[CVE-2026-74739][MODERATE 7.0] net/sched: cls_u32: skip hash tables in u32_bind_class() [ Upstream

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

CVE: CVE-2026-74739
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: net/sched: cls_u32: skip hash tables in u32_bind_class() [ Upstream
Commit: ec5f3005586a785689fd568361b0c5925cb1548b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ec5f3005586a785689fd568361b0c5925cb1548b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74739
Analysis date: Wed, 26 Aug 2026 12:42:15 -0400
ActionableScore: 5
ActionableScore lower bound: 2
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A local traffic-control configuration can cause `cls_u32` to treat a hash table object as a knode, producing a slab out-of-bounds read in the class binding path with DoS risk and limited manual-review concern for broader corruption impact.

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

The original Linux kernel CVE announcement for CVE-2026-74739 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74739

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-2026-74739	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';CWE-125;CWE-843;CWE-119;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:L/I:N/A:L';BEST CVSS score: '5.8';DESCR 'A slab out-of-bounds read in cls_u32 occurs because u32_bind_class() assumes that every handle passed by u32_walk() is a tc_u_knode, while the walker can also pass tc_u_hnode objects. When a hash table object is treated as a knode, the code reads a non existent tcf_result field and tc_cls_bind_class can access res classid outside the real object. For the CVSS the PR:L is used for the paranoid score because reliable triggering needs local traffic control configuration ability, but CAP_NET_ADMIN is often delegated to containers or can be available in user and network namespace setups. The issue is not directly network reachable and is triggered through tc control plane operations rather than packet traffic. Impact is at least a local denial of service or diagnostic crash risk, with limited confidentiality or integrity impact kept for manual review because the bug is an out-of-bounds kernel memory access in a class binding path.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 4)	YES	REMOTE READ OOB SIMPLEFIX NETWORK QDISC  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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

ActionableScore=5
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**

## 2. Signal breakdown

Conservative signals:

* Memory corruption, weak/indirect corruption candidate: +1
  `u32_bind_class()` treats a `tc_u_hnode` as `tc_u_knode`, causing invalid struct interpretation and a slab out-of-bounds read of `res->classid`.
* Availability impact realistic: +1
  The issue is reproducible through `tc` configuration and can trigger KASAN or diagnostic failure. It is not clearly a reliable production panic.
* Requires CAP_NET_ADMIN in typical deployments: -2
  Traffic control configuration normally requires administrative network privileges.

Conservative total: 0 to 2 depending on whether diagnostic crash risk is counted. I keep 2 because the reproducer reaches a real kernel OOB access.

Paranoid additional signals:

* Local unprivileged trigger in some namespace configurations: +1
  On systems with unprivileged user namespaces and delegated network namespace control, a local user may obtain effective `CAP_NET_ADMIN` inside a namespace and exercise `tc`.
* Weak LPE concern: +1
  This is not a proven privilege escalation primitive, but it is a bogus object interpretation in a class binding path and deserves manual review.
* Broad/common kernel networking control-plane exposure: +1
  `net/sched` and `cls_u32` are common kernel networking components, even though the path is control plane rather than packet RX/TX fast path.

Paranoid total: 5

## 3. Reachability analysis

The bug is triggered locally through traffic-control configuration using `tc qdisc`, `tc class`, and `tc filter` operations. In normal host deployments this requires `CAP_NET_ADMIN`, so the conservative view treats it as privileged local. In container or namespace-heavy deployments, the effective privilege may be lower if an untrusted user can create or control a network namespace with `CAP_NET_ADMIN`, so the paranoid score uses local lower-privilege reachability.

The issue is not remotely network-triggerable by packet traffic. It is reached through netlink control-plane operations that configure qdiscs/classes/filters. The path is not necessarily default-active, but it is part of a broadly available networking subsystem.

Call-site confidence: high. The commit describes the relevant walker behavior and the changed function directly, and the affected call to `tc_cls_bind_class()` is visible in the patch context.

## 4. Severity interpretation

This is more than an ordinary cleanup because it is a real slab out-of-bounds kernel read caused by interpreting one kernel object type as another. However, the demonstrated primitive is read-oriented and constrained. The patch does not show an attacker-controlled overwrite, reclaim primitive, callback control, refcount takeover, or arbitrary memory corruption.

Realistically this behaves like a local privileged or namespace-dependent OOB-read/DoS issue. The paranoid interpretation raises it to Actionable Moderate because kernel OOB access in `net/sched` with possible namespace reachability should not be auto-closed without manual review. It is not a Strong Important candidate based on the supplied evidence.

## 5. One-sentence report phrase

A local traffic-control configuration can cause `cls_u32` to treat a hash table object as a knode, producing a slab out-of-bounds read in the class binding path with DoS risk and limited manual-review concern for broader corruption impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the bug is a real kernel slab out-of-bounds access caused by invalid object interpretation. No strong LPE primitive is demonstrated, but the combination of `net/sched`, namespace-dependent reachability, and object confusion warrants manual triage rather than automatic closure.

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

Patch: net/sched: cls_u32: skip hash tables in u32_bind_class() [ Upstream
Commit: ec5f3005586a785689fd568361b0c5925cb1548b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ec5f3005586a785689fd568361b0c5925cb1548b

Changed files:
  net/sched/cls_u32.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=ec5f3005586a785689fd568361b0c5925cb1548b

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

The original Linux kernel CVE announcement for CVE-2026-74739 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74739

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:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

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: 2
  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: NO
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).