* [CVE-2026-90075][MODERATE REGULAR] net/sched: fq_codel: clamp default quantum and mtu [ Upstream
@ 2026-09-18 12:59 21% AL-KERNEL
0 siblings, 0 replies; 1+ results
From: AL-KERNEL @ 2026-09-18 12:59 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-90075
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: net/sched: fq_codel: clamp default quantum and mtu [ Upstream
Commit: d315ee8a07fd1810880227319cf60e6cd925ef22
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d315ee8a07fd1810880227319cf60e6cd925ef22
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90075
Analysis date: Fri, 18 Sep 2026 08:59:04 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline / manual review recommended
Manual review required: YES
Summary:
No one-sentence report phrase was found.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-90075 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90075
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.
======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================
CVE-2026-90075 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-90075 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:L/A:H';CWE-190;*CWE-835;**CWE-400;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '6.1';DESCR 'fq_codel can enter a CPU soft lockup because fq_codel_init used an unclamped device MTU as the default quantum and CoDel MTU. A very large MTU can make psched_mtu return 0x80000000, which overflows the signed flow deficit to INT_MIN in fq_codel_dequeue and can create an infinite dequeue loop. For the CVSS the PR:L is used because triggering requires local code with CAP_NET_ADMIN in a user namespace or equivalent control over MTU and qdisc configuration. The issue is not directly network reachable and cannot be triggered by packet traffic alone without the vulnerable local configuration. Impact is denial of service via CPU soft lockup. The paranoid score also allows low integrity impact because the same unclamped MTU can silently disable CoDel drops and ECN behavior.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4) YES REMOTE DEADLOCK DANGER NETWORK HARDWARE TEST QDISC DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP NO NO checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=4
ActionableScoreLower=3
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=YES
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
1. ActionableScore
* Raw conservative score: 3
* Conservative score: 3
* Raw paranoid score: 4
* Paranoid score: 4
* Final recommended bucket: **Borderline / manual review recommended**::
No floor or cap changed the score.
2. Signal breakdown
Conservative positive signals:
* +1 Reliable kernel DoS: the patch description establishes an infinite loop in fq_codel_dequeue() and CPU soft lockup caused by signed deficit overflow to INT_MIN.
* +1 Below-host-admin namespace-scoped reachability: the supplied context states that triggering requires CAP_NET_ADMIN in a user namespace, not host-admin-equivalent privilege.
* +1 Attacker-controlled runtime configuration: the attacker can configure a vulnerable qdisc and an extreme MTU on a device such as dummy, making the overflow condition deterministic rather than a narrow race.
Paranoid additional signal:
* +1 Low integrity concern: the same unclamped MTU can silently disable CoDel drops and ECN behavior, weakening the intended queue-management policy.
Non-awarded signals:
* +0 Real UAF: no use-after-free, stale pointer, double free, or refcount lifetime primitive is established.
* +0 Strong corruption: the signed integer overflow causes invalid scheduler arithmetic and loop behavior, but no arbitrary write, controlled overwrite, controlled reclaim, or object replacement is shown.
* +0 Confidentiality: no disclosure path is established.
* +0 Strong LPE: no privilege-escalation primitive is demonstrated.
* +0 Remote reachability: packet traffic alone does not trigger the bug without local qdisc and MTU configuration.
* +0 Extra Availability: not awarded because the same soft lockup is already counted as the reliable DoS signal.
* +0 Race penalty: not applicable because this is not a timing race.
3. Reachability analysis
The trigger requires configuring a network device with an extreme MTU and attaching or initializing fq_codel so that fq_codel_init() uses the unclamped psched_mtu() value. The supplied context explicitly says this requires CAP_NET_ADMIN in a user namespace.
This supports HostAdminRequired=NO and NamespaceScopedPrivilege=YES for the provided scenario. The relevant attack model is a local actor with namespace-scoped network administration capability, or an equivalent delegated network-management context. It is not a direct remote packet-processing vulnerability because incoming traffic does not by itself create the vulnerable MTU and qdisc configuration.
The dummy device example suggests no rare physical hardware is required. The condition is configuration-dependent but not a hard race. The attacker-controlled MTU value makes the overflow condition deterministic once the required namespace and network capabilities are available.
4. Primitive and severity interpretation
The exact primitive is an unchecked MTU-derived value used as fq_codel quantum and CoDel MTU. A huge value can wrap into the sign bit and overflow the signed flow deficit, producing INT_MIN and an infinite dequeue loop. This is an arithmetic and logic error with strong DoS behavior.
PrimitiveClass=CLASS_C because the demonstrated behavior is CPU soft lockup and invalid queue-management state, not memory corruption or a lifetime violation. RealUAF=NO. The CoDel disablement gives a plausible low integrity policy concern in the paranoid view, but it does not establish a security-boundary bypass or privilege escalation.
ImportantCandidate=NO because there is no Class A primitive and no Class B lifetime or corruption primitive requiring Important-vs-Moderate review. ManualReview=RECOMMENDED because the bug is below-host-admin reachable in a namespace-scoped networking control path and can cause host-impacting soft lockup. AutoDowngradeAllowed=YES because the established class remains DoS and low policy-integrity impact without unresolved kernel corruption leverage.
5. One-sentence report phrase
fq_codel can enter an infinite dequeue loop and CPU soft lockup when a namespace CAP_NET_ADMIN user configures an extreme MTU that overflows the signed flow deficit, with additional low integrity concern because CoDel drops and ECN may be disabled.
6. Manual review recommendation
MANUAL CHECK RECOMMENDED
Reason: below-host-admin namespace-scoped reachability and host-impacting soft lockup justify review, but no UAF, memory corruption, authorization bypass, or LPE primitive is established.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: net/sched: fq_codel: clamp default quantum and mtu [ Upstream
Commit: d315ee8a07fd1810880227319cf60e6cd925ef22
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d315ee8a07fd1810880227319cf60e6cd925ef22
Changed files:
net/sched/sch_fq_codel.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=d315ee8a07fd1810880227319cf60e6cd925ef22
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-90075 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90075
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:H
The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:H
The Best / paranoid CVSS score: 6.1
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: 3
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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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