[CVE-2026-98863][IMPORTANT] KVM: x86: Re-pend GET_NESTED_STATE_PAGES if getting said pages fails [ Upstream

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

CVE: CVE-2026-98863
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: NO
Patch: KVM: x86: Re-pend GET_NESTED_STATE_PAGES if getting said pages fails [ Upstream
Commit: 525694a6021ec6c8f7ce1f38acdf2dd77698fec1
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=525694a6021ec6c8f7ce1f38acdf2dd77698fec1
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98863
Analysis date: Sat, 24 Oct 2026 12:14:09 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
Failed nested-page repinning can leave stale, unpinned host PFNs active across KVM_RUN retries, allowing later L2 execution to access a reallocated host page as nested control state.

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

The original Linux kernel CVE announcement for CVE-2026-98863 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98863

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: IMPORTANT
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-2026-98863	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-98863 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=3 LEGACY=IMPORTANT RESULT=DIFF INDEPENDENT_LEGACY=IMPORTANT INDEPENDENT_RESULT=DIFF TEMP_KPANIC_MERGE=IMPORTANT_WINS_DISAGREEMENT PRE_MERGE_LEGACY=IMPORTANT (final Legacy and Aegis differ; FINAL: IMPORTANT, 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:H/I:H/A:H';*CWE-672;*CWE-416;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A failed GET_NESTED_STATE_PAGES operation can return from KVM_RUN after clearing the request while vmcs02 still contains PFNs from a previous L2 run. If userspace runs the vCPU again, KVM may reenter L2 lacking repinning those pages and hardware may use a stale PFN as the vAPIC page or another nested control page after the host page has been reallocated. For the CVSS the PR:L value reflects that an attacker needs guest code execution or comparable access to a KVM vCPU with nested virtualization, but does not need host administrative privileges. AC:H reflects the required page acquisition failure, vCPU rerun, stale PFN lifetime, and useful page reuse. Impact is at least host denial of service and may include confidentiality or integrity impact across the guest to host boundary because the stale mapping is a plausible memory corruption primitive. 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. final reconciliation stage IMPORTANT to IMPORTANT because KP specialist did not establish a qualifying panic. 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 TEMP_KPANIC_MERGE_IMPORTANT_WINS and KPANIC_UNSUPPORTED_REMOVED. The independent weaker Aegis shadow evaluated CVSS 3.1 and ActionableScore 3 with primitive class CLASS_B 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 IMPORTANT.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	MAYBE	VIRT SIMPLEFIX LEAK HARDWARE LINUS INCREASED_TO_HIGH_BASED_ON_GUESSCVSS TEMP_KPANIC_MERGE_IMPORTANT_WINS KPANIC_UNSUPPORTED_REMOVED	-	-	checked

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

ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=UNKNOWN
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 applied.
* Raw paranoid score: 5.
* Paranoid score: 5; no floor or cap applied.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

* +2 — Real page-lifetime violation: failed repinning clears the request, yet a later KVM_RUN can reuse stale, unpinned PFNs after the original host page has been released or reallocated.
* +1 — Active-state consumption: the stale PFN remains installed in vmcs02 and is later used as an active nested control mapping, e.g. the vAPIC page.
* +1 — Plausible host crash/DoS from hardware or KVM accessing a stale host PFN. No additional availability point is counted.
* Paranoid +1 — A vAPIC/control page is writable operational state, so later hardware use provides a technically credible constrained write into a reallocated host page. The target, payload, and useful adjacent object are not shown.

No controlled reclaim, arbitrary write, chosen replacement page, callback influence, disclosure, LPE, or confirmed guest escape is shown. C:H/I:H are therefore not awarded. NEW52/B5-R does not apply because there is no retry-accumulated heap OOB write. B4 authorization review does not apply.

3. Reachability analysis

The explicit retry is performed by host userspace rerunning `KVM_RUN`; thus a local VMM process with access to the affected vCPU can complete that stage without host-administrator powers. Numeric host UID 0 is not required by the supplied evidence.

A malicious nested guest can influence nested state and APIC activity, but guest-only ability to force page acquisition failure and the userspace rerun is not established. The pure guest model is therefore unresolved; the proven model is principally a below-host-admin VMM/vCPU owner, or a compound guest-plus-VMM sequence. No namespace capability chain, special hardware, or hard timing race is shown.

4. Primitive and severity reading

The primitive is an active stale host-page translation: old PFNs survive without repinning and are consumed during later L2 execution after their backing pages may have been reallocated. This is a CLASS_B lifetime/ownership corruption, not a cleanup-only stale dereference and not fixed stale storage guaranteed merely to fault.

There is no repeated put/refcount operation and no CLASS_A controlled corruption. The shown consequence is stale host-memory access with plausible host DoS; useful host-memory corruption remains credible but incompletely characterized.

ImportantCandidate is YES because attacker-influenced nested state, a privileged host/hardware consumer, and a specific guest/VMM-to-host trust boundary form an unresolved active-state corruption chain. Auto-downgrade is disallowed because treating the issue as panic-only would discard that non-crash integrity candidate.

5. One-sentence report phrase

Failed nested-page repinning can leave stale, unpinned host PFNs active across KVM_RUN retries, allowing later L2 execution to access a reallocated host page as nested control state.

6. Manual review advice

MANUAL CHECK REQUIRED

Confirm whether guest-only actions can induce the failure/rerun sequence and characterize vAPIC or other nested-page writes into reallocated host pages.

7. Call-site confidence

Call-site confidence: medium

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

Patch: KVM: x86: Re-pend GET_NESTED_STATE_PAGES if getting said pages fails [ Upstream
Commit: 525694a6021ec6c8f7ce1f38acdf2dd77698fec1
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=525694a6021ec6c8f7ce1f38acdf2dd77698fec1

Changed files:
  arch/x86/kvm/x86.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=525694a6021ec6c8f7ce1f38acdf2dd77698fec1

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

The original Linux kernel CVE announcement for CVE-2026-98863 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98863

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

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).