[CVE-2026-98638][MODERATE REGULAR] KVM: arm64: Validate the SVE vector length in pkvm_vcpu_init_sve() [ Upstream

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

CVE: CVE-2026-98638
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: KVM: arm64: Validate the SVE vector length in pkvm_vcpu_init_sve() [ Upstream
Commit: faf66e55e87498ab4bf88cdf9a629e61b3f2d3bc
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=faf66e55e87498ab4bf88cdf9a629e61b3f2d3bc
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98638
Analysis date: Mon, 19 Oct 2026 01:55:50 -0400
ActionableScore: 1
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
An invalid host supplied SVE vector length can reach a fatal EL2 assertion during pKVM vCPU initialization, causing virtualization denial of service without evidence of memory corruption or privilege escalation.

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

The original Linux kernel CVE announcement for CVE-2026-98638 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98638

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: NO

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

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-98638	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98638 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 LEGACY=MODERATE RESULT=DIFF V39_AEGIS_ONE_STEP_UP=LOW->MODERATE TEMP_DISTANCE_MERGE_SKIP=AEGIS_NO_POSITIVE_SEVERITY_EVIDENCE (final Legacy and Aegis differ; FINAL: MODERATE, Aegis: 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:N/I:N/A:H';*CWE-20;CWE-617;CWE-754;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '4.7';DESCR 'An input validation flaw in KVM arm64 pKVM allows an invalid host supplied SVE vector length to reach sve_state_size_from_vl before its validity is checked. The resulting WARN_ON is fatal at EL2 and can terminate hypervisor execution, causing denial of service to protected virtual machines and potentially the virtualization host. For the CVSS the PR:L value is used in the paranoid assessment because a local process with access to /dev/kvm may be able to influence vCPU SVE configuration, although reliable control of the invalid internal value is not established and the base assessment therefore uses PR:H. The issue requires pKVM and SVE support and is not network reachable. No UAF, OOB access, information disclosure, or write primitive is demonstrated, so the supported impact is availability loss rather than privilege escalation. Automated impact decision path. The original SimpleNet classifier selected MODERATE. The primary LLM estimated paranoid CVSS 4.7 and the production ActionableScore returned 1 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE to LOW because the CVSS score, vector or bug class threshold rule was applied. final reconciliation stage LOW to MODERATE because the cross model evidence reconciliation rule was applied. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_GUESSCVSS_AND_CIANNH and V39_AEGIS_ONE_STEP_UP and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 4.2 and ActionableScore 1 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. Cross model aggregation changed the production result under an enabled evidence gated merge rule. The final reported impact is MODERATE.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1)	MAYBE	VIRT WARNONLY SIMPLEFIX HARDWARE LINUS TEST DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_GUESSCVSS_AND_CIANNH V39_AEGIS_ONE_STEP_UP MANUAL_REVIEW_REQUIRED	-	-	checked

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

ActionableScore=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw conservative score: 1
* Conservative score: 1
* Raw paranoid score: 1
* Paranoid score: 1
* No floor or cap changed the score.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

Positive signals:

* +1 Reliable strong DoS. An invalid `sve_max_vl` reaches `WARN_ON()` in `sve_state_size_from_vl()`, and the supplied context explicitly states that this warning is fatal at EL2.
* No additional availability point was awarded because termination at EL2 is the same DoS consequence.

Negative signals:

* No negative points were applied. Host-admin privilege, difficult timing, and rare hardware are not positively established.

Material non-awarded signals:

* No UAF, OOB access, heap write, stale reference, or active-state corruption is established.
* No arbitrary or directed write, controlled reclaim, information disclosure, or privilege escalation is demonstrated.
* No local-unprivileged bonus was awarded because the complete path from a `/dev/kvm` process to an invalid internal `sve_max_vl` is unresolved.
* No guest-to-host leverage was awarded. Guest control of the invalid value is not established.
* No authorization bypass exists. `sve_vl_valid()` is input validation, not an access-control decision.
* B5-R does not apply because there is no retry-accumulated heap OOB write or later consumption of corrupted structured state.
* V33 and the checksum metadata rules do not apply because no attacker-influenced active structured-state write exists.

3. Reachability analysis

The vulnerable function consumes `host_vcpu->arch.sve_max_vl` while initializing SVE state in the pKVM hypervisor. Three attacker roles must remain separate:

* A malicious guest is not shown to control this host-side initialization value directly.
* A local VMM with `/dev/kvm` access may influence SVE vCPU configuration, but the supplied evidence does not establish that it can bypass earlier KVM validation and install an invalid `arch.sve_max_vl`.
* A malicious or corrupted host kernel can supply malformed host-side state to EL2, but that implies a substantially stronger actor and does not establish that host administration is required for every trigger.

The complete trigger privilege is therefore unknown. The path requires arm64 pKVM with SVE support. It is not network-triggerable, namespace-scoped, race-dependent, or dependent on a retry sequence. No special physical device is required beyond suitable arm64 SVE hardware and the relevant virtualization configuration.

4. Primitive and severity interpretation

The exact primitive is validation-order failure followed by a fatal EL2 assertion. The code calculates an SVE state size before confirming that the vector length is architecturally valid. The existing zero-size rejection runs too late because the helper has already executed its fatal warning.

This is `CLASS_C`: an invalid-state and DoS path without an independently useful corruption primitive. It is neither active structured corruption nor fixed stale storage, and no lifetime operation is involved.

The demonstrated consequence is EL2 termination and virtualization availability loss. Memory corruption, guest-to-host escape, confidentiality impact, integrity impact, and privilege escalation are unsupported. `ImportantCandidate=NO` because the patch establishes only fatal input handling. `AutoDowngradeAllowed=YES` because no unresolved corruption, ownership, authorization, or security-boundary primitive would be lost by treating it as an availability issue.

5. One-sentence report phrase

An invalid host supplied SVE vector length can reach a fatal EL2 assertion during pKVM vCPU initialization, causing virtualization denial of service without evidence of memory corruption or privilege escalation.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Confirm whether a below-host-admin VMM process can actually place an invalid value in `host_vcpu->arch.sve_max_vl`, or whether the vulnerable state requires malicious host-kernel control.

7. Call-site confidence

Call-site confidence: medium

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

Patch: KVM: arm64: Validate the SVE vector length in pkvm_vcpu_init_sve() [ Upstream
Commit: faf66e55e87498ab4bf88cdf9a629e61b3f2d3bc
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=faf66e55e87498ab4bf88cdf9a629e61b3f2d3bc

Changed files:
  arch/arm64/kvm/hyp/nvhe/pkvm.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=faf66e55e87498ab4bf88cdf9a629e61b3f2d3bc

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

The original Linux kernel CVE announcement for CVE-2026-98638 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98638

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:H/UI:N/S:U/C:N/I:N/A:H
  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: unknown
  Paranoid ActionableScore: 1

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).