* [CVE-2026-98538][LOW] writeback: bound cleanup_offline_cgwb() rescans by rotating scanned inodes
@ 2026-10-19 2:27 21% AL-KERNEL
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From: AL-KERNEL @ 2026-10-19 2:27 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-98538
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: writeback: bound cleanup_offline_cgwb() rescans by rotating scanned inodes
Commit: 2d1da58c79b66646721a644c6eb4dd1f9c5c9327
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2d1da58c79b66646721a644c6eb4dd1f9c5c9327
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98538
Analysis date: Sun, 18 Oct 2026 22:27:55 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO
Summary:
Quadratic rescanning during cgroup writeback teardown can cause prolonged CPU soft lockups with extremely large inode populations, but no memory corruption or security-boundary bypass is established.
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ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98538 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98538
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-2026-98538 LOW https://www.kernelcve.org/list/?q=CVE-2026-98538 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT LOW AEGIS_AS=1 LEGACY=LOW RESULT=SAME (final reconciliation agrees; DECISION: both LOW) 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-400;CWE-407;CWE-834;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'cleanup_offline_cgwb repeatedly scans prepared and unswitchable inodes from the same end of the writeback lists, causing the complete drain to have quadratic complexity. A workload with about 17 million inodes attached to one dying cgroup writeback instance produced CPU soft lockups lasting 21 to 48 seconds. For the CVSS the PR:L value reflects a local user or service process that can create a very large inode workload inside a managed or delegated cgroup, even if cgroup teardown itself is performed by a system manager. AC:H reflects the unusually large inode count and the required cgroup writeback teardown conditions. The issue is not network reachable and does not expose a demonstrated UAF, OOB access, information leak, or write primitive. Impact is denial of service through excessive CPU consumption and delayed writeback cleanup. The paranoid score raises availability to A:H only if sustained workload can make the host substantially unresponsive. Automated impact decision path. The original SimpleNet classifier selected MODERATE. The primary LLM estimated paranoid CVSS 4.7 and the production ActionableScore returned 2 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE to LOW 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_LOW_FROM_MODERATE_BASED_ON_GUESSCVSS_AND_CIANNH. The independent weaker Aegis shadow evaluated CVSS 2.7 and ActionableScore 1 with primitive class CLASS_C and manual review RECOMMENDED. Its independently reconciled impact was LOW. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is LOW.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2) MAYBE DEADLOCK DISK HARDWARE LINUS DECREASED_TO_LOW_FROM_MODERATE_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=NO
AutoDowngradeAllowed=YES
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO
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 Reliable strong DoS. Quadratic inode rescanning produced observed CPU soft lockups lasting 21 to 48 seconds.
* +1 Important filesystem and storage path. The defect affects cgroup writeback teardown and can delay draining a dying writeback instance.
* +0 Additional availability. Not awarded because prolonged CPU stalls are the same demonstrated DoS consequence, not an independent persistent failure.
* +0 Resource leak. No resource or reference is permanently lost.
* +0 Memory corruption. The patch establishes repeated in-bounds list traversal, not UAF, OOB access, invalid write, or list corruption.
* +0 Local unprivileged trigger. Creating many inodes may be attacker-influenced, but control of the complete cgwb creation and teardown sequence is not established.
* +0 B5-R. There is no heap OOB write, retry-accumulated write offset, or later consumption of corrupted structured state.
* +0 V32, V33, V34, and B3-PENDING. No skb checksum metadata, active structured-state corruption, or shared-backing write exists.
* +0 Authorization bypass and B4-PENDING. No mandatory authorization decision is changed from DENY to ALLOW.
* No privilege, race, niche-subsystem, or hardware penalty was applied because none is positively established.
3. Reachability analysis
A local workload can contribute a very large number of inodes to a cgroup writeback instance. The demonstrated case required approximately 17 million inodes and teardown of a dying cgwb. The supplied evidence does not establish whether one ordinary user can control the complete sequence or whether cgroup lifecycle management must be performed by a separate privileged service.
HostAdminRequired and NamespaceScopedPrivilege therefore remain UNKNOWN. Namespace-scoped cgroup operation, delegated management, and prior privileged setup are not proven. No special hardware or network reachability is involved. This is not a timing race. Its practical complexity comes from the extreme inode count and required writeback teardown state.
4. Primitive and severity interpretation
The exact defect is quadratic algorithmic behavior. Each bounded cleanup pass starts from the same list end and repeatedly scans prepared or unswitchable inodes, making the complete drain quadratic rather than linear.
There is no stale post-lifetime object use, repeated release, refcount mutation, active-state corruption, OOB write, or authorization primitive. PrimitiveClass is therefore CLASS_C because the demonstrated security-relevant behavior is availability degradation through soft lockups.
The demonstrated consequence is prolonged CPU stalls during cgwb cleanup. A more severe host-wide outage is plausible under sustained extreme workload but is not an independent corruption or boundary-crossing primitive. ImportantCandidate is NO and automatic downgrade is allowed because no unresolved memory-corruption, ownership, or authorization mechanism remains.
5. One-sentence report phrase
Quadratic rescanning during cgroup writeback teardown can cause prolonged CPU soft lockups with extremely large inode populations, but no memory corruption or security-boundary bypass is established.
6. Manual review recommendation
NO MANUAL CHECK NEEDED
The issue is a bounded algorithmic DoS with an extreme workload requirement and no demonstrated UAF, OOB write, persistent leak, authorization bypass, or cross-boundary corruption primitive.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: writeback: bound cleanup_offline_cgwb() rescans by rotating scanned inodes
Commit: 2d1da58c79b66646721a644c6eb4dd1f9c5c9327
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2d1da58c79b66646721a644c6eb4dd1f9c5c9327
Commit description:
cleanup_offline_cgwb() prepares at most WB_MAX_INODES_PER_ISW inodes
per call and is called again until the dying wb is drained, but every
call walks wb->b_attached and then wb->b_dirty_time from the same end.
Inodes already prepared (they stay on the list with I_WB_SWITCH set
until the switch worker runs) and inodes that cannot be switched
(I_FREEING, I_WILL_FREE, !SB_ACTIVE, DAX, already on the target wb)
stay where they are, so each pass rescans a growing run of them under
wb->list_lock and a full drain is quadratic in the number of inodes on
the list. With ~17M inodes attached to one dying cgwb we saw this end
in soft lockups, with CPUs reported stuck for 21-48s.
Walk both lists from the oldest end and move every scanned inode to
the newest end, so the next pass starts where the previous one stopped
and the drain becomes linear. b_attached is unordered, so nobody sees
the reorder there. b_dirty_time is ordered by dirtied_when, but the
oldest unscanned inode stays at the end move_expired_inodes() picks
from, sync takes the whole list regardless of order, and prepared
inodes leave the list as soon as the switch work runs and get a new
dirtied_time_when on the new wb anyway, so the only inodes left out of
order are the ones that can never switch (DAX), and only on the dying
wb.
Fixes: c22d70a ("writeback, cgroup: release dying cgwbs by switching attached inodes")
Cc: [email protected]
Acked-by: Tejun Heo <[email protected]>
Acked-by: Roman Gushchin <[email protected]>
Signed-off-by: Patrick Lu (Anthropic) <[email protected]>
Link: https://patch.msgid.link/[email protected]
Reviewed-by: Jan Kara <[email protected]>
Signed-off-by: Christian Brauner (Amutable) <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
fs/fs-writeback.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=2d1da58c79b66646721a644c6eb4dd1f9c5c9327
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98538 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98538
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:L/UI:N/S:U/C:N/I:N/A:L
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: 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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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