[CVE-2025-38193][MODERATE REGULAR] net_sched: sch_sfq: reject invalid perturb period

[CVE-2025-38193][MODERATE REGULAR] net_sched: sch_sfq: reject invalid perturb period

[AL-KERNEL]

CVE: CVE-2025-38193
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: net_sched: sch_sfq: reject invalid perturb period
Commit: 956b5aebb349449b38d920d444ca1392d43719d1
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=956b5aebb349449b38d920d444ca1392d43719d1
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2025-38193
Analysis date: Thu, 24 Sep 2026 17:14:12 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
An integer overflow in SFQ perturb_period configuration allows invalid timer periods that can reach a related race condition and may cause local network scheduler denial of service.

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

The original Linux kernel CVE announcement for CVE-2025-38193 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2025-38193

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: NO

A detailed explanation of the methodology and priority rules is included
at the end of this message.

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

CVE-2025-38193	MODERATE	https://www.kernelcve.org/list/?q=CVE-2025-38193 CHECK WITH IMPACT FROM ORIG NN MODERATE	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-190;*CWE-20;CWE-362;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '4.7';DESCR 'An integer overflow in the SFQ perturb_period configuration can occur when an invalid negative or excessively large value is multiplied by HZ during qdisc configuration. The invalid timer period can be used to reach a related race condition and may cause denial of service through network scheduler disruption or excessive timer activity. For the CVSS the PR:L rating is used for the paranoid score because a local user may obtain delegated CAP_NET_ADMIN in a network namespace or container environment. On conventional hosts qdisc configuration normally requires administrative network privileges. The issue is not directly network reachable because it is triggered through local netlink qdisc configuration rather than packet traffic. No confidentiality or integrity impact is established by this patch because it does not demonstrate a use after free out of bounds access information leak or write primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)	MAYBE	REMOTE RACE NETWORK LINUS QDISC  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	-	-	checked

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

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

## 1. ActionableScore

* Raw conservative score: 2
* Conservative score: 2
* Raw paranoid score: 2
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate**::

No floor or cap was applied. The validation-only cap is not material because the score is already below it.

## 2. Signal breakdown

* +1 Concrete attacker-controlled invalid timing configuration. `perturb_period` is supplied through the SFQ qdisc netlink configuration and was multiplied by `HZ` without rejecting negative or overflowing values.
* +1 Availability concern. The commit explicitly states that invalid `perturb_period` values can trigger a race condition addressed by a separate patch. An invalid timer period can therefore affect qdisc and network scheduler behavior.

Important non-awarded signals:

* No UAF, double free, stale dereference, refcount misuse, OOB access, or arbitrary write is established by this patch.
* No controlled kernel corruption or attacker-controlled payload/target is demonstrated.
* No confidentiality or integrity impact is established.
* No reliable kernel panic or host-wide crash is directly shown in the supplied patch.
* No local-unprivileged-trigger bonus was awarded. SFQ qdisc changes require `CAP_NET_ADMIN`, and the supplied evidence does not establish that an ordinary attacker can complete the full trigger sequence in an attacker-created namespace.
* No remote reachability bonus was awarded. The trigger is a local rtnetlink qdisc configuration operation, not packet processing.
* No race difficulty penalty was applied. The supplied evidence says that the value can trigger a race but does not establish a narrow, one-shot, externally uncontrollable timing window.

## 3. Reachability analysis

The direct trigger is a local `tc qdisc add/change` operation configuring an SFQ qdisc with an invalid `perturb` value. Such qdisc operations are normally authorized through rtnetlink capability checks involving `CAP_NET_ADMIN`.

The affected object can be attached to a local interface such as loopback. Rtnetlink and qdisc administration are namespace-aware in normal Linux designs, but the supplied material does not establish the complete attacker path: whether unprivileged user namespaces are enabled, whether the attacker can create the relevant network namespace, and whether all required qdisc operations are available in the evaluated deployment.

Therefore, host-admin-equivalent privilege is not positively established, but below-host-admin reachability is also not fully established. `HostAdminRequired=UNKNOWN` and `NamespaceScopedPrivilege=UNKNOWN` are appropriate.

No special hardware, boot option, or prior host-created device topology is indicated. The separate race fix must be reviewed to determine whether it creates a stronger primitive than timer or scheduler disruption.

## 4. Primitive and severity interpretation

The directly established defect is signed integer overflow and invalid-value acceptance in:

`ctl->perturb_period * HZ`

The patch rejects negative values and values above `INT_MAX / HZ`, preventing an invalid timer period from entering SFQ state.

This is an invalid-state and potential DoS issue, not a demonstrated memory-corruption issue. The commit references a separately fixed race condition, but the supplied patch does not show the race mechanics, stale-object use, post-free operation, refcount mutation, object reuse, or a security-boundary bypass.

There is no concrete repeated release/refcount operation. There is no first lifetime operation that may free an object, no surviving dangling reference, and no second lifetime-mutating operation.

The correct current classification is `CLASS_C`. The demonstrated practical concern is qdisc or scheduler disruption. A stronger consequence remains speculative until the referenced race patch and its call paths are correlated.

`ImportantCandidate=NO` because no Class A primitive or meaningful unresolved Class B lifetime/corruption primitive is shown. `AutoDowngradeAllowed=YES` applies to this patch considered alone, although the referenced separate race fix should be correlated before closing the broader issue family.

## 5. One-sentence report phrase

An integer overflow in SFQ perturb_period configuration allows invalid timer periods that can reach a related race condition and may cause local network scheduler denial of service.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

The patch is validation-only, but its commit message explicitly states that invalid input can trigger a separately fixed race condition whose primitive and impact are not included in the supplied material.

## 7. Call-site confidence

Call-site confidence: medium

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

Patch: net_sched: sch_sfq: reject invalid perturb period
Commit: 956b5aebb349449b38d920d444ca1392d43719d1
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=956b5aebb349449b38d920d444ca1392d43719d1

Commit description:

Gerrard Tai reported that SFQ perturb_period has no range check yet,
and this can be used to trigger a race condition fixed in a separate patch.

We want to make sure ctl->perturb_period * HZ will not overflow
and is positive.

Tested:

tc qd add dev lo root sfq perturb -10   # negative value : error
Error: sch_sfq: invalid perturb period.

tc qd add dev lo root sfq perturb 1000000000 # too big : error
Error: sch_sfq: invalid perturb period.

tc qd add dev lo root sfq perturb 2000000 # acceptable value
tc -s -d qd sh dev lo
qdisc sfq 8005: root refcnt 2 limit 127p quantum 64Kb depth 127 flows 128 divisor 1024 perturb 2000000sec
 Sent 0 bytes 0 pkt (dropped 0, overlimits 0 requeues 0)
 backlog 0b 0p requeues 0

Fixes: 1da177e ("Linux-2.6.12-rc2")
Reported-by: Gerrard Tai <gerrard.tai@starlabs.sg>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20250611083501.1810459-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/sched/sch_sfq.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=956b5aebb349449b38d920d444ca1392d43719d1

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

The original Linux kernel CVE announcement for CVE-2025-38193 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2025-38193

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: 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: 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).