[CVE-2026-64555][MODERATE 7.0] KVM: arm64: nv: Fix SPSR_EL2 restore in kvm_hyp_handle_mops()

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

CVE: CVE-2026-64555
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: KVM: arm64: nv: Fix SPSR_EL2 restore in kvm_hyp_handle_mops()
Commit: 10a568010e827108d149779908850afaec898846
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=10a568010e827108d149779908850afaec898846
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64555
Analysis date: Mon, 27 Jul 2026 22:18:59 -0400
ActionableScore: 5
ActionableScore lower bound: 3
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
KVM arm64 nested virtualization may restore an incorrect hardware SPSR_EL2 value during MOPS exception handling because synthetic vEL2 CPSR state was written directly to hardware, creating a local KVM-triggered denial-of-service and possible virtualization state-isolation concern.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64555 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64555

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 7.0
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-2026-64555	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:C/C:L/I:L/A:H';CWE-269;CWE-670;CWE-703;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:C/C:N/I:L/A:L';BEST CVSS score: '7';DESCR 'KVM arm64 nested virtualization can restore an incorrect hardware SPSR_EL2 value while handling a MOPS exception because kvm_hyp_handle_mops() used vcpu_cpsr(), which may contain synthetic vEL2 state for a guest hypervisor. Writing that synthetic state directly to hardware can corrupt the exception return state after the MOPS rewind and single step cleanup. For the CVSS the PR:L is used for the paranoid score because a local user or tenant able to run a KVM VM or control a nested guest can potentially reach this path without full host administrative privileges. The issue is not directly network reachable and requires local KVM execution context on affected arm64 systems with nested virtualization and MOPS support. Impact is at least guest or host denial of service through incorrect CPU state restoration. In the paranoid interpretation it may also have limited confidentiality or integrity impact because the bug affects privilege level state translation across the nested virtualization boundary.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 4)  SKIP CVE-2026-64555 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	VIRT SIMPLEFIX HARDWARE LINUS  INCREASED_TO_MODERATE7_FROM_MODERATE_BASED_ON_GUESSCVSS DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged or tenant trigger: +1
  A user or tenant with access to KVM on arm64 may be able to run a VM or nested guest that reaches the MOPS exception path.

* Cross-boundary virtualization state concern: +1
  The bug writes synthetic vEL2 CPSR state into real hardware SPSR_EL2, which is a KVM nested virtualization privilege-state boundary issue.

* Availability impact realistic: +1
  Incorrect SPSR_EL2 restore can plausibly break exception return state and cause guest or host-side failure, although the patch does not show a concrete panic trace.

* Rare or configuration-dependent path: -1
  Triggering requires arm64 KVM, nested virtualization, and MOPS-related exception handling, so exposure is narrower than common KVM run paths.

Paranoid additional signals:

* Weak LPE concern: +1
  The bug affects EL2 state translation and exception return state across a nested virtualization boundary, but no concrete arbitrary write, UAF, or controlled object corruption primitive is shown.

* Confidentiality and integrity impact plausible: +1 total
  Limited C/I concern is defensible only in the paranoid interpretation because incorrect privilege-state restoration may affect guest or nested-hypervisor isolation semantics. This is not strong evidence of host code execution.

Not awarded:

* No generic memory corruption bonus. The patch does not show UAF, OOB write, double free, refcount misuse, or type confusion.
* No remote reachability bonus. This is not network-triggerable.
* No strong LPE bonus. There is no demonstrated reclaim, overwrite, stale callback, arbitrary write, or reliable object confusion primitive.

Call-site confidence: medium. The changed code is directly in the hyp exception handler, but the input does not include a full call-path reproducer or downstream failure trace.

## 3. Reachability analysis

The likely trigger is a local user, VM tenant, or management context that can run KVM guests on affected arm64 hardware with nested virtualization and MOPS support. In many deployments, access to `/dev/kvm` is granted to non-root users through a virtualization group, so PR:L is more appropriate than PR:H for practical triage. Containers or delegated virtualization environments may further reduce the practical privilege barrier.

The issue is not directly reachable over the network. A remote attacker would need a separate path to control guest execution or VM management. The path is also not universal because it depends on nested virtualization and MOPS handling, so exposure is narrower than common KVM ioctl or vCPU run bugs.

## 4. Severity interpretation

This behaves like an actionable Moderate virtualization boundary bug, not an ordinary low-risk correctness fix. The conservative view is local DoS or guest state corruption due to incorrect SPSR_EL2 restoration. The paranoid view justifies manual review because the affected state is privilege-level exception-return state in nested virtualization, where small translation mistakes can have isolation consequences.

However, the patch does not demonstrate a concrete memory corruption primitive, arbitrary write, host memory disclosure, or reliable guest-to-host privilege escalation. Therefore it should not be treated as a Strong Important candidate without additional evidence.

## 5. One-sentence report phrase

KVM arm64 nested virtualization may restore an incorrect hardware SPSR_EL2 value during MOPS exception handling because synthetic vEL2 CPSR state was written directly to hardware, creating a local KVM-triggered denial-of-service and possible virtualization state-isolation concern.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Manual review is required because this is a KVM nested virtualization privilege-state restore bug involving SPSR_EL2 and synthetic vEL2 state. The patch does not prove memory corruption or LPE, but the affected boundary is security-sensitive enough that it should not be auto-closed.

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UPSTREAM PATCH SUMMARY
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Patch: KVM: arm64: nv: Fix SPSR_EL2 restore in kvm_hyp_handle_mops()
Commit: 10a568010e827108d149779908850afaec898846
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=10a568010e827108d149779908850afaec898846

Commit description:

kvm_hyp_handle_mops() resets the single-step state machine as part of
rewinding state for a MOPS exception by modifying vcpu_cpsr() and
writing the result directly into hardware.

In the case of nested virtualization, vcpu_cpsr() is a synthetic value
such that the rest of KVM can deal with vEL2 cleanly. That means the
value requires translation before being written into hardware, which is
unfortunately missing from the MOPS handler.

Fix it by directly modifying SPSR_EL2 and avoiding the synthetic state
altogether, which will be resynchronized on the next 'full' exit back
to KVM.

Fixes: 2de451a ("KVM: arm64: Add handler for MOPS exceptions")
Reported-by: Zhong Wang <wangzhong.c0ss4ck@bytedance.com>
Reported-by: Xuanqing Shi <shixuanqing.11@bytedance.com>
Link: https://lore.kernel.org/all/ajE4lHQevXNHpl1M@Air.local/
Cc: stable@vger.kernel.org
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
Link: https://patch.msgid.link/20260617040820.2194831-2-bestswngs@gmail.com
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=10a568010e827108d149779908850afaec898846

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

The original Linux kernel CVE announcement for CVE-2026-64555 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64555

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:C/C:N/I:L/A:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:C/C:L/I:L/A:H
  The Best / paranoid CVSS score: 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: 3
  Paranoid ActionableScore: 5

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 7.0

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).