[CVE-2026-74700][MODERATE 7.0] net/sched: cls_api: Always acquire rtnl_lock when destroying locked classifiers [ Upstream

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

CVE: CVE-2026-74700
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: net/sched: cls_api: Always acquire rtnl_lock when destroying locked classifiers [ Upstream
Commit: 34e77d8e3570f9df3952496ddb402833695662fd
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=34e77d8e3570f9df3952496ddb402833695662fd
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74700
Analysis date: Sat, 22 Aug 2026 17:08:26 -0400
ActionableScore: 7
ActionableScore lower bound: 4
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in tc classifier creation can drop the last reference on a locked tcf_proto from an unlocked classifier path and run its destroy callback without rtnl_lock, leading to a demonstrated slab use after free and possible local DoS or privilege escalation concern.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-74700 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74700

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-74700	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:H/I:H/A:H';*CWE-416;CWE-362;CWE-667;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A race in net sched classifier creation can let an unlocked classifier request lose tcf_chain_tp_insert_unique() and drop the last reference on a different locked classifier proto. The destroy callback can then run without rtnl_lock while locked classifier code such as u32_destroy expects rtnl protection, which can leave shared classifier state inconsistent and lead to a slab use after free later observed in u32_init(). For the CVSS the PR:L is used for the paranoid score because reliable triggering requires local code that can perform tc filter operations, typically CAP_NET_ADMIN or delegated net namespace control, but this is not full host administrator access in many deployments. The issue is not reachable by ordinary network packets and is triggered through local netlink control plane operations. Impact is at least local denial of service via kernel crash and in the worst case may allow confidentiality or integrity impact due to a real UAF primitive, so manual review is recommended.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 6)	YES	REMOTE READ BPF RACE UAF NETWORK  KPANIC INCREASED_TO_MODERATE7_FROM_MODERATE_BASED_ON_GUESSCVSS KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN6 DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**

## 2. Signal breakdown

Paranoid scoring signals:

* Local unprivileged trigger: +1
  Potentially reachable by a local unprivileged user when unprivileged user namespaces allow creation of a network namespace where the user gains CAP_NET_ADMIN for tc operations.

* Weak LPE concern: +1
  The patch and PoC show a real slab use after free in net sched classifier handling. No attacker controlled reclaim or overwrite is demonstrated, so this is not strong LPE evidence, but it is enough for manual review sensitivity.

* Memory corruption, weak primitive: +1
  The observed primitive is a race dependent UAF read in u32_init after tcf_proto destruction. This fits the race UAF downgrade rule rather than a strong controlled corruption primitive.

* Real lifetime corruption: +1
  The race drops the last reference on a tcf_proto and runs destroy without the expected rtnl_lock protection.

* Reliable kernel crash / strong DoS: +1
  The commit includes a PoC and KASAN slab use after free trace, making crash impact realistic.

* Local high control netlink control plane: +1
  tc netlink lets an attacker shape classifier types, chain priority, insertion order, and object lifetime timing.

* Broad networking subsystem exposure: +1
  net/sched and tc classifiers are common kernel networking infrastructure, even though this is a control plane path rather than ordinary packet receive processing.

Conservative modifiers:

* Requires CAP_NET_ADMIN in typical deployments: -1
  Not treated as full -2 because CAP_NET_ADMIN may be delegated inside network namespaces or containers and is not always equivalent to full host root.

* Hard race / timing window: -1
  The bug requires a two thread race between classifier creation paths, although the provided PoC suggests it is reproducible.

## 3. Reachability analysis

The bug is triggered locally through tc filter creation, not by ordinary network packets. In a traditional host configuration, the attacker needs CAP_NET_ADMIN to modify qdisc or classifier state. In environments with unprivileged user namespaces or delegated network namespace administration, a non root user may obtain enough namespace scoped CAP_NET_ADMIN to exercise this path, while the resulting kernel UAF still affects the host kernel. The path is not remotely reachable over the network, but it is a common local networking control plane API.

Call-site confidence: high. The commit describes the exact tc_new_tfilter race, tcf_chain_tp_insert_unique behavior, final tcf_proto_put path, tcf_proto_destroy call, and KASAN UAF stack.

## 4. Severity interpretation

This is stronger than an ordinary Moderate because it is not only a warning or resource leak. It is a demonstrated race dependent lifetime bug with a real slab use after free in kernel networking code. Realistic demonstrated impact is local DoS via crash. Privilege escalation is plausible but not proven because the patch does not show attacker controlled reclaim, write after free, callback control, or type confusion. Therefore the conservative score remains borderline, but the paranoid score reaches Strong Important candidate level and should not be auto closed.

## 5. One-sentence report phrase

A race in tc classifier creation can drop the last reference on a locked tcf_proto from an unlocked classifier path and run its destroy callback without rtnl_lock, leading to a demonstrated slab use after free and possible local DoS or privilege escalation concern.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED. YES REQUIRES MANUAL CHECK.

Reason: the issue is a demonstrated kernel UAF in a common networking control plane path, with namespace dependent reachability and plausible but unproven LPE impact.

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

Patch: net/sched: cls_api: Always acquire rtnl_lock when destroying locked classifiers [ Upstream
Commit: 34e77d8e3570f9df3952496ddb402833695662fd
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=34e77d8e3570f9df3952496ddb402833695662fd

Changed files:
  net/sched/cls_u32.c
  net/sched/cls_api.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=34e77d8e3570f9df3952496ddb402833695662fd

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

The original Linux kernel CVE announcement for CVE-2026-74700 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74700

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: 4
  Paranoid ActionableScore: 7

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).