[CVE-2023-53624][LOW] net/sched: sch_fq: fix integer overflow of "credit" [ Upstream

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

CVE: CVE-2023-53624
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: net/sched: sch_fq: fix integer overflow of "credit" [ Upstream
Commit: 4b8a05e3801661a0438fcd0cdef181030d966a5a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4b8a05e3801661a0438fcd0cdef181030d966a5a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2023-53624
Analysis date: Mon, 05 Oct 2026 05:33:24 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2023-53624 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2023-53624

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2023-53624	LOW	https://www.kernelcve.org/list/?q=CVE-2023-53624 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-190;CWE-681;CWE-834;**CWE-400;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'An integer conversion flaw in sch_fq allows an initial_quantum value greater than INT_MAX to initialize the signed flow credit with a very large negative value. Processing a new flow can then consume excessive CPU time while the scheduler recovers the credit, producing a CPU soft lockup even with only a small number of sockets. For the CVSS the PR:L value is used because configuring the FQ qdisc requires CAP_NET_ADMIN, but this capability may be available to a less privileged user inside an allowed user and network namespace. The issue is not directly network reachable because crafted packet traffic alone cannot set initial_quantum. Impact is denial of service through prolonged CPU processing. There is no UAF, out of bounds access, information leak, or write primitive supporting confidentiality or integrity impact.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2) 	MAYBE	REMOTE DEADLOCK INIT SIMPLEFIX NETWORK HARDWARE CVEOUTDATED QDISC PACKET  DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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

ActionableScore=2
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=YES
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 2
* Conservative score: 2
* Raw paranoid score: 2
* Paranoid score: 2
* No floor or cap was applied.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Local unprivileged trigger. Where unprivileged user namespaces are enabled, an ordinary user can create a user and network namespace, obtain namespace-scoped CAP_NET_ADMIN, create a namespace-local device, and configure its FQ qdisc.
* +1 Strong availability effect. An invalid initial_quantum can make flow credit approximately minus 2 GB and keep a CPU in prolonged scheduler processing until the soft-lockup watchdog fires. This was reproduced with few sockets.
* +0 Integer overflow. The signed conversion produces an invalid scheduler value, but no memory corruption primitive is established.
* No remote-reachability point. Network packets alone cannot configure initial_quantum.
* No common packet-processing point. The vulnerable state requires an explicitly configured invalid FQ parameter.
* No UAF, OOB access, arbitrary write, controlled reclaim, information disclosure, integrity violation, or privilege-escalation primitive is demonstrated.
* No additional availability point was awarded because the CPU stall and soft-lockup report describe the same consequence.
* No race or hardware penalty applies.

3. Reachability analysis

The trigger requires an RTM_NEWQDISC style control-plane operation with CAP_NET_ADMIN for the target network namespace. Linux qdisc configuration is namespace-aware, and the complete sequence can be performed on a namespace-local dummy device when an ordinary user is allowed to create user and network namespaces. This establishes a below-host-admin path and namespace-scoped privilege, although systems that disable unprivileged user namespaces restrict the trigger to a delegated network administrator or host administrator.

No prior host-created qdisc or special hardware is required. The attacker can instantiate the FQ qdisc and create flows locally. Incoming traffic cannot independently set the malformed configuration value. There is no race or narrow timing requirement.

4. Primitive and severity interpretation

The exact primitive is a signed integer conversion or overflow in which a U32 initial_quantum above INT_MAX initializes signed flow credit with a large negative value. The resulting invalid accounting state causes excessive scheduler work but does not escape into a memory-safety primitive.

This is CLASS_C because the demonstrated behavior is CPU soft lockup and denial of service without UAF, overwrite, disclosure, ownership violation, or authorization bypass. The practical consequence is temporary or potentially repeated CPU unavailability. Stronger effects such as memory corruption or privilege escalation are unsupported.

ImportantCandidate is NO because there is no unresolved Important-grade primitive. AutoDowngradeAllowed is YES because the issue is a bounded logic and availability flaw with no corruption or security-boundary consequence.

5. One-sentence report phrase

An attacker with namespace-scoped CAP_NET_ADMIN can configure sch_fq with an initial_quantum above INT_MAX, producing a large negative flow credit and prolonged CPU processing that can trigger a soft lockup and denial of service.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

The patch establishes a configuration-induced CPU stall without memory corruption, lifetime misuse, confidentiality or integrity impact, or an unresolved security-boundary primitive.

7. Call-site confidence

Call-site confidence: medium

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

Patch: net/sched: sch_fq: fix integer overflow of "credit" [ Upstream
Commit: 4b8a05e3801661a0438fcd0cdef181030d966a5a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4b8a05e3801661a0438fcd0cdef181030d966a5a

Changed files:
  net/sched/sch_fq.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=4b8a05e3801661a0438fcd0cdef181030d966a5a

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

The original Linux kernel CVE announcement for CVE-2023-53624 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2023-53624

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:L/UI:N/S:U/C:N/I:N/A:L
  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: unknown
  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).