[CVE-2026-64288][MODERATE REGULAR] KVM: arm64: nv: Avoid dereferencing NULL VNCR pseudo-TLB

[CVE-2026-64288][MODERATE REGULAR] KVM: arm64: nv: Avoid dereferencing NULL VNCR pseudo-TLB

[AL-KERNEL]

CVE: CVE-2026-64288
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: KVM: arm64: nv: Avoid dereferencing NULL VNCR pseudo-TLB
Commit: 7c73a269a880b1399baacfb9d521415e6ef7ecc2
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7c73a269a880b1399baacfb9d521415e6ef7ecc2
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64288
Analysis date: Sat, 25 Jul 2026 11:33:34 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline manual review recommended / Actionable Moderate not proven
Manual review required: YES

Summary:
KVM arm64 nested virtualization can dereference a NULL VNCR pseudo TLB pointer during VNCR TLB invalidation when TLBI or MMU notifier activity races with vCPU online state, allowing a local KVM user or guest controlled context to crash the host kernel.

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ABOUT THIS REPORT
======================================================================

The original Linux kernel CVE announcement for CVE-2026-64288 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64288

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-2026-64288	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';**CWE-476;CWE-362;CWE-703;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'KVM arm64 nested virtualization can dereference a NULL VNCR pseudo TLB pointer during VNCR TLB invalidation when MMU notifiers or guest TLBI instructions race with a vcpu that is not fully onlined yet. This can crash the host kernel and cause denial of service for the host or other guests. For the CVSS the PR:L reflects that reliable triggering requires local KVM access or control of a privileged guest context, not only network reachability. The issue is not directly network reachable and does not show a concrete information leak or write primitive. Impact is denial of service only in the base assessment, with the paranoid score increasing attack complexity assumptions but staying within the NULL pointer dereference class.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review recommended / Actionable Moderate not proven (with actual score 4)  SKIP CVE-2026-64288 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 4ffa72ad8f37e73bbb6c0baa88557bcb4fd39929 YES  NO NO unknown 	MAYBE	VIRT SIMPLEFIX RACE HARDWARE LINUS 	-	-	checked

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ACTIONABLESCORE ANALYSIS
======================================================================

ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline manual review recommended / Actionable Moderate not proven**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. Triggering requires local KVM access or control of a guest path able to issue relevant TLBI activity. This is not remote network reachable.
* Reliable kernel crash / strong DoS: +1. The patch prevents a NULL dereference of vcpu->arch.vncr_tlb in the host kernel.
* Availability impact realistic: +1. A host kernel crash can affect the host and other guests, so this is stronger than a process-local failure.
* Hard or unreliable race / special timing required: -1. The bug depends on VNCR invalidation racing with a vCPU that is not fully onlined yet.

Paranoid adjustment:

* Remove the race penalty: +1. If VM lifecycle and guest TLBI or MMU notifier activity can be coordinated reliably, the race may be easier to hit than the conservative assessment assumes.

Not applied:

* No remote reachable bonus. The affected path is KVM control or guest execution, not a network protocol parser.
* No generic memory corruption bonus. The demonstrated primitive is a NULL pointer dereference, not UAF, OOB write, double free, type confusion, or arbitrary write.
* No privilege escalation bonus. The patch does not show attacker controlled reclaim, write after free, callback control, refcount abuse, or another LPE primitive.
* No rare subsystem penalty in the numeric score. arm64 nested virtualization is configuration dependent, but once enabled the path is reachable through normal KVM guest activity.

## 3. Reachability analysis

The likely attacker is a local user with access to /dev/kvm, a VM management context, or a guest able to trigger nested virtualization TLBI paths while vCPU state is changing. Namespace behavior depends on whether KVM device access is delegated into containers, but this should not be treated as PR:N because local code execution or guest control is still required. The issue is not directly network reachable. Practical exploitation requires arm64 KVM nested virtualization with VNCR pseudo TLB support, making exposure narrower than common x86 KVM paths but still security relevant for affected virtualization hosts.

## 4. Severity interpretation

This behaves like a KVM host DoS rather than an Important class memory corruption issue. The theoretical concern is elevated because a guest influenced path can crash the host kernel, but the concrete primitive shown by the patch is only a NULL pointer dereference. There is no evidence of UAF reuse, stale callback dispatch, arbitrary write, object confusion, or confidentiality and integrity impact. Conservative handling is borderline Moderate with manual review recommended, while the paranoid score reaches Actionable Moderate threshold only if the race is considered practically controllable.

## 5. One-sentence report phrase

KVM arm64 nested virtualization can dereference a NULL VNCR pseudo TLB pointer during VNCR TLB invalidation when TLBI or MMU notifier activity races with vCPU online state, allowing a local KVM user or guest controlled context to crash the host kernel.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: this is a KVM race with host kernel crash impact and guest influenced reachability, but the demonstrated primitive is NULL dereference only and does not currently justify Important class treatment.

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

Patch: KVM: arm64: nv: Avoid dereferencing NULL VNCR pseudo-TLB
Commit: 7c73a269a880b1399baacfb9d521415e6ef7ecc2
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7c73a269a880b1399baacfb9d521415e6ef7ecc2

Commit description:

VNCR TLB invalidation occurs from MMU notifiers or TLBI instructions,
and either can race against a vcpu not being onlined yet (no pseudo-TLB
allocated). Similarly, the TLB might be invalid, and the invalidation
should be skipped in this case.

Both kvm_invalidate_vncr_ipa() and kvm_invalidate_vncr_va() are
expected to perform the same checks, except that the latter doesn't
check for the allocation and blindly dereferences the pointer.

Solve this by introducing a new iterator built on top of the usual
kvm_for_each_vcpu() that checks for both of the above conditions,
and convert the two users to it.

Reported-by: Hyunwoo Kim <imv4bel@gmail.com>
Link: https://lore.kernel.org/r/aiUvSbrWndQeUPc8@v4bel
Fixes: 4ffa72a ("KVM: arm64: nv: Add S1 TLB invalidation primitive for VNCR_EL2")
Cc: stable@vger.kernel.org
Reviewed-by: Oliver Upton <oupton@kernel.org>
Link: https://patch.msgid.link/20260607175745.297793-1-maz@kernel.org
Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  arch/arm64/kvm/nested.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=7c73a269a880b1399baacfb9d521415e6ef7ecc2

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

The original Linux kernel CVE announcement for CVE-2026-64288 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64288

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:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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).