[CVE-2025-38477][MODERATE 7.0] net/sched: sch_qfq: Fix race condition on qfq_aggregate [ Upstream

[CVE-2025-38477][MODERATE 7.0] net/sched: sch_qfq: Fix race condition on qfq_aggregate [ Upstream

[AL-KERNEL]

CVE: CVE-2025-38477
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: net/sched: sch_qfq: Fix race condition on qfq_aggregate [ Upstream
Commit: 466e10194ab81caa2ee6a332d33ba16bcceeeba6
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=466e10194ab81caa2ee6a332d33ba16bcceeeba6
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2025-38477
Analysis date: Mon, 12 Oct 2026 20:35:30 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2025-38477 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2025-38477

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-2025-38477	MODERATE	https://www.kernelcve.org/list/?q=CVE-2025-38477 CHECK WITH IMPACT FROM ORIG NN IMPORTANT | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=4 LEGACY=MODERATE_KPANIC RESULT=SAME (legacy and Aegis reconciliation agree; DECISION: both MODERATE_KPANIC)	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-362;*CWE-416;**CWE-476;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 sch_qfq can allow qfq_aggregate to be modified or destroyed while another thread reads it through qfq_dump_class, qfq_dump_class_stats, or an enqueue related path. The demonstrated outcomes include a NULL pointer dereference and a use after free, which can crash the kernel and may permit memory corruption if the freed aggregate is reused before the stale access completes. For the CVSS the PR:L value is selected because reliable triggering requires local control of QFQ class operations, normally through CAP_NET_ADMIN, but that capability may be delegated to a restricted service or a user inside a network namespace. AC:H is selected because exploitation depends on winning a concurrent timing window between class deletion or aggregate modification and another access. The issue is not directly reachable using ordinary remote packets alone, although network traffic can drive qfq_enqueue while a local actor performs the required control plane operations. Impact is at least a local denial of service via kernel crash and in the worst case may allow confidentiality, integrity, or privilege impact due to the confirmed use after free. Manual review is required to determine whether object reuse can provide a practical exploitation primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	MAYBE	REMOTE OOB NULLPTR RACE UAF NETWORK SKB LINUS QDISC KPANIC  KPANIC LOWERED_FROM_HIGH_BASED_ON_NULLPTR	-	-	checked

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

ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw conservative score: 3.
* Conservative score: 4. REAL UAF FLOOR 4 changes 3 to 4 because an active structured aggregate can be used after its lifetime ends and host-admin-equivalent privilege is not positively established.
* Raw paranoid score: 4.
* Paranoid score: 5. REAL UAF FLOOR 5 changes 4 to 5 because the QFQ object is runtime configurable, the lifetime transition can be repeatedly driven, and stale use can occur in active enqueue or class-management processing rather than only shallow cleanup.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Conservative positive signals:

* +2 Real UAF. The commit directly states that concurrent aggregate modification and class deletion can cause use-after-free.
* +1 Reliable kernel crash or strong DoS. The documented NULL dereference and stale aggregate access can produce an Oops or kernel crash.
* Total: 3.

Paranoid additional signal:

* +1 Attacker-influenced active-object state and repeatable lifetime manipulation. An actor able to manage the QFQ configuration can repeatedly modify or delete classes while traffic drives `qfq_enqueue`. This is scored only in the paranoid interpretation because the complete below-host-admin capability chain is not established.
* Total: 4 before the floor.

Negative signals:

* No race penalty. The evidence does not establish that the timing window is one-shot, exceptionally narrow, or difficult to retry.
* No host-admin penalty. Host-admin-equivalent privilege is not positively established.
* No niche-subsystem penalty. QFQ is optional, but an authorized actor can instantiate it at runtime and no unusual hardware is required.

Important non-awarded signals:

* Controlled reclaim or attacker-shaped replacement contents are not demonstrated.
* Arbitrary or directed kernel write is not demonstrated.
* Structured stale-object leverage is not separately awarded because the supplied context does not show a distinct post-reuse pointer, callback, offset, or ownership operation.
* Weak LPE is not awarded merely because the defect is a UAF.
* Confidentiality and integrity bonuses are not awarded from theoretical reuse.
* Extra Availability is not awarded because it would duplicate the same crash.
* Remote reachability is not established. Packet traffic can exercise enqueue, but class modification or deletion requires a local control-plane actor.
* No concrete repeated release or refcount signal exists. The patch does not establish an initial release followed by a second lifetime mutation through the exact same dangling reference.
* The first class destruction can end aggregate lifetime, but no second `put`, `free`, or refcount operation on the exact stale reference is shown.
* Repeated-release +1 and paranoid repeated-refcount +1 are therefore withheld.
* FLOOR 5 is not based on repeated release. It applies only to the paranoid active-object and repeatable-lifetime interpretation.
* B5-R does not apply because no retry-accumulated heap OOB write is established.
* B3-PENDING does not apply because no shared-backing write is established.
* B4-PENDING does not apply because there is no authorization decision bypass.

3. Reachability analysis

The complete trigger requires concurrent QFQ data-plane and control-plane activity. Traffic can drive `qfq_enqueue`, while rtnetlink class dump, change, or deletion operations access or retire the same aggregate.

Qdisc management normally requires `CAP_NET_ADMIN`, but the supplied material does not resolve whether every required operation can be completed with a capability obtained in an attacker-created user and network namespace. Consequently, host-admin-equivalent privilege and complete namespace-scoped reachability both remain unknown.

Numeric host UID 0 is not intrinsically required because Linux capability checks can authorize a non-UID-0 process. This does not prove that an ordinary user can complete the trigger on the evaluated system. User namespace availability, capability scope, target network namespace ownership, and the ability to instantiate QFQ all require deployment-specific confirmation.

No special hardware, boot mode, or firmware is required. QFQ must be configured before the vulnerable path exists. The race appears repeatable through class operations and traffic generation, but the supplied evidence does not quantify its practical reliability.

4. Primitive and severity interpretation

The exact primitive is a race-induced use-after-free involving `qfq_aggregate`, with a separate NULL-dereference outcome. The aggregate is an active structured scheduler object, not fixed stale storage. It is accessed by class dump and statistics paths and participates in active enqueue-related state changes.

This is broader than an ordinary destructor-only stale dereference because concurrent packet scheduling can overlap aggregate modification or destruction. However, the supplied patch does not establish controlled slab reuse, attacker-controlled post-free contents, callback dispatch, arbitrary kernel read or write, or privilege escalation.

No concrete repeated release or refcount mutation is shown. This is therefore an active-object CLASS_B UAF, not CLASS_A and not the repeated-lifetime category.

The demonstrated consequence is kernel crash or DoS. Memory corruption and privilege escalation remain possible review questions but are not established consequences. ImportantCandidate is YES because runtime-configurable active-object lifetime and stale-use behavior leave meaningful exploitation leverage unresolved. AutoDowngradeAllowed is NO because automatically treating the issue as ordinary crash-only Moderate would discard that unresolved active-object UAF surface.

Likely operational severity is Actionable Moderate. Important classification is not proven, but it requires manual exploitation-leverage and namespace-reachability review.

5. One-sentence report phrase

A race between QFQ aggregate modification or destruction and concurrent class or enqueue processing can cause a NULL dereference or active-object use-after-free, resulting in kernel DoS and requiring manual review for post-free reuse leverage.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Confirm the exact stale-use instruction, aggregate allocation cache, reclaim opportunities, capability namespace governing QFQ operations, and whether an ordinary user can create the complete QFQ and traffic trigger inside an attacker-controlled namespace.

7. Call-site confidence

Call-site confidence: medium

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

Patch: net/sched: sch_qfq: Fix race condition on qfq_aggregate [ Upstream
Commit: 466e10194ab81caa2ee6a332d33ba16bcceeeba6
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=466e10194ab81caa2ee6a332d33ba16bcceeeba6

Changed files:
  net/sched/sch_qfq.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=466e10194ab81caa2ee6a332d33ba16bcceeeba6

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

The original Linux kernel CVE announcement for CVE-2025-38477 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2025-38477

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: 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: 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).