[CVE-2026-74663][LOW] net/sched: reject overly deep qdisc hierarchies

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

CVE: CVE-2026-74663
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: net/sched: reject overly deep qdisc hierarchies
Commit: 9f69bb9fdaa2fe64b68bb62fb84d405784bee540
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9f69bb9fdaa2fe64b68bb62fb84d405784bee540
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74663
Analysis date: Sat, 22 Aug 2026 12:51:48 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
Overly deep qdisc hierarchies in `net/sched` can exhaust the kernel stack during recursive qdisc tree walks, allowing a local actor with traffic-control configuration privileges to cause a denial of service.

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

The original Linux kernel CVE announcement for CVE-2026-74663 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74663

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: LOW
Manual review required: NO

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
======================================================================

CVE-2026-74663	LOW	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-674;**CWE-400;CWE-770;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'Overly deep qdisc hierarchies in net/sched can lead to excessive recursion in qdisc tree walkers and exhaust the kernel stack. A local actor able to configure traffic control can extend the hierarchy through the create and graft path because the existing loop check did not prevent adding a new child below an already deep parent. For the CVSS the PR:L is used for the paranoid score because CAP_NET_ADMIN may be delegated to a container root, service account, or reduced capability administrator rather than full host root. The issue is not directly network reachable and is triggered by qdisc configuration changes rather than packet traffic. Impact is denial of service via kernel stack exhaustion and possible kernel crash. No concrete UAF, OOB write, or information leak primitive is indicated, so confidentiality and integrity remain unchanged.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2)  SKIP CVE-2026-74663 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	REMOTE OOB SIMPLEFIX QDISC  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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

ActionableScore=2
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: **1**
* Paranoid score: **2**
* Final recommended bucket: **Ordinary Moderate / Low-like**::

## 2. Signal breakdown

Conservative signals:

* **Reliable kernel crash / strong DoS: +1**
  Deep qdisc hierarchies can cause excessive recursion in qdisc tree walkers and exhaust the kernel stack.
* **Availability impact realistic: +1**
  Kernel stack exhaustion can plausibly cause a host-wide crash or severe stall.
* **Broad/default/common subsystem exposure: +1**
  `net/sched` and qdisc handling are common kernel networking components.
* **Requires admin/root/CAP_NET_ADMIN in typical deployments: -2**
  Creating and grafting qdiscs through traffic control normally requires `CAP_NET_ADMIN`.

Paranoid adjustment:

* **Reduced privilege CAP_NET_ADMIN interpretation: +1 relative to conservative**
  In some deployments `CAP_NET_ADMIN` may be delegated to container root, a service account, or a reduced-capability administrator. This makes the practical risk higher than full host-root-only, but still not equivalent to a true unprivileged trigger.

Not applied:

* No remote/network-triggerable signal. The bug is triggered through qdisc configuration, not packet traffic.
* No generic memory corruption signal. The patch indicates recursion and stack exhaustion, not UAF, OOB write, double free, refcount misuse, or attacker-controlled overwrite.
* No privilege escalation signal. No concrete LPE primitive is supported.
* No confidentiality or integrity signal. The described impact is availability only.

Call-site confidence: **high**. The relevant `qdisc_graft()` path and the new depth enforcement are visible in the supplied patch.

## 3. Reachability analysis

A local actor able to configure traffic control qdiscs can trigger the issue by building an overly deep hierarchy through the create-and-graft path. In typical host deployments this requires `CAP_NET_ADMIN`, so conservative scoring treats it as privileged local control-plane access. Namespaces or containers may reduce the effective privilege barrier if `CAP_NET_ADMIN` is delegated, which justifies the slightly higher paranoid score. The path is not remotely reachable through normal packet processing.

## 4. Severity interpretation

This behaves like an **ordinary Moderate / Low-like local DoS** issue. The realistic impact is kernel stack exhaustion leading to crash or lockup. Theoretical memory-corruption concern from stack exhaustion is not enough here to score C or I, because the patch and description do not show a controlled overwrite, UAF, object reuse, or privilege-escalation primitive.

## 5. One-sentence report phrase

Overly deep qdisc hierarchies in `net/sched` can exhaust the kernel stack during recursive qdisc tree walks, allowing a local actor with traffic-control configuration privileges to cause a denial of service.

## 6. Manual review recommendation

**NO MANUAL CHECK NEEDED**

This is a CAP_NET_ADMIN-gated local DoS with no supported memory-corruption or privilege-escalation primitive, so it can be handled as non-urgent unless the product delegates traffic-control administration to untrusted containers or tenants.

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

Patch: net/sched: reject overly deep qdisc hierarchies
Commit: 9f69bb9fdaa2fe64b68bb62fb84d405784bee540
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9f69bb9fdaa2fe64b68bb62fb84d405784bee540

Commit description:

Deep qdisc hierarchies can lead to excessive recursion in qdisc tree
walkers and exhaust the kernel stack. The existing loop check does not
cover the create-and-graft path, so a hierarchy can still be extended by
creating a new child qdisc below an already deep parent.

Store the hierarchy depth in struct Qdisc and update it when qdiscs are
grafted. Reject new child qdiscs once the parent is already at the maximum
allowed depth.

Fixes: 1da177e ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Suggested-by: Jamal Hadi Salim <jhs@mojatatu.com>
Reported-by: Vega <vega@nebusec.ai>
Assisted-by: Codex:gpt-5.4
Signed-off-by: Zijie Huang <milkory@outlook.com>
Signed-off-by: Ren Wei <enjou1224z@gmail.com>
Reviewed-by: Victor Nogueira <victor@mojatatu.com>
Link: https://patch.msgid.link/1e9ab39597423fd5d13cfaaf52279b8ee3d9fc3c.1785434373.git.milkory@outlook.com
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  include/net/sch_generic.h
  net/sched/sch_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=9f69bb9fdaa2fe64b68bb62fb84d405784bee540

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

The original Linux kernel CVE announcement for CVE-2026-74663 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74663

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:H
  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: 2

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).