* [CVE-2026-98653][LOW] KVM: s390: Fix _gaccess_shadow_fault() [ Upstream
@ 2026-10-24 8:33 21% AL-KERNEL
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From: AL-KERNEL @ 2026-10-24 8:33 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-98653
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: KVM: s390: Fix _gaccess_shadow_fault() [ Upstream
Commit: 94c43c7df46433ed4d1e18f13770b019ba977bd8
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=94c43c7df46433ed4d1e18f13770b019ba977bd8
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98653
Analysis date: Sat, 24 Oct 2026 04:33:50 -0400
ActionableScore: 0
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO
Summary:
A missing parent-gmap mapping can incorrectly propagate `-ENOENT` to the s390 KVM VMM and disrupt the affected nested VM without evidence of host memory corruption or security-boundary bypass.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98653 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98653
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: LOW
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-2026-98653 LOW https://www.kernelcve.org/list/?q=CVE-2026-98653 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT LOW AEGIS_AS=1 LEGACY=LOW RESULT=SAME (final reconciliation agrees; DECISION: both LOW) 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-755;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'A logic error in KVM s390 nested guest address translation can propagate ENOENT to userspace when the nested guest page being shadowed has no leaf mapping in the parent guest gmap. This can interrupt execution of the affected virtual machine and may terminate it if the userspace VMM treats the KVM error as fatal. For the CVSS the PR:L value reflects the need to execute or control a guest context using nested virtualization rather than merely send network traffic. No invalid memory access, stale rmap, UAF, OOB operation, information leak, or write primitive is established by the changed path. The issue is not directly network reachable and requires KVM s390 nested virtualization plus the missing parent gmap mapping condition. Automated impact decision path. The original SimpleNet classifier selected MODERATE. The primary LLM estimated paranoid CVSS 5.5 and the production ActionableScore returned 0 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were ActionableScore thresholds stage MODERATE to LOW 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_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3. The independent weaker Aegis shadow evaluated CVSS 3.0 and ActionableScore 1 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was LOW. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is LOW.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 0) MAYBE DANGER VIRT SIMPLEFIX HARDWARE LINUS DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=0
PrimitiveClass=NONE
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO
1. ActionableScore
* Raw base score: 0.
* Conservative score: 0; no floor or cap applied.
* Raw paranoid score: 0.
* Paranoid score: 0; no additional technically supported primitive.
* Final recommended bucket: **Ordinary Moderate**::
2. Signal breakdown
* +1 Availability impact: `_gaccess_do_shadow()` can return `-ENOENT` for an absent parent-gmap leaf, and that error is propagated to userspace, potentially interrupting or terminating the affected VM.
* -1 Rare and difficult-to-reach configuration: the path requires KVM on s390, nested virtualization, and a missing parent guest gmap mapping; the guest cannot independently provision the complete host-side environment.
* No memory-corruption points: the patch shows no OOB access, UAF, stale rmap, invalid pointer, or write outside intended state.
* No crash point: host kernel panic or host-wide DoS is not shown.
* No authorization or integrity point: `gmap_link()` restores a missing translation link; it does not restore an omitted security check.
* NEW52/B5-R does not apply: there is retry after successful linking, but no accumulated index and no heap OOB write or corrupted-state consumption.
* No guest-to-host corrupt leverage, disclosure, arbitrary write, or privilege escalation is established.
3. Reachability analysis
A nested guest can plausibly create the missing-mapping condition while executing nested memory accesses. Host/VMM setup must first provide s390 KVM nested virtualization, but the supplied evidence supports a guest-context trigger rather than effective host-administrator control; therefore `HostAdminRequired=NO`. No namespace capability is involved. This is not network reachable, and no race is shown.
4. Primitive and severity reading
The defect is incorrect error handling for a missing parent-gmap leaf. The fix links the nested guest memory into the parent gmap and retries the shadow operation. There is no stale object, active-state corruption, authorization bypass, or memory-write primitive. The supported consequence is VM-local interruption or termination if userspace treats the KVM error as fatal; stronger host effects are unproven. Thus the primitive class is NONE, Important review is unwarranted, and automatic Moderate downgrade is allowed.
5. One-sentence report phrase
A missing parent-gmap mapping can incorrectly propagate `-ENOENT` to the s390 KVM VMM and disrupt the affected nested VM without evidence of host memory corruption or security-boundary bypass.
6. Manual review advice
NO MANUAL CHECK NEEDED
The patch establishes a constrained translation/error-propagation bug with VM availability impact only.
7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: KVM: s390: Fix _gaccess_shadow_fault() [ Upstream
Commit: 94c43c7df46433ed4d1e18f13770b019ba977bd8
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=94c43c7df46433ed4d1e18f13770b019ba977bd8
Changed files:
arch/s390/kvm/gaccess.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=94c43c7df46433ed4d1e18f13770b019ba977bd8
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98653 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98653
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:L/UI:N/S:U/C:N/I:N/A:L
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: unknown
Paranoid ActionableScore: 0
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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW
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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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