[CVE-2026-45886][MODERATE 7.0] bpf: Fix bpf_xdp_store_bytes proto for read-only arg [ Upstream

[CVE-2026-45886][MODERATE 7.0] bpf: Fix bpf_xdp_store_bytes proto for read-only arg [ Upstream

[AL-KERNEL]

CVE: CVE-2026-45886
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: bpf: Fix bpf_xdp_store_bytes proto for read-only arg [ Upstream
Commit: ffb5d1c5e3933b947fc7303ad68bf0c536d0c85e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ffb5d1c5e3933b947fc7303ad68bf0c536d0c85e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45886
Analysis date: Wed, 27 May 2026 23:07:33 -0400
ActionableScore: 6
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
An incorrect `bpf_xdp_store_bytes()` helper prototype can make the verifier treat a read source buffer as writable/uninitialized, potentially allowing uninitialized memory to be copied into XDP packet output.

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

The original Linux kernel CVE announcement for CVE-2026-45886 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45886

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-2026-45886	MODERATE	CHECK	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:L/A:L';*CWE-457;*CWE-200;CWE-693;CWE-682;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:L/I:N/A:L';BEST CVSS score: '6.6';DESCR 'bpf_xdp_store_bytes had an incorrect helper prototype for its source buffer. The verifier treated the source as ARG_PTR_TO_UNINIT_MEM, which can reject valid read-only map values and may allow the helper to read uninitialized memory that the verifier believed was only written by the helper. For the CVSS the PR:L is used in the paranoid score because delegated BPF loaders, Cilium-like agents, or service accounts may have BPF program load capability below full host root. The issue is not directly network reachable, although packet output may expose data copied by the XDP helper. Impact is primarily confidentiality risk from possible uninitialized memory disclosure, plus limited integrity or availability risk from verifier mis-modeling and false rejection.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5) 	MAYBE	REMOTE INIT BPF LINUS  KPANIC INCREASED_TO_MODERATE7_FROM_MODERATE_BASED_ON_GUESSCVSS	-	-	checked

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

ActionableScore=6
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 6
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

* Local trigger with BPF load capability: +1
  The affected path is reachable by loading an XDP/BPF program using `bpf_xdp_store_bytes()`.

* Local high-control kernel data-plane API: +1
  BPF/XDP is a high-control local kernel API, and programs can shape verifier state and helper arguments.

* Confidentiality impact plausible: +1
  The incorrect prototype marks the source buffer as `ARG_PTR_TO_UNINIT_MEM`, while the helper actually reads from it and copies data into a packet. This may expose uninitialized memory if accepted by the verifier.

* Integrity or availability impact plausible: +1
  The wrong access type can also cause false verifier rejection of valid read-only map usage and verifier memory-model inconsistency.

* Security-boundary concern: +1 in paranoid scoring
  The BPF verifier is a security boundary. Helper prototype mistakes can sometimes become verifier unsoundness beyond the immediately shown false reject.

* Broad/security-sensitive subsystem exposure: +1
  BPF/XDP is widely used in networking stacks such as Cilium and other production datapaths.

* Requires BPF permissions in many deployments: -2 conservative
  BPF program loading is often limited to CAP_BPF, CAP_NET_ADMIN, CAP_SYS_ADMIN, or privileged loaders.

* No demonstrated kernel memory corruption or LPE: +0
  The patch indicates possible uninitialized read / verifier modeling bug, not a proven arbitrary write, UAF, or privilege escalation primitive.

## 3. Reachability analysis

A local user or service with BPF/XDP program loading capability can trigger this by using `bpf_xdp_store_bytes()` with a source buffer affected by the verifier prototype mismatch. In typical hardened systems, direct BPF loading is privileged. In real deployments, however, BPF loading may be delegated to agents, service accounts, or container/networking frameworks such as Cilium, making PR:L plausible for triage.

The issue is not directly network reachable. Network packets may carry out data copied by the XDP helper, but the vulnerable condition is created by local BPF program loading and verifier modeling.

## 4. Severity interpretation

This is an actionable Moderate BPF verifier/helper prototype issue. The strongest concrete concern is possible uninitialized memory disclosure through packet writes and false rejection of valid read-only map values. It is not yet an Important memory-corruption case because there is no demonstrated arbitrary write, UAF, or LPE chain. Manual analysis is still warranted because BPF verifier metadata bugs can be security-sensitive.

## 5. One-sentence report phrase

An incorrect `bpf_xdp_store_bytes()` helper prototype can make the verifier treat a read source buffer as writable/uninitialized, potentially allowing uninitialized memory to be copied into XDP packet output.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Manual review is required because this affects BPF verifier helper semantics and may create an information disclosure path, even though no privilege escalation or arbitrary write primitive is shown.

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

Patch: bpf: Fix bpf_xdp_store_bytes proto for read-only arg [ Upstream
Commit: ffb5d1c5e3933b947fc7303ad68bf0c536d0c85e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ffb5d1c5e3933b947fc7303ad68bf0c536d0c85e

Changed files:
  net/core/filter.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=ffb5d1c5e3933b947fc7303ad68bf0c536d0c85e

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

The original Linux kernel CVE announcement for CVE-2026-45886 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45886

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:L/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:L
  The Best / paranoid CVSS score: 6.6

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: 4
  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: 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).