[CVE-2026-98651][MODERATE REGULAR] KVM: s390: Properly handle NULL pointer in dat_cond_set_storage_key() [ Upstream

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

CVE: CVE-2026-98651
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: KVM: s390: Properly handle NULL pointer in dat_cond_set_storage_key() [ Upstream
Commit: 75867536746ab2d034488a6a93ee6cde83553342
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=75867536746ab2d034488a6a93ee6cde83553342
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98651
Analysis date: Mon, 19 Oct 2026 03:49:52 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
A NULL oldkey pointer can reach page_cond_set_storage_key on the KVM s390 large mapping path and crash the host kernel, while no UAF, controlled memory corruption, information disclosure, or privilege escalation primitive is established.

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

The original Linux kernel CVE announcement for CVE-2026-98651 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98651

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-98651	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98651 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 LEGACY=MODERATE RESULT=DIFF (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:L/PR:L/UI:N/S:U/C:N/I:N/A:H';**CWE-476;CWE-754;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'A NULL pointer dereference can occur in KVM s390 when dat_cond_set_storage_key passes a NULL oldkey pointer to page_cond_set_storage_key on the large mapping path. A guest that can reach the affected storage key operation may cause the host kernel to dereference NULL and crash, resulting in denial of service across the guest host isolation boundary. For the CVSS the PR:L value is used because control of a guest workload or guest privileged operation is required, while host administrator privileges are not required. The attack vector is local because the vulnerable interface is exposed through KVM rather than a network protocol. The realistic trigger has high complexity because it depends on the s390 KVM configuration, the relevant large mapping path, and a caller that supplies NULL as oldkey. No UAF, OOB access, information disclosure, or controllable memory corruption primitive is demonstrated, so the supported impact is host availability loss only. Automated impact decision path. The original SimpleNet classifier selected MODERATE. The primary LLM estimated paranoid CVSS 5.5 and the production ActionableScore returned 2 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. No severity changing production threshold or specialist override was recorded. 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 V41_INHERITED_KPANIC_PROMOTION_BLOCKED and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 4.8 and ActionableScore 1 with primitive class CLASS_C and manual review RECOMMENDED. Its independently reconciled impact was MODERATE_KPANIC. The models disagreed but the aggregation rules retained the production result. The final reported impact is MODERATE.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)	MAYBE	DANGER VIRT SIMPLEFIX HARDWARE  V41_INHERITED_KPANIC_PROMOTION_BLOCKED MANUAL_REVIEW_REQUIRED	-	-	checked

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

ActionableScore=2
ActionableScoreLower=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
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: 2
* Paranoid score: 2
* No score floor or cap was applied.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

Conservative signals:

* +1 Reliable kernel crash or strong DoS. A NULL `oldkey` reaches `page_cond_set_storage_key()`, which dereferences it and can crash the host kernel.
* Total: 1.

Paranoid additional signal:

* +1 Credible below-host-admin guest trigger. KVM s390 storage-key handling is plausibly reachable through guest operations, but the relevant callers are not included, so this is retained only in the paranoid score.
* Total: 2.

Not awarded:

* No UAF, OOB access, stale object, refcount corruption, or repeated release is established.
* No controlled write exists. The helper writes through a fixed NULL output pointer rather than an attacker-selected destination.
* No confidentiality, integrity, authorization-bypass, or privilege-escalation primitive is established.
* No strong guest-to-host corruption bonus is awarded. The demonstrated primitive is a host NULL dereference and possible host DoS.
* No additional availability point is awarded because it would double-count the same crash.
* No remote-reachability point is awarded.
* No hardware or niche-subsystem penalty is applied merely because this is s390-specific.

3. Reachability analysis

The vulnerable branch is entered when `ptep` is absent, indicating the large-CRSTE mapping path, and a caller has supplied `oldkey == NULL`. The commit establishes that such callers exist but does not identify them.

A malicious guest may plausibly reach the path through an s390 storage-key operation. However, the supplied code does not prove whether the complete trigger is guest-only, requires a host VMM operation, or requires host-side KVM configuration beyond creation of the VM. A local VMM process with `/dev/kvm` access is a separate attacker role and must not be merged with guest capabilities.

An existing s390 KVM virtual machine and the relevant large mapping state are prerequisites. No namespace capability is involved, no direct network trigger is established, and no timing race is required. Because complete caller and privilege information is missing, `HostAdminRequired` and `NamespaceScopedPrivilege` remain unknown.

4. Primitive and severity interpretation

The exact primitive is a NULL-pointer dereference caused by passing a NULL output pointer to `page_cond_set_storage_key()`. The affected value is not stale storage and no object lifetime has ended. This is therefore `CLASS_C`, not a UAF or active-state corruption case.

The demonstrated consequence is a kernel fault and possible host denial of service. A guest-triggered host crash is credible but not fully established from the supplied call sites. Arbitrary host memory access, controlled corruption, VM escape, information disclosure, and privilege escalation are not supported.

`ImportantCandidate=NO` because there is no independently useful corruption or boundary-bypass primitive beyond the crash. `AutoDowngradeAllowed=YES` because the defect is a constrained NULL dereference with no unresolved Class A or strong Class B mechanism.

The B4 authorization profile does not apply. No security check changes from DENY to ALLOW, and no protected resource is exposed.

5. One-sentence report phrase

A NULL oldkey pointer can reach page_cond_set_storage_key on the KVM s390 large mapping path and crash the host kernel, while no UAF, controlled memory corruption, information disclosure, or privilege escalation primitive is established.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Confirm the NULL-passing callers and determine whether malicious guest code alone can reliably reach the vulnerable large-mapping path or whether host VMM participation is required.

7. Call-site confidence

Call-site confidence: low

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

Patch: KVM: s390: Properly handle NULL pointer in dat_cond_set_storage_key() [ Upstream
Commit: 75867536746ab2d034488a6a93ee6cde83553342
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=75867536746ab2d034488a6a93ee6cde83553342

Changed files:
  arch/s390/kvm/dat.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=75867536746ab2d034488a6a93ee6cde83553342

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

The original Linux kernel CVE announcement for CVE-2026-98651 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98651

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: 1
  Paranoid ActionableScore: 2

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).