[CVE-2026-72280][MODERATE REGULAR] KVM: arm64: nv: Drop bogus WARN for write to ZCR_EL2

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

CVE: CVE-2026-72280
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: KVM: arm64: nv: Drop bogus WARN for write to ZCR_EL2
Commit: 6561597dba97e3e8479a9919bda172cf43eb902c
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6561597dba97e3e8479a9919bda172cf43eb902c
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72280
Analysis date: Sat, 15 Aug 2026 04:13:22 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline, manual review required / Actionable Moderate candidate
Manual review required: YES

Summary:
A bogus WARN_ON_ONCE in the KVM arm64 nested virtualization ZCR_EL2 handling path can be triggered by a guest write to a valid sysreg alias and cause a host hyp_panic, resulting in guest-to-host denial of service.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-72280 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72280

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: MODERATE REGULAR
Manual review required: YES

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-2026-72280	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:C/C:N/I:N/A:H';CWE-617;CWE-755;CWE-703;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '6.5';DESCR 'KVM arm64 nested virtualization can hit a host hyp_panic because a legitimate guest write to the ZCR_EL2 sysreg alias reaches kvm_hyp_handle_fpsimd and triggers a bogus WARN_ON_ONCE when FEAT_NV2 is exposed to the L1 hypervisor. A malicious or buggy guest that can run nested virtualization code can use this path to crash the host kernel or hypervisor. For the CVSS the PR:L is used because the attacker needs the ability to run code in a guest or nested guest context, but does not need host administrator privileges. The issue is not directly network reachable and is triggered from the virtualization control path rather than packet traffic. Impact is denial of service via host crash. No memory corruption or information disclosure primitive is visible from the patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review required / Actionable Moderate candidate (with actual score 4)  SKIP CVE-2026-72280 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	VIRT WARNONLY HARDWARE LINUS  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScoreLower=3
ActionableScore=4

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline, manual review required / Actionable Moderate candidate**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. A guest or nested guest workload can trigger the path without host administrator privileges, assuming it can execute the relevant nested virtualization code.
* Reliable kernel crash / strong DoS: +1. The commit explicitly says the bogus WARN has the effect of a hyp_panic once hyp vectors are installed.
* Cross-boundary isolation impact: +1. The failure crosses the guest to host boundary by allowing guest controlled behavior to crash the host hypervisor.
* Availability impact realistic: +1. A hyp_panic is host-wide availability impact, not only a local guest crash.
* Legacy / non-default execution mode: -1. The issue depends on arm64 nested virtualization context and FEAT_NV2 exposure, which is not a universal/default KVM deployment mode.

Paranoid difference:

* Paranoid score removes some deployment discount because KVM nested virtualization may be intentionally enabled in cloud, CI, or virtualization hosting environments where guest-to-host DoS is operationally significant.

## 3. Reachability analysis

The bug is triggerable by code running in a guest or nested guest context when FEAT_NV2 is exposed to the L1 hypervisor. It is not network reachable by itself and does not involve packet parsing. It does not require host root privileges, but it does require access to a VM environment where nested virtualization is enabled and the relevant arm64 feature path is reachable. Containers do not directly change reachability unless they provide access to KVM or to a guest execution environment.

Call-site confidence: high, because the patch directly removes the WARN_ON_ONCE in the KVM hyp FPSIMD trap handling path and the commit explains the hyp_panic consequence.

## 4. Severity interpretation

This behaves more like an actionable Moderate guest-to-host DoS than an Important memory-corruption issue. There is no evidence of UAF, OOB access, arbitrary write, type confusion, information disclosure, or privilege escalation. The practical concern is that guest controlled execution can crash the host hypervisor in a specific nested virtualization configuration.

## 5. One-sentence report phrase

A bogus WARN_ON_ONCE in the KVM arm64 nested virtualization ZCR_EL2 handling path can be triggered by a guest write to a valid sysreg alias and cause a host hyp_panic, resulting in guest-to-host denial of service.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is not memory corruption, but it is a KVM guest-to-host host-crash condition with a defensible paranoid score above ordinary low-priority cleanup.

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

Patch: KVM: arm64: nv: Drop bogus WARN for write to ZCR_EL2
Commit: 6561597dba97e3e8479a9919bda172cf43eb902c
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6561597dba97e3e8479a9919bda172cf43eb902c

Commit description:

It is entirely possible for a guest to write to the ZCR_EL2 sysreg alias
while in a nested context, as it is expected if FEAT_NV2 is advertised
to the L1 hypervisor.

Get rid of the bogus WARN which, since the hyp vectors were installed at
this point, has the effect of a hyp_panic...

Cc: stable@vger.kernel.org
Fixes: 0cfc85b ("KVM: arm64: nv: Load guest FP state for ZCR_EL2 trap")
Signed-off-by: Oliver Upton <oupton@kernel.org>
Link: https://patch.msgid.link/20260615051324.830045-1-oupton@kernel.org
Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  arch/arm64/kvm/hyp/include/hyp/switch.h

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=6561597dba97e3e8479a9919bda172cf43eb902c

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

The original Linux kernel CVE announcement for CVE-2026-72280 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72280

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:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:C/C:N/I:N/A:H
  The Best / paranoid CVSS score: 6.5

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: 3
  Paranoid ActionableScore: 4

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: MODERATE
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

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).