[CVE-2026-90211][LOW] bpf, s390: Clear fetch destination on faulting arena atomic [ Upstream

[CVE-2026-90211][LOW] bpf, s390: Clear fetch destination on faulting arena atomic [ Upstream

[AL-KERNEL]

CVE: CVE-2026-90211
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: bpf, s390: Clear fetch destination on faulting arena atomic [ Upstream
Commit: 1a26541b3f60ce447433a4c4dd56edb10d2cd580
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1a26541b3f60ce447433a4c4dd56edb10d2cd580
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90211
Analysis date: Fri, 18 Sep 2026 00:12:47 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate
Manual review required: YES

Summary:
A faulting s390 BPF arena RMW atomic with BPF_FETCH can resume with a stale destination register instead of zero, causing incorrect BPF runtime semantics without a demonstrated kernel memory corruption or privilege escalation primitive.

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

The original Linux kernel CVE announcement for CVE-2026-90211 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90211

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: 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-2026-90211	LOW	https://www.kernelcve.org/list/?q=CVE-2026-90211 CHECK WITH IMPACT FROM ORIG NN 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:L/I:L/A:N';CWE-665;*CWE-457;CWE-754;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:L/A:N';BEST CVSS score: '4.4';DESCR 'A faulting s390 BPF arena RMW atomic with BPF_FETCH can resume with a stale destination register because the exception table entry does not record which register must be cleared. This affects atomics that read the old value into src_reg or r0 for BPF_CMPXCHG, while plain write only atomics do not need such clearing. A local attacker would need the ability to load or run a BPF program that uses arena atomics on s390, so there is no direct network reachability. For the CVSS the PR:L is used for the paranoid score because delegated BPF capability or an environment with unprivileged BPF could make the trigger available to a non root local user. Impact is mainly incorrect BPF program semantics, with possible low confidentiality or integrity impact from stale register propagation. No clear kernel memory corruption primitive is shown by the patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)	YES	INIT BPF SIMPLEFIX HARDWARE 	YES	NO	checked

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

ActionableScore=2
ActionableScoreLower=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

## 1. ActionableScore

* Raw conservative score: 1
* Conservative score: 1
* Raw paranoid score: 2
* Paranoid score: 2
* No floor or cap changed the score.
* Final recommended bucket: **Ordinary Moderate**::

## 2. Signal breakdown

Conservative signals:

* +1 Weak invalid-state / semantic fault-handling primitive. A faulting s390 BPF arena RMW atomic with `BPF_FETCH` may resume with the old destination register value instead of zero.
* 0 No Real UAF. The patch does not show post-lifetime object use, double release, refcount corruption, or stale freed object traversal.
* 0 No reliable kernel crash is established. The demonstrated issue is incorrect register clearing on a fault path, not panic or Oops.
* 0 No C/I bonus. There is no demonstrated disclosure path, arbitrary write, verifier escape, controlled kernel corruption, or privilege escalation.
* 0 No remote reachability. The trigger is via BPF program execution on s390, not network packet input.

Paranoid additional signal:

* +1 BPF JIT exception-handling semantic concern. The stale register value may affect BPF program semantics after a fault and deserves review for verifier/runtime assumptions, but this is still not a proven kernel memory corruption primitive.

Non-awarded material signals:

* Controlled reclaim not demonstrated.
* Arbitrary or directed kernel write not demonstrated.
* Real UAF not demonstrated.
* Concrete repeated release/refcount mutation not present.
* Structured stale-object leverage not awarded because no stale kernel object is used after free.
* Weak LPE not awarded as a separate point because no concrete privilege-escalation path is shown.
* Extra Availability not awarded because crash or persistent DoS is not established.
* BPF object complexity not used as an exploitation-leverage point.

## 3. Reachability analysis

The trigger requires a BPF program using s390 arena atomics with a faulting arena pointer and an RMW atomic carrying `BPF_FETCH`, `BPF_XCHG`, or `BPF_CMPXCHG`.

BPF loading and relevant arena/JIT usage are commonly privilege-gated by `CAP_BPF`, `CAP_SYS_ADMIN`, BPF sysctls, or product policy. The supplied evidence does not prove that an ordinary local user can perform the complete load and trigger sequence. It also does not prove that host-admin-equivalent privilege is strictly required in all deployments. Therefore `HostAdminRequired=UNKNOWN`.

No complete namespace-scoped trigger chain is established. The patch has no netns-specific component, and BPF privilege scope is unresolved. Therefore `NamespaceScopedPrivilege=UNKNOWN`.

There is no direct network trigger. No special external device input is involved. The issue is architecture-specific to s390 BPF JIT arena atomics, which limits exposure, but I am not applying a separate rare-hardware penalty because s390 is a supported Linux architecture and the prompt requires concrete evidence for such penalties.

Race repeatability is not relevant. This is not a timing race.

## 4. Primitive and severity interpretation

The exact primitive is a stale register-value propagation after a handled BPF probe fault. The exception table entry lacks the destination register to clear, so the BPF program continues with whatever value was previously in `src_reg` or `r0` instead of the expected zero.

This is not active structured-object UAF and not fixed stale storage UAF. It is an invalid runtime state / semantic mismatch in BPF JIT fault handling.

Primitive class is `CLASS_C`: demonstrated behavior is primarily invalid-state behavior without an independently useful kernel corruption primitive.

Demonstrated consequence: incorrect BPF program semantics after a fault on an unmapped arena page.

Speculative consequence: possible low confidentiality or integrity relevance if stale register propagation violates verifier or runtime assumptions in a security-relevant way. The supplied patch does not establish such a chain.

`ImportantCandidate=NO` because no Class A primitive, Real UAF, authorization bypass, controlled corruption, shared-page ownership violation, or demonstrated LPE is present.

`AutoDowngradeAllowed=YES` because the issue is constrained to a BPF JIT semantic fault-handling mismatch and lacks concrete unresolved memory-corruption leverage.

Likely operational severity: ordinary Moderate / Low-like, with manual review recommended due to BPF JIT semantics.

## 5. One-sentence report phrase

A faulting s390 BPF arena RMW atomic with BPF_FETCH can resume with a stale destination register instead of zero, causing incorrect BPF runtime semantics without a demonstrated kernel memory corruption or privilege escalation primitive.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: BPF JIT exception handling bugs can be security-sensitive, and stale register propagation should be checked against verifier and arena semantics, but the supplied evidence does not establish UAF, arbitrary write, disclosure, LPE, or a direct DoS.

## 7. Call-site confidence

Call-site confidence: medium

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

Patch: bpf, s390: Clear fetch destination on faulting arena atomic [ Upstream
Commit: 1a26541b3f60ce447433a4c4dd56edb10d2cd580
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1a26541b3f60ce447433a4c4dd56edb10d2cd580

Changed files:
  arch/s390/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=1a26541b3f60ce447433a4c4dd56edb10d2cd580

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

The original Linux kernel CVE announcement for CVE-2026-90211 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90211

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:L/A:N
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N
  The Best / paranoid CVSS score: 4.4

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: 1
  Paranoid ActionableScore: 2

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).