[CVE-2026-98818][MODERATE REGULAR] mm/slab: do not wake up kswapd in __kfree_rcu_sheaf() [ Upstream

[CVE-2026-98818][MODERATE REGULAR] mm/slab: do not wake up kswapd in __kfree_rcu_sheaf() [ Upstream

[AL-KERNEL]

CVE: CVE-2026-98818
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: mm/slab: do not wake up kswapd in __kfree_rcu_sheaf() [ Upstream
Commit: 46545c86b54ffb969b6e3662942b4a6191619eaa
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=46545c86b54ffb969b6e3662942b4a6191619eaa
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98818
Analysis date: Mon, 19 Oct 2026 01:44:04 -0400
ActionableScore: 0
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
A rare lock inversion can occur when the kfree_rcu sheaf allocation path wakes kswapd while holding scheduler pi_lock, potentially causing a local system deadlock or denial of service without a demonstrated memory corruption or privilege escalation primitive.

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

The original Linux kernel CVE announcement for CVE-2026-98818 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98818

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-2026-98818	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98818 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=0 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:H/PR:L/UI:N/S:U/C:N/I:N/A:H';*CWE-833;CWE-667;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '4.7';DESCR 'A circular locking dependency can occur because __kfree_rcu_sheaf() may allocate a slab sheaf and wake kswapd while the scheduler pi_lock is held. The kswapd wakeup path can acquire pgdat kswapd_wait and later attempt to acquire pi_lock, which creates a lock inversion and may deadlock a CPU or stall the system. Triggering requires a rare scheduler fallback path, a failed sheaf refill, and memory conditions that would normally wake kswapd, so the attack complexity is high. For the CVSS the PR:L is used for the paranoid score because a local user can contribute task and memory churn while system scheduling or cpuset events provide the remaining preconditions. The issue is not network reachable. Impact is denial of service only, with no demonstrated UAF, out of bounds access, information disclosure, arbitrary write, or privilege escalation primitive. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 4.7 and the production ActionableScore returned 0 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 5.0 and ActionableScore 0 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 0)	MAYBE	LOCK DEADLOCK HARDWARE LINUS MEMORY  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH V41_INHERITED_KPANIC_PROMOTION_BLOCKED MANUAL_REVIEW_REQUIRED	-	-	checked

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

ActionableScore=0
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw conservative score: 0
* Conservative score: 0
* Raw paranoid score: 0
* Paranoid score: 0
* No floor or cap was applied.
* Final recommended bucket: **Borderline / manual review recommended**::

Arithmetic for both scores:

* +1 potential strong DoS from the concrete circular locking dependency.
* -1 materially difficult timing and state combination explicitly established by the commit.
* Total: 0.

2. Signal breakdown

Positive signals:

* +1 Strong DoS concern. Waking kswapd while holding `pi_lock` creates the documented chain `pi_lock -> kswapd_wakeup -> kswapd_wait -> pi_lock`. A successful interleaving can deadlock a CPU or stall the system.

Negative signals:

* -1 Hard and unreliable timing. The commit explicitly says intentional reproduction was not straightforward, `set_cpus_allowed_force()` is exercised very rarely, and competing allocations will usually wake kswapd before the vulnerable path does.

Material signals not awarded:

* No UAF, OOB access, double release, refcount corruption, or active-object lifetime violation is established.
* No attacker-controlled write, controlled reclaim, arbitrary write, or disclosure exists.
* No local-unprivileged bonus was awarded because control over the complete scheduler fallback, sheaf refill failure, and kswapd wakeup sequence is not established.
* No host-admin penalty was applied because host-admin-equivalent privilege is also not positively established.
* No remote or namespace-scoped reachability is demonstrated.
* No extra availability point was awarded because it would duplicate the same deadlock consequence.
* NEW52 B5-R does not apply because there is no heap OOB write or retry-accumulated offset.
* NEW53 B3-PENDING does not apply because there is no shared-backing write.
* NEW54 B4-PENDING does not apply because no authorization decision is changed from DENY to ALLOW.
* V33 and V32 do not apply because there is no attacker-influenced active structured state or checksum metadata corruption.

3. Reachability analysis

The complete trigger requires `kfree_rcu()` to execute under scheduler `pi_lock`, entry through a rare CPU fallback path, failure to obtain or refill a slab sheaf, and allocator state that attempts to wake kswapd.

A local process can contribute task churn and memory pressure, but the supplied evidence does not show that an ordinary user can independently force the complete sequence. CPU hotplug and cpuset reconfiguration may involve administrative control, but the shown `select_fallback_rq()` path can also arise from system scheduling events. Therefore host-admin requirements remain unknown rather than YES or NO.

No relevant namespace capability check, prior delegated interface, boot option, special hardware, or network trigger is established. The issue is timing-sensitive and difficult to repeat.

4. Primitive and severity interpretation

The exact primitive is a circular locking dependency caused by allocation flags that permit kswapd wakeup from a context that may already hold `pi_lock`. This is a locking and availability defect, not memory corruption.

There is no stale storage, active structured UAF, repeated lifetime mutation, or authorization bypass. The demonstrated security consequence is potential deadlock or system stall. Privilege escalation, confidentiality impact, integrity impact, and useful exploitation leverage beyond denial of service are unsupported.

`PrimitiveClass=CLASS_C` is appropriate because the behavior is limited to a difficult deadlock path. `ImportantCandidate=NO` follows from the absence of an Important-preserving corruption or boundary primitive. `AutoDowngradeAllowed=YES` is appropriate because no unresolved memory-corruption, lifetime, authorization, or cross-boundary mechanism would be lost by ordinary Moderate handling.

5. One-sentence report phrase

A rare lock inversion can occur when the kfree_rcu sheaf allocation path wakes kswapd while holding scheduler pi_lock, potentially causing a local system deadlock or denial of service without a demonstrated memory corruption or privilege escalation primitive.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Review is useful to confirm product-specific reachability and whether unprivileged workload can materially force the complete scheduler and allocator sequence, but no mandatory Important-preserving primitive is established.

7. Call-site confidence

Call-site confidence: medium

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

Patch: mm/slab: do not wake up kswapd in __kfree_rcu_sheaf() [ Upstream
Commit: 46545c86b54ffb969b6e3662942b4a6191619eaa
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=46545c86b54ffb969b6e3662942b4a6191619eaa

Changed files:
  mm/20260831143500.x-saxdAs
  mm/slub.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=46545c86b54ffb969b6e3662942b4a6191619eaa

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

The original Linux kernel CVE announcement for CVE-2026-98818 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98818

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: 0

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).