[CVE-2026-98641][IMPORTANT] bpf, arm64: set up the frame pointer for the exception callback [ Upstream

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

CVE: CVE-2026-98641
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: bpf, arm64: set up the frame pointer for the exception callback [ Upstream
Commit: ecdd770a9fc3eaf6c3127c5991312b84774a7b0f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ecdd770a9fc3eaf6c3127c5991312b84774a7b0f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98641
Analysis date: Sat, 24 Oct 2026 12:17:39 -0400
ActionableScore: 6
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
ARM64 BPF exception callbacks can retain a dead subprogram frame pointer, allowing helper writes to overwrite active kernel return addresses and trigger a panic.

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

The original Linux kernel CVE announcement for CVE-2026-98641 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98641

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
======================================================================

CVE-2026-98641	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-98641 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT IMPORTANT AEGIS_AS=7 LEGACY=IMPORTANT RESULT=SAME (final reconciliation agrees; DECISION: both IMPORTANT)	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-787;CWE-121;CWE-670;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'The ARM64 BPF JIT can leave BPF_REG_FP pointing into a dead subprogram frame when an exception callback starts after bpf_throw. If the callback materializes the address of a local variable and passes it to a helper, the helper can write into its own active kernel call stack and overwrite saved return addresses. The demonstrated value 0x1234 reached both PC and LR and caused a kernel panic, which establishes a controlled kernel stack corruption primitive rather than a crash only condition. For the CVSS the PR:L value reflects that an attacker must already be able to load and execute a crafted BPF program, while full administrative control is not inherently required by the vulnerable JIT path. The base impact is limited to C:L and I:L because reliable code execution is not demonstrated, while the paranoid C:H and I:H values account for the plausible privilege escalation path created by a controlled return address overwrite. The issue is local and is not directly network reachable. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 7 and the production ActionableScore returned 6 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 LOW to MODERATE_KPANIC because the manual review requirement was applied. ActionableScore thresholds stage MODERATE_KPANIC to IMPORTANT because the ActionableScore threshold rule was applied. 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_FROM_LOW_BASED_ON_REQUIREMANUALCHECK and INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHERTHAN5. The independent weaker Aegis shadow evaluated CVSS 6.1 and ActionableScore 7 with primitive class CLASS_A and manual review REQUIRED. Its independently reconciled impact was IMPORTANT. The shadow agreed with the final production impact and aggregation did not change that impact. 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 6)	MAYBE	OOB BPF HARDWARE LINUS  KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHERTHAN5	-	-	checked

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

ActionableScore=6
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 6.
* Conservative score: 6; no floor or cap applied.
* Raw paranoid score: 6.
* Paranoid score: 6; stronger effects would require unsupported LPE or code-execution assumptions.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

* +2 strong corruption primitive: the stale frame pointer directs a helper write into its active kernel call stack.
* +2 strong exploit leverage: attacker-selected helper data demonstrably overwrites saved return state; `0x1234` reached both PC and LR.
* +1 reliable kernel crash: the supplied trace demonstrates a fatal exception and kernel panic.
* +1 integrity impact: active kernel control-flow state was actually modified, rather than merely read or causing an unrelated fault.
* No privilege penalty: host-admin-equivalent privilege is not established, but neither is a complete below-host-admin loading and execution path.
* No remote or ordinary-unprivileged bonus; no arbitrary write, reliable code execution, confidentiality impact, UAF, or LPE is shown.
* No extra availability point: it would duplicate the demonstrated panic.
* B5-R and B3-PENDING do not apply: this is neither retry-accumulated heap OOB nor shared-backing corruption.

3. Reachability analysis

A crafted ARM64 JIT-compiled BPF program must use an exception boundary, throw from a stack-using subprogram, and have the callback pass a local address to a writing helper. The evidence demonstrates this through `bpf_test_run`, but does not establish the complete BPF load/JIT/test-run privilege checks for an ordinary user or user namespace. Accordingly, host-admin and namespace-scoped reachability remain unknown. No network reachability, special hardware, race, or retry sequence is required.

4. Primitive and severity reading

The primitive is a controlled write into an active kernel call stack through an incorrectly retained BPF frame pointer. It is not a UAF or cleanup-only stale dereference. Saved PC/LR corruption is proven, making this CLASS_A and materially stronger than crash-only behavior. The shown result is panic; redirecting execution for LPE remains unproven. ImportantCandidate is YES and auto-downgrade is prohibited because a controlled return-state overwrite requires exploitability review.

There is no authorization decision bypass, protected-resource access, or B4 authorization-state issue.

5. One-sentence report phrase

ARM64 BPF exception callbacks can retain a dead subprogram frame pointer, allowing helper writes to overwrite active kernel return addresses and trigger a panic.

6. Manual review advice

MANUAL CHECK REQUIRED

The demonstrated attacker-influenced return-address overwrite is a strong kernel-corruption primitive despite unresolved BPF privilege scope and absent proven LPE.

7. Call-site confidence

Call-site confidence: high

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

Patch: bpf, arm64: set up the frame pointer for the exception callback [ Upstream
Commit: ecdd770a9fc3eaf6c3127c5991312b84774a7b0f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ecdd770a9fc3eaf6c3127c5991312b84774a7b0f

Changed files:
  arch/arm64/net/bpf_jit_comp.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=ecdd770a9fc3eaf6c3127c5991312b84774a7b0f

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

The original Linux kernel CVE announcement for CVE-2026-98641 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98641

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: unknown
  Paranoid ActionableScore: 6

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: YES
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).