[CVE-2026-74736][MODERATE REGULAR] net/sched: cls_bpf: reject dev-bound programs bound to a different device [ Upstream

[CVE-2026-74736][MODERATE REGULAR] net/sched: cls_bpf: reject dev-bound programs bound to a different device [ Upstream

[AL-KERNEL]

CVE: CVE-2026-74736
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: net/sched: cls_bpf: reject dev-bound programs bound to a different device [ Upstream
Commit: ec5a552f4b2d841c6c021752450716e1a9676661
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ec5a552f4b2d841c6c021752450716e1a9676661
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74736
Analysis date: Wed, 26 Aug 2026 12:45:47 -0400
ActionableScore: 3
ActionableScore lower bound: 1
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
A cls_bpf validation bug allowed a device-bound offloaded SCHED_CLS BPF program for one netdev to be attached to another netdev, leaving offload state inconsistent after the original device is removed and potentially causing a WARN or kernel panic under panic_on_warn.

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

The original Linux kernel CVE announcement for CVE-2026-74736 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74736

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 REGULAR
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
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CVE-2026-74736	MODERATE	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:N/I:N/A:H';CWE-863;CWE-284;*CWE-664;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'A TC cls_bpf logic bug allowed a device bound offloaded SCHED_CLS BPF program loaded for device A to be attached with skip_sw to device B without checking that the bound device matched the target TC device. When device A is deleted, the program offload state can be destroyed while the program is still attached to device B, which can trigger a netdevsim WARN and can become a kernel panic when panic_on_warn is enabled. For the CVSS the PR:L is used for the paranoid score because TC BPF offload control may be available to a reduced capability root, container root, or delegated network administration service rather than only to a full host administrator. The issue is not directly network reachable and is triggered through local control plane operations for BPF and traffic control configuration. Impact is denial of service, with no concrete evidence in the patch of information disclosure or privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 2)	YES	REMOTE BPF SIMPLEFIX NETWORK HARDWARE QDISC 	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=3
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 3
* Final recommended bucket: **Borderline, manual review recommended**::

## 2. Signal breakdown

Conservative signals:

* Real lifetime corruption: +1. A device-bound BPF program can remain attached to device B after device A deletion destroys the program offload state.
* Reliable kernel crash / strong DoS: +1. The demonstrated failure is a netdevsim WARN that can become a panic when panic_on_warn=1.
* Requires admin/root/CAP_* in typical deployments: -2. Loading and attaching TC BPF offload programs normally requires CAP_NET_ADMIN and BPF-related privileges.
* Rare or difficult-to-reach configuration: -1. The path depends on device-bound offloaded BPF, skip_sw, and an offload-capable or simulated netdev setup.
* Floor applied at 1 because this is a real kernel lifetime/state mismatch in BPF/netdev offload code, not a pure cosmetic validation fix.

Paranoid signals:

* Real lifetime corruption: +1.
* Reliable kernel crash / strong DoS: +1.
* Privileged device-management lifetime/state corruption path: +1. The mismatch affects netdev offload state lifetime during device teardown.
* Privilege penalty reduced in paranoid interpretation: 0 instead of -2. TC/BPF network control may be delegated to container root, reduced-capability root, or a network-management service account.
* Rare/config-dependent exposure: 0 in paranoid interpretation, because BPF and TC offload bugs deserve manual review even when not broadly exposed.

Not awarded:

* No remote/network-triggerable score. This is a local control-plane operation.
* No local unprivileged trigger. Unprivileged BPF should not be assumed.
* No generic strong memory corruption. The patch shows stale offload state and WARN, but not attacker-controlled reclaim, arbitrary write, type confusion, or object replacement.
* No privilege escalation score. LPE is not supported by the patch context.

## 3. Reachability analysis

The bug is triggered by loading a device-bound SCHED_CLS BPF program for one netdev and attaching it through cls_bpf with skip_sw to another TC netdev. In typical deployments this requires CAP_NET_ADMIN and BPF/offload privileges, so conservative scoring treats it as privileged local. Namespaces or delegated network administration can lower the practical privilege level if a container root or service account can manage TC and BPF offload objects. The path is not remotely reachable by packet traffic and depends on device-bound offload support, making broad default exposure limited.

## 4. Severity interpretation

This behaves more like a borderline actionable Moderate than an Important-class issue. The realistic demonstrated impact is DoS through WARN or panic_on_warn after offload state is destroyed while still referenced by another device attachment. Theoretical lifetime corruption exists, but the patch does not show a concrete memory corruption primitive such as UAF reclaim, controlled replacement, arbitrary write, type confusion, or callback control. Manual review is still useful because BPF plus netdev offload lifetime mismatches are historically sensitive and may have product-specific reachability through delegated network control.

## 5. One-sentence report phrase

A cls_bpf validation bug allowed a device-bound offloaded SCHED_CLS BPF program for one netdev to be attached to another netdev, leaving offload state inconsistent after the original device is removed and potentially causing a WARN or kernel panic under panic_on_warn.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended because the issue combines BPF, TC, device-bound offload state, and netdev teardown lifetime behavior. It should not be treated as a strong Important candidate without evidence of real memory corruption or privilege escalation, but it is also not ideal for automatic closure in environments that delegate TC/BPF control to reduced-privilege services or containers.

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

Patch: net/sched: cls_bpf: reject dev-bound programs bound to a different device [ Upstream
Commit: ec5a552f4b2d841c6c021752450716e1a9676661
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ec5a552f4b2d841c6c021752450716e1a9676661

Changed files:
  net/core/dev.c
  net/sched/cls_bpf.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=ec5a552f4b2d841c6c021752450716e1a9676661

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

The original Linux kernel CVE announcement for CVE-2026-74736 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74736

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: 1
  Paranoid ActionableScore: 3

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: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

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).