[CVE-2026-98628][LOW] bpf: Make post-verification instruction rewrites killable [ Upstream

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

CVE: CVE-2026-98628
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: bpf: Make post-verification instruction rewrites killable [ Upstream
Commit: 56dc616d7a1f7d134d3e101c455e62b0a24d0cfd
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=56dc616d7a1f7d134d3e101c455e62b0a24d0cfd
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98628
Analysis date: Sun, 18 Oct 2026 22:25:07 -0400
ActionableScore: 1
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
A privileged BPF loader can submit a maximum-length program that causes quadratic post-verification rewriting, monopolizing a CPU and preventing prompt SIGKILL handling until the rewrite completes.

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

The original Linux kernel CVE announcement for CVE-2026-98628 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98628

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-2026-98628	LOW	https://www.kernelcve.org/list/?q=CVE-2026-98628 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-400;CWE-834;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'Post-verification BPF instruction rewrites can perform quadratic work because individual patch or removal operations repeatedly move the remaining instruction and auxiliary data arrays and adjust branch offsets. A privileged loader can submit a maximum length program containing many removable jumps and cause the rewrite stage to monopolize a CPU for a long period. A pending SIGKILL cannot terminate the loader until this processing finishes because the affected paths do not check for fatal signals or provide rescheduling points. For the CVSS the PR:L value in the paranoid score reflects environments where BPF loading is delegated to a reduced capability process, service account, or container root rather than a fully privileged host administrator. The issue is local and is not directly network reachable. Impact is denial of service through CPU starvation and delayed task termination. There is no supported UAF, OOB access, information disclosure, or 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 2.4 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	INIT BPF NOMEMCHK IMPROVEONLY LINUS  INCREASED_FROM_LOW_BASED_ON_HIGHCVSSSCORE DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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

ActionableScore=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES
RequiresHostUID0=UNKNOWN
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 strong DoS: a maximum-length BPF program can deterministically cause quadratic post-verification work and monopolize a CPU for a long period while delaying SIGKILL.
* No additional Availability point: delayed termination and CPU starvation are the same demonstrated consequence.
* No privilege penalty: the commit establishes a privileged BPF loader but does not establish whether effective host-admin authority or a reduced delegated capability is required.
* No corruption signal: array movement and branch adjustment are expensive but remain intentional operations. No OOB access, UAF, stale reference, arbitrary write, or invalid lifetime mutation is shown.
* No local-unprivileged bonus: complete BPF_PROG_LOAD reachability for an ordinary user is not established.
* No remote bonus: the trigger is a local BPF control-plane operation.
* NEW52 B5-R does not apply because there is no retry-accumulated heap OOB write or later consumption of corrupted state.
* NEW53, V32, V33, and V34 do not apply because no skb, checksum-placement, or shared-backing write exists.
* NEW54 B4-PENDING does not apply because no authorization decision changes from DENY to ALLOW.

3. Reachability analysis

The trigger is BPF_PROG_LOAD with a program containing up to 131072 removable unconditional jumps followed by a valid return. Loading is explicitly described as privileged, but the supplied evidence does not resolve whether the complete operation requires host-admin-equivalent authority or can be performed using delegated CAP_BPF or another reduced capability. It also does not establish that the complete operation is available from an attacker-created user namespace.

No program attachment, netdevice control, network traffic, special hardware, race, or retry timing is required for the documented path. The workload is deterministic once an authorized loader can submit the program. Root-controlled BPF policy or sysctl settings may affect exposure, but their exact requirements are not supplied.

4. Primitive and severity interpretation

The exact primitive is attacker-selected algorithmic complexity in post-verification instruction rewriting. Removing instructions individually causes repeated movement of the remaining instruction and auxiliary-data arrays and repeated branch-offset adjustment, producing quadratic work without signal checks or rescheduling points.

This is CLASS_C denial-of-service behavior, not memory corruption. The demonstrated consequences are prolonged CPU monopolization and delayed termination of the loader. Broader host starvation from concurrent loaders is plausible but not independently demonstrated and does not add another point. There is no evidence of confidentiality loss, integrity modification, privilege escalation, authorization bypass, or an Important-preserving primitive.

ImportantCandidate is NO because no unresolved corruption or security-boundary mechanism exists. AutoDowngradeAllowed is YES because the established issue is a privilege-gated, local, algorithmic DoS with no useful primitive beyond resource starvation.

5. One-sentence report phrase

A privileged BPF loader can submit a maximum-length program that causes quadratic post-verification rewriting, monopolizing a CPU and preventing prompt SIGKILL handling until the rewrite completes.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

The patch establishes only local algorithmic DoS through privileged BPF loading, with no memory-corruption, authorization, cross-boundary, or remote primitive.

7. Call-site confidence

Call-site confidence: high

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

Patch: bpf: Make post-verification instruction rewrites killable [ Upstream
Commit: 56dc616d7a1f7d134d3e101c455e62b0a24d0cfd
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=56dc616d7a1f7d134d3e101c455e62b0a24d0cfd

Changed files:
  kernel/bpf/core.c
  kernel/bpf/fixups.c
  net/xdp.h

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=56dc616d7a1f7d134d3e101c455e62b0a24d0cfd

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

The original Linux kernel CVE announcement for CVE-2026-98628 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98628

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: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: 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).