* [CVE-2026-98610][MODERATE REGULAR] net/sched: sch_hfsc: bound the classify inner-filter walk with a drift budget [ Upstream
@ 2026-10-19 9:40 21% AL-KERNEL
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From: AL-KERNEL @ 2026-10-19 9:40 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-98610
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: net/sched: sch_hfsc: bound the classify inner-filter walk with a drift budget [ Upstream
Commit: d4fd251e3311727c32af89344d3332f788c153a2
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d4fd251e3311727c32af89344d3332f788c153a2
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98610
Analysis date: Mon, 19 Oct 2026 05:40:31 -0400
ActionableScore: 3
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES
Summary:
An unprivileged local user with access to user and network namespaces can construct cyclic HFSC flowid bindings that make one packet enter an infinite classification walk, causing a host CPU soft lockup and potentially a kernel panic when watchdog panic handling is enabled.
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ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98610 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98610
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
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CVE-2026-98610 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-98610 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=2 LEGACY=MODERATE RESULT=DIFF (final Legacy and Aegis differ; FINAL: MODERATE, Aegis: MODERATE_KPANIC) 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-835;**CWE-400;CWE-834;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'A logic flaw in sch_hfsc can create an unbounded classification walk when flowid bindings that were valid at bind time become lateral after class level changes. Two affected bindings can point back to each other and make hfsc_classify loop forever while the qdisc lock is held and BH processing is disabled. A single packet can then cause a CPU soft lockup, block processing through the affected qdisc, and trigger a kernel panic when softlockup_panic is enabled. For the CVSS the PR:L value is used because a local user or process is required, although an unprivileged user may obtain CAP_NET_ADMIN inside new user and network namespaces through unshare -Urn when that functionality is enabled. The issue is not directly network reachable because the malicious HFSC and U32 configuration must first be created locally. Impact is denial of service only, with no supported memory corruption, information disclosure, or privilege escalation primitive. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 5.5 and the production ActionableScore returned 3 with recommendation Borderline / manual review recommended. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE_KPANIC to MODERATE because the CVSS score, vector or bug class threshold rule was applied. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH and V41_INHERITED_KPANIC_PROMOTION_BLOCKED and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 5.5 and ActionableScore 2 with primitive class CLASS_C and manual review RECOMMENDED. Its independently reconciled impact was MODERATE_KPANIC. The models disagreed but the aggregation rules retained the production result. The final reported impact is MODERATE.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3) MAYBE REMOTE DEADLOCK DANGER NETWORK HARDWARE LINUS QDISC PACKET DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH V41_INHERITED_KPANIC_PROMOTION_BLOCKED MANUAL_REVIEW_REQUIRED - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=3
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=YES
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO
## 1. ActionableScore
* Raw conservative score: 3
* Conservative score: 3
* Raw paranoid score: 3
* Paranoid score: 3
* No floor or cap was applied.
* Final recommended bucket: **Borderline / manual review recommended**::
## 2. Signal breakdown
Positive signals:
* **+1 Local unprivileged trigger:** the supplied evidence explicitly states that an ordinary user can create user and network namespaces with `unshare -Urn`, obtain namespace-scoped `CAP_NET_ADMIN`, configure the HFSC and U32 cycle, and trigger it with one packet.
* **+1 Reliable strong DoS:** the cyclic classification walk holds the qdisc lock with BH disabled and causes a CPU soft lockup. It can panic the kernel when `softlockup_panic=1`.
* **+1 Cross-boundary interference:** configuration inside an attacker-created network namespace can stall a host CPU and potentially panic the host, so the effect is not confined to the namespace-local qdisc.
No negative signals were applied:
* This is deterministic configuration and traversal, not a hard race.
* No rare-hardware penalty applies.
* HFSC is optional, but the attacker can instantiate the required qdisc under the evidenced namespace model, so no niche-subsystem penalty applies.
* No host-admin penalty applies because the complete trigger is explicitly described as reachable through attacker-created namespaces.
Important non-awarded signals:
* No UAF, OOB access, heap overwrite, refcount mutation, or controlled corruption is established.
* No active-state trust-boundary corruption under V33 is established. The class graph becomes cyclic, but no protected structured state is overwritten.
* No confidentiality, integrity, authorization-bypass, or privilege-escalation primitive is shown.
* No remote-reachability bonus applies because the attacker must first configure the qdisc locally.
* No additional Availability point is awarded because the soft lockup and optional panic are the same DoS mechanism.
* B3-PENDING, B4-PENDING, B5-R, V32, and V34 do not apply.
## 3. Reachability analysis
The complete trigger is described as available to an ordinary local user when unprivileged user namespaces are enabled. `unshare -Urn` creates attacker-owned user and network namespaces, and the resulting namespace-scoped `CAP_NET_ADMIN` permits creation of the HFSC classes and U32 flowid bindings on a namespace-local interface.
No separate host-administrator action is required for the soft lockup path. Enabling `softlockup_panic=1` is a host-controlled prerequisite only for converting the lockup into an automatic kernel panic.
Required kernel features include `CONFIG_NET_SCHED`, `CONFIG_NET_SCH_HFSC`, and `CONFIG_NET_CLS_U32`. `CONFIG_LOCKUP_DETECTOR` is relevant to watchdog detection. No special hardware or timing race is required. Once the cyclic bindings exist, a single locally generated packet deterministically enters the unbounded walk.
## 4. Primitive and severity interpretation
The exact primitive is an attacker-created cyclic control-flow path in `hfsc_classify()`. Flowid bindings retain a bound `res.class`, bypassing the dynamic downward-level check. Later class-level changes can therefore turn two individually valid bindings into a cycle.
This is a logic-driven infinite loop and DoS primitive, classified as **CLASS_C**. It is not a lifetime violation, memory-corruption primitive, authorization bypass, or repeated release case.
The demonstrated consequence is a CPU soft lockup while the qdisc lock is held and BH processing is disabled. Host panic is supported when the watchdog panic policy is enabled. There is no evidenced leverage beyond availability impact.
`ImportantCandidate=NO` because no corruption, protected-state modification, authorization bypass, or unresolved exploitation primitive exists. `AutoDowngradeAllowed=YES` because this remains a bounded security classification case involving local namespace-reachable DoS, although manual review is recommended to validate the host-wide operational effect and product namespace policy.
## 5. One-sentence report phrase
An unprivileged local user with access to user and network namespaces can construct cyclic HFSC flowid bindings that make one packet enter an infinite classification walk, causing a host CPU soft lockup and potentially a kernel panic when watchdog panic handling is enabled.
## 6. Manual review recommendation
MANUAL CHECK RECOMMENDED
Confirm whether unprivileged user namespaces are enabled in the evaluated product and whether the observed CPU lockup produces host-wide service loss under its watchdog and panic configuration.
## 7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: net/sched: sch_hfsc: bound the classify inner-filter walk with a drift budget [ Upstream
Commit: d4fd251e3311727c32af89344d3332f788c153a2
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d4fd251e3311727c32af89344d3332f788c153a2
Changed files:
net/sched/sch_hfsc.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=d4fd251e3311727c32af89344d3332f788c153a2
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98610 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98610
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: 3
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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