[CVE-2026-98440][MODERATE 7.0] KVM: arm64: nv: Delay freeing of shadow S2 structures until VM destruction [ Upstream

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

CVE: CVE-2026-98440
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: KVM: arm64: nv: Delay freeing of shadow S2 structures until VM destruction [ Upstream
Commit: 876049f026b4dfaca0cc78ec81be0e639ca8525d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=876049f026b4dfaca0cc78ec81be0e639ca8525d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98440
Analysis date: Sat, 24 Oct 2026 11:48:42 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
KVM arm64 prematurely freed active nested Stage-2 MMU containers during shadow flushing, leaving host KVM paths able to consume stale structured references before final VM destruction.

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

The original Linux kernel CVE announcement for CVE-2026-98440 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98440

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-2026-98440	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98440 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=2 LEGACY=MODERATE_KPANIC RESULT=SAME (final reconciliation agrees; DECISION: both MODERATE_KPANIC)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;*CWE-664;CWE-362;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'KVM arm64 could free nested shadow Stage 2 MMU containers from kvm_arch_flush_shadow_all while another KVM path still retained references to them. A later access through such a stale reference is a host kernel use after free candidate that can cause a host crash and may permit memory corruption if the freed allocation is reused. For the CVSS the PR:L value reflects that reliable triggering requires local access to KVM or control of a process managing a nested virtualization VM rather than network reachability alone. AC:H reflects the likely need to coordinate shadow MMU flushing with a remaining reference. The issue is not proven to be directly triggerable by an untrusted guest lacking assistance from the host VMM. Automated impact decision path. The original SimpleNet classifier selected MODERATE. The primary LLM estimated paranoid CVSS 7 and the production ActionableScore returned 5 with recommendation Actionable Moderate / Strong Important candidate requiring manual review. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE to IMPORTANT because the CVSS score, vector or bug class threshold rule was applied. KP and FD specialist checks stage IMPORTANT to MODERATE_KPANIC because a specific evidence based protection or manual review veto was applied and FD false positive review allowed a downgrade. The legacy KP specialist produced a review artifact and its decision was processed by the applicable KP rules. The legacy FD or DH downgrade specialist produced a review artifact and its decision was processed by the applicable downgrade and preservation rules. Exact triggered rule markers were INCREASED_TO_HIGH_BASED_ON_GUESSCVSS and V33_ACTIVE_STATE_BOUNDARY_CANDIDATE and IMPORTANT_CANDIDATE_MANUAL_REVIEW and KP_ONLY_DOWNGRADE_VETO and DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH. The independent weaker Aegis shadow evaluated CVSS 3.1 and ActionableScore 2 with primitive class CLASS_B and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is MODERATE_KPANIC.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	MAYBE	VIRT SIMPLEFIX HARDWARE LINUS  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS V33_ACTIVE_STATE_BOUNDARY_CANDIDATE IMPORTANT_CANDIDATE_MANUAL_REVIEW KP_ONLY_DOWNGRADE_VETO DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH	-	-	checked

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

ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 4.
* Conservative score: 4; no floor or cap changed it.
* Raw paranoid score: 5.
* Paranoid score: 5; no floor or cap changed it.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

* +2 Real UAF/lifetime violation: `kvm_arch_flush_shadow_all()` freed nested Stage-2 MMU containers while other KVM paths could retain references; the fix postpones container destruction until vCPUs are gone.
* +1 Reliable host-kernel crash plausible: later host access through a freed MMU container can fault; this is not proof of guaranteed panic.
* +1 Local below-host-admin trigger: supplied evidence grounds access through `/dev/kvm` or control of a VM-management process, without requiring host-administrator powers.
* Paranoid +1 attacker-influenced lifetime sequence: a KVM-capable VMM can plausibly coordinate shadow flushing with retained references. This is omitted from the lower score because the exact initiating ioctl/call sequence is not shown.
* No timing penalty: AC:H is described only as a likely coordination requirement, not a proven narrow or one-shot race.
* Controlled reclaim, attacker-shaped replacement contents, arbitrary host write, callback control, guest escape, confidentiality, and integrity impact are not shown.
* No extra availability point: it would duplicate the same possible host crash.
* No repeated release/refcount mutation is shown; FLOOR 5 does not apply on that basis.

3. Reachability analysis

The supported attacker is a local process permitted to use `/dev/kvm`, or a compromised VMM managing an arm64 nested-virtualization VM. Numeric host UID 0 and host-admin capabilities are not required by the supplied PR:L evidence. No namespace capability is involved.

A malicious guest alone is not shown able to invoke the necessary host-side shadow flush or coordinate the lifetime transition. The VMM creates and manages the VM; host KVM code frees the containers and performs the later stale traversal. Exact flush callers and retry reliability are absent, so pure guest-to-host reachability remains unproven.

4. Primitive and severity reading

This is an active structured-object UAF, not fixed stale storage or a destructor-only dereference. The vulnerable flush frees `kvm_s2_mmu` containers before VM destruction while references may remain operational. The fix preserves those containers until vCPUs have been destroyed.

The patch supports CLASS_B lifetime corruption and possible host crash. It does not show controlled reuse or a useful guest-to-host memory-corruption primitive, so CLASS_A is unwarranted. ImportantCandidate is YES because a below-host-admin VMM can influence active host MMU lifetime state and a later privileged KVM consumer remains credible but incompletely characterized. Auto-downgrade is unsafe until the stale consumers and reclaim behavior are reviewed.

5. One-sentence report phrase

KVM arm64 prematurely freed active nested Stage-2 MMU containers during shadow flushing, leaving host KVM paths able to consume stale structured references before final VM destruction.

6. Manual review advice

MANUAL CHECK REQUIRED

Identify the flush caller, surviving reference holders, stale fields consumed, and whether a KVM-capable VMM can reliably obtain useful post-free reuse rather than only a host crash.

7. Call-site confidence

Call-site confidence: medium

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

Patch: KVM: arm64: nv: Delay freeing of shadow S2 structures until VM destruction [ Upstream
Commit: 876049f026b4dfaca0cc78ec81be0e639ca8525d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=876049f026b4dfaca0cc78ec81be0e639ca8525d

Changed files:
  arch/arm64/include/asm/kvm_nested.h
  arch/arm64/kvm/arm.c
  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=876049f026b4dfaca0cc78ec81be0e639ca8525d

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

The original Linux kernel CVE announcement for CVE-2026-98440 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98440

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:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/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: 4
  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: YES
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).