[CVE-2026-64012][MODERATE REGULAR] net/sched: sch_sfb: Replace direct dequeue call with peek and qdisc_dequeue_peeked [ Upstream

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

CVE: CVE-2026-64012
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: net/sched: sch_sfb: Replace direct dequeue call with peek and qdisc_dequeue_peeked [ Upstream
Commit: e5ea51e5f3fbba41d50cd84a530f33bc1c8f4d57
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e5ea51e5f3fbba41d50cd84a530f33bc1c8f4d57
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64012
Analysis date: Sun, 19 Jul 2026 21:49:11 -0400
ActionableScore: 4
ActionableScore lower bound: 1
Actionable bucket: Borderline, manual review recommended. Not an Important candidate by itself
Manual review required: YES

Summary:
A qdisc API mismatch in sch_sfb can call a child dequeue path after a prior peek operation, causing qfq_dequeue to dereference NULL and panic the kernel when a non-default SFB/QFQ hierarchy is used.

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

The original Linux kernel CVE announcement for CVE-2026-64012 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64012

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-2026-64012	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';**CWE-476;CWE-754;CWE-703;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.9';DESCR 'sch_sfb can call a child qdisc dequeue path directly after a prior peek operation, while qdiscs using qdisc_peek_dequeued may have already staged the packet in the peeked skb state. With a non-default hierarchy such as tbf over sfb over qfq, this mismatch can make qfq_dequeue operate on an invalid NULL internal state and crash the kernel. For the CVSS the PR:N in the paranoid network scenario because a remote peer may only need network reachability to make the host transmit packets if the vulnerable qdisc stack is already configured. In the typical administrative scenario the practical PR is H because creating this qdisc hierarchy requires CAP_NET_ADMIN. The issue is not a demonstrated privilege escalation or information leak. Impact is denial of service via kernel panic. This is not expected to be exposed on default systems and usually depends on local traffic control configuration.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended. Not an Important candidate by itself (with actual score 3)	YES	REMOTE KASAN NULLPTR NETWORK SKB KERNEL_PANIC_PLUS_UAF HARDWARE LINUS QDISC PACKET  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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

ActionableScore=4
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 4
* Final recommended bucket: **Borderline, manual review recommended. Not an Important candidate by itself**::

## 2. Signal breakdown

Conservative signals:

* Reliable kernel crash / strong DoS: +1
  The commit includes a KASAN NULL pointer dereference and kernel panic path through `qfq_dequeue() -> sfb_dequeue() -> tbf_dequeue()`.
* Requires admin/root/CAP_NET_ADMIN in typical deployments: -2
  Creating the vulnerable qdisc hierarchy such as `tbf -> sfb -> qfq` normally requires `CAP_NET_ADMIN`.
* Common networking packet-processing path: +1
  The crash occurs in the qdisc dequeue path, which is part of packet scheduling and transmit processing.
* No memory corruption primitive: +0
  The evidence shows NULL pointer dereference, not UAF, OOB write, type confusion, double free, refcount takeover, or attacker-controlled overwrite.
* No confidentiality or integrity impact: +0
  The patch and trace do not show information disclosure or data corruption.

Conservative total: 1

Paranoid signals:

* Remote reachable / network-triggerable: +2
  If the vulnerable qdisc hierarchy is already configured, a remote peer may be able to induce transmit traffic and reach the dequeue path without local privileges.
* Reliable kernel crash / strong DoS: +1
  The observed failure is a kernel panic from NULL pointer dereference.
* Common networking packet-processing path: +1
  The affected code is in net/sched packet processing.
* Non-default specialized runtime configuration: -1
  The issue depends on a specific qdisc stack involving SFB and a child qdisc using `qdisc_peek_dequeued()`, such as QFQ.
* No LPE or memory corruption bonus: +0
  This remains a NULL dereference / API sequencing bug, not a demonstrated memory corruption primitive.

Paranoid total: 3 to 4. I choose 4 to avoid auto-closing a remotely triggerable kernel panic in a networking path when the vulnerable configuration exists.

## 3. Reachability analysis

Typical triggering requires local administrative control over traffic control configuration, because setting up the vulnerable qdisc hierarchy requires `CAP_NET_ADMIN`. In that conservative model, the attacker is already privileged for network configuration, so the practical score is low.

The paranoid model is different: if an administrator or orchestration system has already installed the vulnerable `tbf -> sfb -> qfq` style hierarchy, then ordinary packet transmission can enter the affected dequeue path. A remote peer may be able to cause the host to transmit packets and trigger the crash, but only under that pre-existing configuration. This is not a default exposure and not a generic Internet-reachable parser bug.

Namespaces may reduce the practical privilege requirement if untrusted users are granted `CAP_NET_ADMIN` in a network namespace that controls a relevant qdisc. That supports manual review, but not an Important classification without stronger evidence.

Call-site confidence: high, because the commit message provides the dequeue/peek call sequence and the patch directly changes the call from `child->dequeue()` to `qdisc_dequeue_peeked(child)`.

## 4. Severity interpretation

This behaves like an actionable DoS issue in a networking scheduling path, not like a privilege escalation vulnerability. The theoretical risk is limited to a kernel crash caused by inconsistent qdisc peek/dequeue state. The patch does not support UAF, object reuse, callback corruption, arbitrary write, information leak, or integrity impact.

Conservative interpretation: ordinary Moderate / low actionable priority because typical setup requires `CAP_NET_ADMIN`.

Paranoid interpretation: borderline actionable Moderate because a remote peer may trigger the panic if the vulnerable qdisc stack is already configured.

## 5. One-sentence report phrase

A qdisc API mismatch in sch_sfb can call a child dequeue path after a prior peek operation, causing qfq_dequeue to dereference NULL and panic the kernel when a non-default SFB/QFQ hierarchy is used.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended because the affected code is in a networking packet scheduling path and the paranoid model allows network-induced triggering when the vulnerable qdisc hierarchy is preconfigured. It is not a strong Important candidate because the demonstrated primitive is NULL pointer dereference DoS only, with no supported LPE, UAF, OOB write, or confidentiality/integrity impact.

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

Patch: net/sched: sch_sfb: Replace direct dequeue call with peek and qdisc_dequeue_peeked [ Upstream
Commit: e5ea51e5f3fbba41d50cd84a530f33bc1c8f4d57
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e5ea51e5f3fbba41d50cd84a530f33bc1c8f4d57

Changed files:
  net/sched/sch_sfb.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=e5ea51e5f3fbba41d50cd84a530f33bc1c8f4d57

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

The original Linux kernel CVE announcement for CVE-2026-64012 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64012

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:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.9

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: 4

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).