[CVE-2026-90177][MODERATE 7.0] bpf: Check pointer type for all atomic RMW paths [ Upstream

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

CVE: CVE-2026-90177
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: bpf: Check pointer type for all atomic RMW paths [ Upstream
Commit: eb287c6e81dedef92da01eb947f380d0aae513c3
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=eb287c6e81dedef92da01eb947f380d0aae513c3
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90177
Analysis date: Fri, 18 Sep 2026 01:48:40 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-90177 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90177

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: MODERATE 7.0
Manual review required: YES

A detailed explanation of the methodology and priority rules is included
at the end of this message.

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

CVE-2026-90177	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-90177 CHECK WITH IMPACT FROM ORIG NN LOW	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-843;*CWE-20;CWE-693;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A BPF verifier type tracking flaw can allow the same atomic RMW instruction to be reached with incompatible destination pointer types, because the old logic recorded the pointer type only for PTR_TO_ARENA paths. The post verification fixup then uses the saved type for every path and may rewrite the instruction to BPF_PROBE_ATOMIC even when another path uses a non arena pointer. For the CVSS the PR:L is used because an attacker needs local ability to load or run BPF programs, for example through BPF permissions or delegated capabilities, but not necessarily full root privileges. The issue is not network reachable and requires local code execution or a local process context. Impact is at least local denial of service via verifier or JIT miscompilation, and in the paranoid case may include confidentiality and integrity impact due to a plausible kernel memory safety risk.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	YES	WARNONLY BPF SIMPLEFIX IMPROVEONLY LINUS  KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK	YES	NO	checked

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

ActionableScore=5
PrimitiveClass=CLASS_B
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 5
* Raw paranoid score: 5
* Paranoid score: 5
* Applied conservative floor/cap: semantic reinterpretation / verifier type-confusion manual-review floor 4 -> 5. BPF privileged-control-plane cap does not lower the score.
* Applied paranoid floor/cap: none.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Positive signals:

* +2: BPF verifier semantic/type-tracking mismatch. The same atomic RMW instruction can be reached with incompatible destination pointer types, while post-verification fixup uses the saved type for every path.
* +1: Attacker-controlled BPF program structure. The BPF author can shape branches, instruction reachability, and pointer-state convergence relevant to this verifier bug.
* +1: Active memory-access sink. The saved pointer type affects rewrite to `BPF_PROBE_ATOMIC`, so the mismatch is not passive metadata and can affect generated/runtime memory-access behavior.
* +1 paranoid only: The established verifier/JIT mismatch plausibly affects kernel memory safety beyond a simple rejection failure, but arbitrary read/write or LPE is not demonstrated.

Negative / non-awarded signals:

* 0: No RealUAF. This is not a lifetime or stale-reference bug.
* 0: Controlled reclaim is not demonstrated.
* 0: Arbitrary or directed kernel write is not demonstrated.
* 0: Reliable kernel crash is not directly demonstrated by the supplied patch.
* 0: Remote reachability is not established.
* 0: Local unprivileged trigger is not awarded because BPF loading policy is unresolved.
* 0: Host-admin penalty is not applied because host-admin-equivalent privilege is not positively established.
* 0: Namespace-scoped privilege is not awarded because the complete BPF load/JIT/trigger chain is not shown to work inside attacker-created namespaces.
* 0: Strong LPE / CLASS_A is not awarded because no controlled memory primitive, callback control, credential impact, arbitrary R/W, or demonstrated escalation chain is supplied.

3. Reachability analysis

The vulnerable path is reached through loading or verifying a crafted BPF program that uses atomic RMW paths involving `PTR_TO_ARENA` and a non-arena pointer reaching the same instruction. This requires local ability to submit BPF programs to the kernel.

The exact privilege boundary is unresolved from the supplied input. On many systems, BPF loading is gated by `CAP_BPF`, `CAP_SYS_ADMIN`, BPF-related sysctls such as unprivileged BPF disablement, and JIT policy. The supplied evidence supports a local BPF attacker model but does not prove ordinary unprivileged reachability and does not prove host-admin-equivalent privilege either.

NamespaceScopedPrivilege is UNKNOWN. A namespace or delegated capability could matter in some deployments, but the complete operation including BPF load, verifier acceptance, possible JIT/fixup behavior, and execution is not shown to be fully namespace-scoped.

No network trigger is established. No timing race is required. The issue is a verifier state/path consistency bug rather than a race.

4. Primitive and severity interpretation

The exact primitive is a BPF verifier pointer-type confusion across control-flow paths for one atomic RMW instruction. The old code recorded an auxiliary pointer type only when the current destination was `PTR_TO_ARENA`. A second verifier path could reach the same instruction with an ordinary pointer without comparing against the already saved arena type. Later post-verification fixup uses the saved type globally for that instruction and may rewrite it as `BPF_PROBE_ATOMIC` for paths where that interpretation is incompatible.

This is not a UAF, not a double release, and not fixed stale storage. It is an active verifier/JIT semantic mismatch affecting memory-access behavior. The demonstrated consequence is an unsafe verifier acceptance/fixup condition. The speculative consequence is kernel memory corruption or privilege escalation if the mismatched atomic operation can be turned into a useful memory primitive.

PrimitiveClass is CLASS_B because a real verifier type-confusion / semantic mismatch primitive is established, but controlled arbitrary kernel read/write, controlled object corruption, or demonstrated LPE is not established. ImportantCandidate is YES because BPF verifier type confusion with active post-verification memory-access rewriting is a high-value kernel primitive requiring manual analysis. AutoDowngradeAllowed is NO because automatic closure would risk discarding a concrete verifier memory-safety primitive whose exploitation leverage is unresolved.

Likely operational severity is Actionable Moderate at minimum, with Important-class potential depending on whether the mismatched `BPF_PROBE_ATOMIC` path can be transformed into a controlled kernel memory access primitive under real BPF policy and JIT configuration.

5. One-sentence report phrase

A BPF verifier type-tracking flaw allows one atomic RMW instruction to be validated across incompatible pointer-type paths and then fixed up using the saved arena type for all paths, creating a local verifier/JIT memory-safety primitive that requires manual review.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is not a simple crash-only bug or resource leak. It is a BPF verifier type-confusion issue affecting post-verification instruction rewriting and kernel memory-access semantics.

7. Call-site confidence

Call-site confidence: medium

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

Patch: bpf: Check pointer type for all atomic RMW paths [ Upstream
Commit: eb287c6e81dedef92da01eb947f380d0aae513c3
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=eb287c6e81dedef92da01eb947f380d0aae513c3

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=eb287c6e81dedef92da01eb947f380d0aae513c3

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

The original Linux kernel CVE announcement for CVE-2026-90177 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90177

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: 5

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: MODERATE
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): MODERATE 7.0

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).