[CVE-2026-98647][LOW] bpf: Fix divide-by-zero in btf_struct_walk() [ Upstream

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

CVE: CVE-2026-98647
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: bpf: Fix divide-by-zero in btf_struct_walk() [ Upstream
Commit: 1ea3820937f8022e044c41c6e95c84b02741b45f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1ea3820937f8022e044c41c6e95c84b02741b45f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98647
Analysis date: Mon, 19 Oct 2026 04:44:46 -0400
ActionableScore: 1
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
A zero-sized flexible-array element can make `btf_struct_walk()` perform a modulo operation with a zero divisor during BPF program verification, causing a kernel Oops and a possible host panic without establishing memory corruption or privilege escalation.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-98647 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98647

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.

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AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-98647	LOW	https://www.kernelcve.org/list/?q=CVE-2026-98647 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT LOW AEGIS_AS=0 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-369;*CWE-20;CWE-754;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'A divide by zero can occur in btf_struct_walk during BPF program verification when an access passes the end of a structure and its final flexible array member has a zero sized structure element. The verifier calculates an offset with a modulo operation using t size without first rejecting zero, which can generate a kernel Oops during BPF_PROG_LOAD and may cause a host panic when panic_on_oops is enabled. For the CVSS the PR:L value is used in the paranoid score because reliable triggering requires a local process that can load the required sleepable BPF program, and this access may be delegated to a container or restricted service account without granting full host administration. The base score uses PR:H because typical systems disable unprivileged BPF and restrict the relevant tracing program types through CAP_BPF, CAP_PERFMON, CAP_SYS_ADMIN, or equivalent privileges. The issue is not network reachable. Impact is denial of service only because the demonstrated primitive is a synchronous divide error and there is no supported UAF, OOB access, information disclosure, or arbitrary write primitive. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 5.5 and the production ActionableScore returned 1 with recommendation Ordinary Moderate. 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 CVSS score, vector or bug class threshold rule was applied. ActionableScore thresholds stage MODERATE_KPANIC 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 INCREASED_FROM_LOW_BASED_ON_HIGHCVSSSCORE and DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 and DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3. The independent weaker Aegis shadow evaluated CVSS 2.3 and ActionableScore 0 with primitive class CLASS_C and manual review NO. 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 1)	MAYBE	BPF UAF IMPROVEONLY LINUS  INCREASED_FROM_LOW_BASED_ON_HIGHCVSSSCORE DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw conservative score: 1
* Conservative score: 1
* Raw paranoid score: 1
* Paranoid score: 1
* No floor or cap changed either score.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Reliable kernel fault or DoS. A zero-sized flexible-array element reaches `(off - moff) % t->size`, producing the demonstrated divide-error Oops. A host panic is possible when `panic_on_oops` is enabled.
* No additional Availability point. The Oops and possible panic are the same failure mechanism.
* No corruption point. The patch establishes division by zero, not an OOB access, UAF, stale pointer, or write.
* No local-unprivileged point. Loading the required sleepable BPF program below host-admin privilege is not established.
* No privilege penalty. The complete privilege requirement is unresolved, so host-admin access is not assumed.
* No race penalty. The trigger is deterministic verifier input processing rather than a timing race.
* No remote-reachability point. The path is reached through local `BPF_PROG_LOAD`.
* NEW52 B5-R does not apply. There is no heap OOB write, retry-accumulated offset, or later consumption of corrupted structured state.
* NEW53 B3-PENDING does not apply. No shared-backing write exists.
* V33 and V34 do not apply. No attacker-influenced active-state corruption or checksum-metadata write exists.
* NEW54 B4-PENDING does not apply. No authorization decision or mandatory security guard is changed.
* Confidentiality, integrity, LPE, controlled reclaim, arbitrary write, and semantic reinterpretation signals are not established.

3. Reachability analysis

The trigger is a crafted BPF program and BTF description submitted through `BPF_PROG_LOAD`. Reaching the vulnerable verifier path additionally requires a non-`MEM_ALLOC` `PTR_TO_BTF_ID`, such as a local kptr read from a map by a sleepable program.

The interface is capability-gated on typical systems, and unprivileged BPF is commonly disabled. However, the supplied evidence does not resolve the exact combination of `CAP_BPF`, `CAP_PERFMON`, `CAP_SYS_ADMIN`, program-type checks, and product policy required for the complete trigger. Therefore `HostAdminRequired=UNKNOWN`.

Numeric host UID 0 is not intrinsically required because BPF authorization is capability-based, hence `RequiresHostUID0=NO`. The evidence does not show that an attacker-created user namespace grants all capabilities needed for loading the program and exercising this path, so `NamespaceScopedPrivilege=UNKNOWN`. No network, special hardware, retry loop, or race is required.

4. Primitive and severity interpretation

The exact primitive is a synchronous integer divide-by-zero exception in the BPF verifier. The verifier accepts a zero-sized structure as a flexible-array element and later uses its size as the modulo divisor.

This is `CLASS_C`: demonstrated crash or invalid-state behavior without memory corruption or an independently useful security primitive. It is neither an active structured-object UAF nor fixed stale storage. There is no repeated release, refcount mutation, OOB write, corrupted descriptor consumption, or authorization bypass.

The demonstrated consequence is a kernel Oops during program loading. Host-wide panic is configuration-dependent. Privilege escalation, confidentiality impact, integrity impact, and post-fault exploitation leverage are unsupported.

`ImportantCandidate=NO` because the bug has no Important-preserving corruption or boundary primitive. `AutoDowngradeAllowed=YES` because the established behavior is a local, privilege-gated verifier fault with no unresolved non-crash leverage.

5. One-sentence report phrase

A zero-sized flexible-array element can make `btf_struct_walk()` perform a modulo operation with a zero divisor during BPF program verification, causing a kernel Oops and a possible host panic without establishing memory corruption or privilege escalation.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

The patch establishes a deterministic divide-by-zero DoS path only, with no UAF, OOB write, authorization bypass, shared-backing corruption, or network exposure.

7. Call-site confidence

Call-site confidence: high

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

Patch: bpf: Fix divide-by-zero in btf_struct_walk() [ Upstream
Commit: 1ea3820937f8022e044c41c6e95c84b02741b45f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1ea3820937f8022e044c41c6e95c84b02741b45f

Changed files:
  kernel/bpf/btf.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=1ea3820937f8022e044c41c6e95c84b02741b45f

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

The original Linux kernel CVE announcement for CVE-2026-98647 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98647

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:H/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: unknown
  Paranoid ActionableScore: 1

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 alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).