCPU and can cause hung guest vCPUs or unbounded delays in guest execution. For the CVSS the PR:L is used for the paranoid score because a process inside a guest may be able to induce guest kernel IPI activity through normal workload patterns, even though direct APIC control is privileged inside the guest. The issue is not network reachable by itself and requires code execution in the affected VM environment on specific Hygon hardware. Impact is denial of service to guest execution only, with no evidence of host memory corruption, information disclosure, or privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2)  SKIP CVE-2026-64172 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 8de4a1c8164e5b2e40d1df764840a31de983f40b YES  NO NO unknown 	MAYBE	LOCK VIRT INIT SIMPLEFIX HARDWARE LINUS 	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=2
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like**::

## 2. Signal breakdown

Conservative signals:

* Reliable guest DoS / strong availability concern: +1
  The missed AVIC IPI failure VM-Exit can cause KVM to miss wakeups for blocking target vCPUs, leading to hung vCPUs or unbounded guest execution delays.
* Availability impact realistic: +1
  The impact can persist as stalled guest execution, not merely a transient warning.
* Broad subsystem context: +1
  KVM is a broadly deployed virtualization subsystem, although this specific issue is hardware-limited.
* Physical-only or rare hardware-only condition: -2
  The bug only applies to Hygon Family 18h systems affected by the AVIC erratum.
* Conservative total: 1

Paranoid additional interpretation:

* Local unprivileged trigger inside guest: +1
  A guest user process may indirectly induce IPI activity through normal guest workload patterns, even if direct APIC control is privileged inside the guest.
* Paranoid total: 2

Not applied:

* No remote/network-triggerable signal. The path is not directly reachable through network packets.
* No generic memory corruption signal. The patch describes missed VM-Exits and missed vCPU wakeups, not UAF, OOB access, double free, refcount misuse, or type confusion.
* No privilege escalation signal. There is no evidence of guest-to-host escape, host memory corruption, or host code execution.
* No confidentiality or integrity impact. The described failure mode is guest execution delay or hang only.

## 3. Reachability analysis

The most plausible trigger is code running inside an affected guest that causes IPI activity while AVIC IPI virtualization is enabled on Hygon Family 18h hardware. A host administrator must already be running KVM on this specific hardware class, but the attacker model can be a guest-local user in the paranoid interpretation. Namespaces or containers inside the guest do not materially change host reachability, because the issue is in virtual CPU interrupt delivery behavior rather than a host userspace API. The affected path is hardware and feature dependent, and realistic exploitation conditions require a vulnerable Hygon Family 18h host with AVIC IPI virtualization active.

## 4. Severity interpretation

This behaves like a hardware-specific guest availability bug, not an Important-class memory corruption vulnerability. The theoretical impact is a guest vCPU hang or unbounded guest delay caused by missed wakeups. Realistic exploitation evidence supports DoS against guest execution only. There is no demonstrated primitive for host memory corruption, privilege escalation, information disclosure, or integrity compromise. The issue should not be treated as an actionable Moderate or Important candidate unless local policy treats guest availability on affected virtualization hosts as high priority.

## 5. One-sentence report phrase

KVM SVM on Hygon Family 18h CPUs may miss AVIC IPI failure VM-Exits due to erratum 1235, causing missed vCPU wakeups and guest execution hangs, with impact limited to denial of service on affected hardware.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This can be auto-closed or deferred in most generic CVE triage workflows because it is hardware-specific, DoS-only, not network reachable, and does not show memory corruption or privilege escalation evidence.

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UPSTREAM PATCH SUMMARY
======================================================================

Patch: KVM: SVM: Disable AVIC IPI virtualization on Hygon Family 18h (erratum #1235)
Commit: 94ade38f317ea086a181db0e6b69c574b3b70a5e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=94ade38f317ea086a181db0e6b69c574b3b70a5e

Commit description:

Hygon Family 18h CPUs are derived from AMD Family 17h (Zen1) silicon and
share the same erratum #1235: hardware may read a stale IsRunning=1 bit
during ICR write emulation and silently fail to generate an
AVIC_IPI_FAILURE_TARGET_NOT_RUNNING VM-Exit on the sending vCPU.

The absence of the VM-Exit causes KVM to miss the required wakeup of
blocking target vCPUs, leading to hung vCPUs and unbounded delays in
guest execution.

Extend the existing AMD Family 17h erratum #1235 workaround to also cover
Hygon Family 18h.  With IPI virtualization disabled, KVM never sets
IsRunning=1 in the Physical ID table, so every non-self IPI generates a
VM-Exit and is correctly emulated.

Fixes: 8de4a1c ("KVM: SVM: Disable (x2)AVIC IPI virtualization if CPU has erratum #1235")
Cc: <stable@vger.kernel.org>
Signed-off-by: Tina Zhang <zhang_wei@open-hieco.net>
Message-ID: <20260522040014.3380201-1-zhang_wei@open-hieco.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  arch/x86/kvm/svm/avic.c

Diff excerpt:

Not included in this email. See the upstream URL for the full patch.

Full patch:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=94ade38f317ea086a181db0e6b69c574b3b70a5e

======================================================================
DETAILED REPORT METHODOLOGY
======================================================================

The original Linux kernel CVE announcement for CVE-2026-64172 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64172

The original CVE announcement normally does not include a security-level
estimate. In particular, it may not contain a CVSS assessment, an impact
level, or enough information to determine whether the reported bug is a
practically relevant security issue. One purpose of this parallel CVE list
is to provide that missing technical and prioritization information.

The original goal of the AL-KERNEL project was to prioritize Linux kernel
CVE analysis automatically before manual review. The system can also help
identify non-security issues that may be suitable for automatic closure.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin.

The first analysis stage combines an autonomous classifier with additional
LLM-based analysis. The autonomous classifier runs locally on a CPU and is
based on a backpropagation neural network. Together, these mechanisms
produce a technical vulnerability description, identify likely weakness
types, estimate CVSS severity, and provide input for ActionableScore.

Two CVSS estimates are retained because incomplete kernel vulnerability
information often permits more than one defensible interpretation:

  Conservative CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

The conservative vector represents a lower-impact interpretation.
The Best/paranoid vector intentionally represents a plausible upper-bound
interpretation and should not automatically be treated as demonstrated
real-world impact.

CVSS may also need to be adjusted for a particular Linux deployment,
because actual reachability, privileges, enabled kernel configuration,
hardware, namespaces, exposed device nodes, and other environmental
conditions can differ significantly between systems.

A separate ActionableScore mechanism evaluates practical remediation
urgency. Its analysis may include reachability, attack prerequisites,
subsystem exposure, memory-corruption characteristics, denial-of-service
reliability, and possible confidentiality, integrity, or
privilege-escalation impact.

  Conservative ActionableScore: 1
  Paranoid ActionableScore: 2

The final base severity is taken directly from the second tab-separated
field of the AL-KERNEL classification result. ActionableScore does not
replace or independently override that final AL-KERNEL decision, and
if ActionableScore adjusted impact level of ALKERNEL, then you would see
self-readable flags above like INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7.

For an AL-KERNEL result of MODERATE, this report uses the following
additional presentation split:

  ActionableScore below 5   -> MODERATE REGULAR
  ActionableScore 5 or more -> MODERATE 7.0

The distinction between MODERATE REGULAR and MODERATE 7.0 makes it
possible to identify Moderate issues that should receive manual analysis
and fixes before lower-priority MODERATE REGULAR issues. In many cases,
MODERATE REGULAR fixes may wait for a later rebase or routine update.

There is one override in which MODERATE REGULAR becomes MODERATE 7.0
even when the ActionableScore is below 5. When the AL-KERNEL result
contains the KPANIC flag, a MODERATE result is always presented as
MODERATE 7.0. The KPANIC flag selected with few regexps without
usage of AI at all, so it helps to detect cases when Kernel Crash happens
and similar (to filter False-Negative results from the LLM usage).

KPANIC indicates that a reliable kernel crash, kernel panic, or similarly
serious kernel availability impact was identified by the classification
workflow.

AL-KERNEL base severity for this report: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

These results are intended to support engineering triage. They are
machine-generated estimates, and cases marked for manual review should
be validated by a human security engineer before final disposition.
For more info read docs linked from here: https://kernelcve.org/
(and you can submit you own patch there to generate such a report
for non-existant CVE-id yet).

Note that in many cases this AI tool selects higher severity, than
real is (means you can expect Importants instead of Moderate 7.0 or
Moderates 7.0 instead of regular Moderates). If you see such cases,
please use reply email interface to add additional manual analyses
info to this particular CVE.
And please, please, let me know when you see Lows instead of Importants
or Important instead of Low (because particular for such cases I
need to tune this AI tool to make it better for this one and next similar).
My contact email for such notifications is alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).