[CVE-2026-98649][MODERATE REGULAR] KVM: s390: Fix race in _destroy_pages_crste() [ Upstream

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

CVE: CVE-2026-98649
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: KVM: s390: Fix race in _destroy_pages_crste() [ Upstream
Commit: 90bd832b2bc367991d424d847003ce345b454ffc
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=90bd832b2bc367991d424d847003ce345b454ffc
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98649
Analysis date: Mon, 19 Oct 2026 03:14:24 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
A race in KVM s390 page teardown can combine CRSTE flags and an origin from different entry versions, potentially directing destructive processing toward an unintended page range, but an actual UAF, guest-only trigger, or guest-to-host corruption primitive is not established.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-98649 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98649

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-2026-98649	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98649 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:H/PR:L/UI:N/S:U/C:H/I:H/A:H';CWE-362;CWE-367;*CWE-416;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A race in KVM s390 page teardown can make _destroy_pages_crste() validate flags from one CRSTE value and calculate the page table origin from another value. This inconsistent snapshot may cause page destruction to operate on an unintended backing range, resulting at least in a KVM or host kernel crash and potentially creating stale page references or a use after free condition. For the CVSS the PR:L value is used because reliable triggering requires access to a KVM guest or a local process that can drive the affected virtual machine state, but full host administrative privileges are not clearly required. AC:H is used because the vulnerable reads must overlap a concurrent CRSTE update. The issue is not directly network reachable. Impact is at least a local denial of service via kernel crash and in the worst case may allow confidentiality or integrity impact through incorrect page teardown, so the memory corruption potential should be reviewed manually. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 7 and the production ActionableScore returned 3 with recommendation Borderline / manual review recommended. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were ActionableScore thresholds stage MODERATE_KPANIC to MODERATE because the ActionableScore threshold 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_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 and V41_INHERITED_KPANIC_PROMOTION_BLOCKED and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 2.7 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 Borderline / manual review recommended (with actual score 3)	MAYBE	VIRT SIMPLEFIX RACE HARDWARE LINUS MEMORY  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 V41_INHERITED_KPANIC_PROMOTION_BLOCKED MANUAL_REVIEW_REQUIRED	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=3
ActionableScoreLower=2
PrimitiveClass=CLASS_B
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: 2.
* Conservative score: 2. No floor or cap applied.
* Raw paranoid score: 3.
* Paranoid score: 3. No floor or cap applied.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

Conservative positive signals:

* +1 weak or indirect corruption candidate. Separate reads can combine CRSTE flags from one version with an origin from another version.
* +1 active KVM translation and page-lifetime state. The inconsistent origin is consumed by a destructive page traversal rather than used only for diagnostics.

Paranoid additional signal:

* +1 plausible host kernel crash or strong DoS. Processing an unintended page range during teardown can plausibly fault or damage KVM mapping state, although the supplied patch does not demonstrate an actual panic.

Negative signals:

* No negative privilege score. Host-admin-equivalent privilege is not established.
* No race penalty. The commit identifies a race but does not establish that the timing window is narrow, one-shot, or difficult to repeat.
* No subsystem rarity penalty. s390 is architecture-specific, but the supplied evidence does not establish a rare hardware prerequisite relevant to an existing s390 KVM deployment.

Material non-awarded signals:

* Real UAF is not established. CWE-416 remains only a possible downstream consequence.
* No controlled reclaim, OOB write, arbitrary write, repeated release, refcount mutation, or callback influence is shown.
* No confidentiality or integrity point is awarded merely because incorrect teardown could theoretically produce stale references.
* No guest-to-host leverage point is awarded. The evidence does not show that guest code alone controls both the CRSTE update and the later teardown traversal.
* No local-unprivileged point is awarded because below-host-admin completion of the trigger is unresolved.
* No extra Availability point is awarded for the same possible crash.
* NEW52 B5-R does not apply because no confirmed retry-accumulated heap OOB write exists.
* NEW53 B3-PENDING does not apply because no shared-backing write is established.
* V33 does not apply because attacker-influenced active-state corruption and a credible complete trust-boundary trigger are not both established.
* B4-PENDING does not apply because there is no authorization DENY-to-ALLOW change.

3. Reachability analysis

The evaluated attacker model is unresolved and potentially compound. A malicious guest may influence guest translation activity, but the patch does not show that guest code can directly replace the host CRSTE while also invoking the destructive walk. A local VMM process with access to the VM may have broader control, but the necessary ioctl or lifecycle sequence is not supplied. Host-administrator involvement is therefore neither proven nor excluded.

No namespace-scoped capability path is identified. No network trigger exists. No prior host setup beyond an active s390 KVM VM can be established from the supplied context. Race repeatability is unknown because the concurrent CRSTE writer and callers of the teardown path are not shown.

4. Primitive and severity interpretation

The exact primitive is a time-of-check to time-of-use style inconsistent snapshot of an active CRSTE. The vulnerable code can validate flags from one entry version and derive the page-table origin from another version. That origin is then used by page teardown.

This is active structured translation state, not fixed stale storage. It is not a demonstrated UAF and no concrete repeated release or refcount operation exists. The first lifetime-ending operation, an exact surviving dangling reference, and a second lifetime mutation are all unproven.

CLASS_B is used conservatively because the mismatched active entry can direct destructive processing toward an unintended backing range. The demonstrated root cause is stronger than a passive bookkeeping error, but the supplied evidence does not establish a specific corrupted page, stale post-lifetime use, or guest-controlled host-memory primitive.

The patch-supported consequence is inconsistent KVM page teardown. A host crash or stale-page condition is plausible but not demonstrated. Privilege escalation, disclosure, and guest escape remain speculative.

ImportantCandidate is NO because meaningful exploitation leverage beyond invalid page selection is not established. AutoDowngradeAllowed is YES because no concrete UAF, controlled corruption, authorization bypass, or confirmed guest-to-host boundary consequence remains. Operationally this is a borderline Moderate KVM race requiring source review rather than an Important-preserving primitive.

5. One-sentence report phrase

A race in KVM s390 page teardown can combine CRSTE flags and an origin from different entry versions, potentially directing destructive processing toward an unintended page range, but an actual UAF, guest-only trigger, or guest-to-host corruption primitive is not established.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Review the concurrent CRSTE writer, callers of `_destroy_pages_crste()`, and the operation performed for each calculated page to determine whether the mismatch can release or modify unrelated host pages and whether a guest or below-host-admin VMM can drive the complete race.

7. Call-site confidence

Call-site confidence: medium

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

Patch: KVM: s390: Fix race in _destroy_pages_crste() [ Upstream
Commit: 90bd832b2bc367991d424d847003ce345b454ffc
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=90bd832b2bc367991d424d847003ce345b454ffc

Changed files:
  arch/s390/kvm/gmap.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=90bd832b2bc367991d424d847003ce345b454ffc

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

The original Linux kernel CVE announcement for CVE-2026-98649 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98649

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: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: 2
  Paranoid ActionableScore: 3

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).