[CVE-2026-74740][MODERATE REGULAR] net/sched: act_api: fix TOCTOU NULL deref on a->goto_chain [ Upstream

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

CVE: CVE-2026-74740
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: net/sched: act_api: fix TOCTOU NULL deref on a->goto_chain [ Upstream
Commit: 91d55fd1fdb85c8371ca8793c788ea7d5192383a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=91d55fd1fdb85c8371ca8793c788ea7d5192383a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74740
Analysis date: Wed, 26 Aug 2026 12:50:55 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Borderline Moderate. Manual review recommended, but not Strong Important candidate and not Actionable Moderate by score
Manual review required: YES

Summary:
A TOCTOU race in `net/sched` can let a concurrent tc action update clear `goto_chain` between two independent RCU reads, causing a NULL pointer dereference and local kernel DoS when traffic exercises the affected action.

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

The original Linux kernel CVE announcement for CVE-2026-74740 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74740

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-2026-74740	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-367;CWE-362;**CWE-476;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '4.7';DESCR 'A TOCTOU race in net sched act_api can occur when tcf_action_exec checks a goto_chain pointer and tcf_action_goto_chain_exec later performs a second independent RCU read of the same pointer. A concurrent tc action update can clear the pointer between the two reads, causing the second read to return NULL and leading to a NULL pointer dereference when chain filter_chain is accessed. For the CVSS the PR:L is used for the paranoid score because reduced capability root, container root, or delegated CAP_NET_ADMIN may be able to modify tc actions without full host administrator privileges. The issue is not directly network reachable by packets alone because reliable triggering requires control plane modification of tc actions. Remote traffic may only help exercise the data path if the affected tc action is already configured. Impact is denial of service via kernel crash. The patch context supports NULL dereference and race classification, but does not show a stronger memory corruption primitive or a direct privilege escalation path.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline Moderate. Manual review recommended, but not Strong Important candidate and not Actionable Moderate by score (with actual score 3)	YES	REMOTE SIMPLEFIX RACE NETWORK LINUS  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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

ActionableScore=3
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 3
* Final recommended bucket: **Borderline Moderate. Manual review recommended, but not Strong Important candidate and not Actionable Moderate by score**

## 2. Signal breakdown

Conservative signals:

* Reliable kernel crash / strong DoS: +1
  The patch fixes a concrete NULL pointer dereference in `tcf_action_goto_chain_exec()`.

* Common networking packet-processing path: +1
  `tcf_action_exec()` is in the traffic control action execution path, although the vulnerable state requires specific tc action configuration.

* Hard or unreliable race / special timing required: -1
  Triggering requires a TOCTOU window between the check of `a->goto_chain` and the second RCU dereference.

* Requires admin/root/CAP_NET_ADMIN in typical deployments: -2
  Reliable setup and replacement of tc actions normally requires `CAP_NET_ADMIN`.

Paranoid adjustment:

* Local unprivileged trigger: +1
  In systems with unprivileged user namespaces or delegated network administration, an attacker may obtain `CAP_NET_ADMIN` inside a namespace and configure tc objects without full host-root privileges.

Not awarded:

* Generic memory corruption: +0
  The demonstrated primitive is NULL dereference, not UAF, stale object reuse, OOB write, type confusion, or arbitrary write.

* Privilege escalation plausible: +0
  The patch context does not show attacker-controlled reclaim, object replacement, callback control, refcount abuse, or writable stale object access.

* Confidentiality / Integrity impact: +0
  No info leak or write primitive is supported by the patch.

## 3. Reachability analysis

The realistic trigger requires control-plane access to configure or replace tc actions using `TC_ACT_GOTO_CHAIN`, plus concurrent packet-path execution. In typical deployments this requires `CAP_NET_ADMIN`, so the conservative interpretation treats it as privileged local DoS. In containerized or namespace-heavy environments, delegated `CAP_NET_ADMIN` or unprivileged user namespaces may reduce the practical privilege requirement, which justifies the paranoid score. The issue is not directly network-triggerable by packets alone. Remote traffic can help exercise the data path only after the vulnerable tc action setup already exists.

Call-site confidence: high, because the affected caller and callee are both shown in the patch.

## 4. Severity interpretation

This behaves like a Borderline Moderate kernel DoS issue. It is an RCU/TOCTOU race in a networking subsystem and therefore deserves manual review, but the demonstrated failure mode is a NULL pointer dereference. There is no supported evidence of UAF, attacker-controlled reuse, object confusion, or privilege escalation. The paranoid score accounts for delegated `CAP_NET_ADMIN` scenarios, but still does not justify Important-class handling.

## 5. One-sentence report phrase

A TOCTOU race in `net/sched` can let a concurrent tc action update clear `goto_chain` between two independent RCU reads, causing a NULL pointer dereference and local kernel DoS when traffic exercises the affected action.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

This is not auto-close quality because it is an RCU race in a networking control/data path and namespace delegation may lower practical privileges, but the current evidence supports DoS only rather than LPE or memory corruption.

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

Patch: net/sched: act_api: fix TOCTOU NULL deref on a->goto_chain [ Upstream
Commit: 91d55fd1fdb85c8371ca8793c788ea7d5192383a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=91d55fd1fdb85c8371ca8793c788ea7d5192383a

Changed files:
  net/sched/act_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=91d55fd1fdb85c8371ca8793c788ea7d5192383a

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

The original Linux kernel CVE announcement for CVE-2026-74740 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74740

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:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: 2
  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).