[CVE-2026-98745][MODERATE REGULAR] arm64/fpsimd: signal: Forbid non-streaming SVE payload on SME-only systems [ Upstream

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

CVE: CVE-2026-98745
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: arm64/fpsimd: signal: Forbid non-streaming SVE payload on SME-only systems [ Upstream
Commit: f2338929c7663859c0c74dd6fef2c0245066f845
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f2338929c7663859c0c74dd6fef2c0245066f845
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98745
Analysis date: Sat, 24 Oct 2026 10:39:06 -0400
ActionableScore: 4
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate
Manual review required: YES

Summary:
A local unprivileged arm64 process can install unsupported SVE task state through a crafted signal frame and cause later privileged FPSIMD restoration to trigger a demonstrated kernel panic.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-98745 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98745

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-2026-98745	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98745 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-20;CWE-754;CWE-682;BEST CVSS score: '5.5';DESCR 'A local process can supply a malformed SVE signal context on an SME-only arm64 system with SVE_SIG_FLAG_SM clear while an SVE payload is present. The kernel can then record unsupported non-streaming SVE state for the task, and later FPSIMD or SVE state restoration can execute an unsupported ZCR_EL1 access and cause a fatal EL1 UNDEF and kernel panic. For the CVSS the PR:L value is selected because reliable triggering requires local user code able to construct and restore its own signal frame. The issue is not network reachable, requires the uncommon SME supported and SVE unsupported hardware configuration, and provides a demonstrated denial of service primitive without evidence of UAF, OOB access, information disclosure, arbitrary write, or privilege escalation. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 5.5 and the production ActionableScore returned 4 with recommendation Actionable Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE_KPANIC to MODERATE because the CVSS score, vector or bug class 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_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH and V41_INHERITED_KPANIC_PROMOTION_BLOCKED and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 5.5 and ActionableScore 1 with primitive class CLASS_C and manual review REQUIRED. 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 Actionable Moderate (with actual score 4)	MAYBE	DANGER HARDWARE  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH V41_INHERITED_KPANIC_PROMOTION_BLOCKED MANUAL_REVIEW_REQUIRED	-	-	checked

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

ActionableScore=4
PrimitiveClass=CLASS_B
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
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: 4.
* Paranoid score: 4.
* Final recommended bucket: **Actionable Moderate**::

2. Signal breakdown

* +1 local unprivileged trigger: an ordinary arm64 process can craft and restore its own signal frame; UID 0 or capabilities are not required by this interface.
* +1 reliable kernel crash/strong DoS: the supplied trace demonstrates an EL1 undefined instruction followed by kernel panic.
* +1 attacker-influenced active structured-state corruption: the malformed context installs unsupported non-streaming SVE state into the task’s live kernel-managed FPSIMD/SVE state.
* +1 privileged downstream consumption across the user/kernel boundary: later kernel state restoration consumes that invalid state and accesses ZCR_EL1, producing the fatal EL1 exception. Other stated consequences remain unspecified.
* No hardware/configuration penalty: SME-without-SVE is unusual, but the evidence includes a virtual platform and does not establish rare physical hardware or an attacker-inaccessible subsystem.

Not awarded: UAF, heap/OOB write, arbitrary write, disclosure, authorization bypass, LPE, or protected-state integrity impact. B5-R and B3-PENDING do not apply. The demonstrated crash receives no duplicate availability point.

3. Reachability analysis

The trigger is local and available through signal-frame restoration by the affected process itself. It requires an arm64 system exposing SME without SVE, but no host-admin privilege, namespace capability, prior privileged setup, special race, or separate vulnerability is shown. It is not remotely reachable.

4. Primitive and severity reading

The primitive is acceptance of an attacker-crafted signal representation that creates unsupported active architectural state, followed by privileged kernel consumption during FPSIMD/SVE restoration. This is CLASS_B active-state corruption, not kernel-memory corruption or a UAF. Host panic is demonstrated; non-crash integrity or privilege effects are unknown rather than proven.

ImportantCandidate is YES under the active-state/trust-boundary rule because attacker-influenced live state reaches a specific privileged consumer and the full consequences of subsequent state manipulation are unresolved. Auto-downgrade is therefore disallowed, although the patch does not prove Important severity, arbitrary access, or privilege escalation.

5. One-sentence report phrase

A local unprivileged arm64 process can install unsupported SVE task state through a crafted signal frame and cause later privileged FPSIMD restoration to trigger a demonstrated kernel panic.

6. Manual review advice

MANUAL CHECK REQUIRED

Review the unresolved consequences of privileged kernel manipulation of the unsupported active task state; the proven impact is host DoS, not memory corruption or LPE.

7. Call-site confidence

Call-site confidence: high

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

Patch: arm64/fpsimd: signal: Forbid non-streaming SVE payload on SME-only systems [ Upstream
Commit: f2338929c7663859c0c74dd6fef2c0245066f845
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f2338929c7663859c0c74dd6fef2c0245066f845

Changed files:
  arch/arm64/kernel/signal.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=f2338929c7663859c0c74dd6fef2c0245066f845

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

The original Linux kernel CVE announcement for CVE-2026-98745 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98745

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: Not available
  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: 4

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).