[CVE-2026-98039][IMPORTANT] bpf: Require MEM_PERCPU for percpu kptr stores [ Upstream

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

CVE: CVE-2026-98039
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: bpf: Require MEM_PERCPU for percpu kptr stores [ Upstream
Commit: aaa9cf7707d1c5c0d2ca9d40c8b71652ac1c3c3f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=aaa9cf7707d1c5c0d2ca9d40c8b71652ac1c3c3f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98039
Analysis date: Sat, 26 Sep 2026 16:30:00 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-98039 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98039

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-2026-98039	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-98039 CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H';CWE-843;CWE-284;*CWE-787;BEST CVSS score: '7.8';DESCR 'A BPF verifier type validation flaw allows a non per CPU pointer to be stored in a BPF_KPTR_PERCPU map field without the required MEM_PERCPU register flag. The value is later treated as a per CPU object, which can result in invalid pointer relocation, an incorrect destruction path, and arbitrary kernel read and write through a plain bpf_obj_new allocation. For the CVSS the PR:L because exploitation requires local access and permission to load and use the relevant BPF program and map features, such as CAP_BPF or delegated equivalent access. The issue is not directly network reachable. Impact can include confidentiality and integrity compromise, kernel crash, and local privilege escalation. YES REQUIRES MANUAL CHECK';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6)	YES	OOB BPF SIMPLEFIX HARDWARE LINUS TEST  KPANIC INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHERTHAN5	YES	NO	checked

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

ActionableScore=6
ActionableScoreLower=5
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 5
* Conservative score: 5
* Raw paranoid score: 6
* Paranoid score: 6
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

* +2 Strong corruption primitive. The commit explicitly states that storing a plain `bpf_obj_new()` allocation in a `BPF_KPTR_PERCPU` field can provide arbitrary kernel read and write.
* +2 Strong attacker-influenced semantic reinterpretation. The verifier accepts a non per CPU pointer, but later loads and consumers interpret it as a per CPU allocation cookie. This changes pointer relocation and destruction semantics.
* +1 Confidentiality impact is plausible. The explicitly stated arbitrary kernel read capability can expose kernel-resident information.
* +1 Integrity impact is plausible. The explicitly stated arbitrary kernel write capability can modify kernel-resident state.
* -1 Reduced privileged API requirement. Reaching the relevant BPF kptr allocation and verifier path normally requires permission to load privileged BPF programs, such as `CAP_BPF` or an equivalent delegated privilege.

Paranoid-only:

* +1 Additional unresolved lifetime and ownership impact. The separate kernel-pointer variant can be per CPU relocated to an invalid address or processed through a missing destructor path. This is not needed to establish the arbitrary kernel read and write primitive, but it supports a higher review priority because the verifier confusion affects both memory access and object ownership handling.

Important non-awarded signals:

* No remote reachability point. The patch describes a local BPF verifier and map operation, not a network-reachable path.
* No Local unprivileged trigger point. The supplied evidence does not establish that an ordinary user without BPF-related privilege can load the required program and complete the full sequence.
* No UAF signal. The defect is a verifier type and semantic confusion, not a demonstrated stale post-free object use.
* No controlled reclaim point. Arbitrary kernel read and write are directly described, so controlled heap reclaim is not necessary for the classification.
* No separate practical LPE-chain point. Privilege escalation is highly plausible from arbitrary kernel read and write, but a complete credential or control-flow exploit chain is not supplied and is not counted separately.
* No reliability or availability point. Invalid relocation can cause faults, but the central demonstrated security primitive is arbitrary read and write, and an additional DoS score would not materially change triage.

3. Reachability analysis

The evaluated attacker is a local process capable of loading a BPF program that uses local kptr allocation and a map containing a `BPF_KPTR_PERCPU` field. The vulnerability is triggered during verifier acceptance of an invalid kptr store and later use of the stored value by per CPU pointer helpers or map teardown.

The relevant BPF operations are normally privilege-gated. Modern Linux systems commonly require `CAP_BPF`, `CAP_SYS_ADMIN`, or another privileged BPF policy path for the required program load and map usage. However, the supplied patch does not establish whether the complete load, map creation, BTF setup, program attach or execution, and trigger sequence requires host-admin-equivalent privilege in the evaluated deployment.

Therefore, `HostAdminRequired=UNKNOWN` is appropriate. The presence of potentially delegated BPF permissions does not prove that the entire sequence is reachable from an attacker-created namespace. `NamespaceScopedPrivilege=UNKNOWN` is also appropriate.

No special hardware, firmware, boot option, or network listener is indicated. The issue does not appear to depend on a race. The attacker controls the BPF program representation and the invalid store type relationship accepted by the verifier.

4. Primitive and severity interpretation

The exact primitive is a verifier-enforced type confusion and semantic reinterpretation involving `BPF_KPTR_PERCPU`. A non per CPU pointer can be accepted for storage in a per CPU kptr field because the old subset test permits `MEM_PERCPU` as an allowed destination flag without requiring that flag on the source register.

When the field is loaded, it is always marked as `MEM_PERCPU`. Subsequent helpers treat the value as a per CPU object allocation cookie. This can relocate a normal pointer as if it were per CPU memory. The commit explicitly states that a plain `bpf_obj_new()` allocation can provide arbitrary kernel read and write. A referenced kernel pointer can also reach invalid relocation or a missing destructor path.

This is not a fixed stale-storage issue and not a cleanup-only crash. It is an active verifier semantic mismatch affecting pointer access and ownership behavior. There is no demonstrated UAF, no concrete repeated release or refcount mutation, and no need to infer such a condition because the patch directly establishes a stronger memory-access primitive.

The issue is `CLASS_A` because arbitrary kernel read and write are explicitly stated in the supplied commit message. A complete privilege-escalation exploit is not supplied, but arbitrary kernel read and write in a local kernel context is sufficient to require Important-versus-Moderate manual review. `ImportantCandidate=YES` and `AutoDowngradeAllowed=NO` are appropriate because automatic closure would discard an explicitly documented kernel memory-access primitive.

Operationally, this should be treated as at least Actionable Moderate and as a strong Important candidate where affected BPF kptr functionality is available to non-host-admin or delegated-capability users.

5. One-sentence report phrase

A BPF verifier type confusion allows non per CPU pointers to be stored in BPF_KPTR_PERCPU fields and later treated as per CPU allocation cookies, enabling arbitrary kernel read and write for an attacker able to load the required privileged BPF program.

6. Manual review recommendation

MANUAL CHECK REQUIRED

The commit explicitly establishes arbitrary kernel read and write through a BPF verifier semantic mismatch, while the exact host versus delegated BPF privilege boundary depends on product configuration and deployment policy.

7. Call-site confidence

Call-site confidence: medium

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

Patch: bpf: Require MEM_PERCPU for percpu kptr stores [ Upstream
Commit: aaa9cf7707d1c5c0d2ca9d40c8b71652ac1c3c3f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=aaa9cf7707d1c5c0d2ca9d40c8b71652ac1c3c3f

Changed files:
  kernel/bpf/verifier.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=aaa9cf7707d1c5c0d2ca9d40c8b71652ac1c3c3f

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

The original Linux kernel CVE announcement for CVE-2026-98039 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98039

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:H/I:H/A:H
  The Best / paranoid CVSS score: 7.8

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: 5
  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).