[CVE-2026-98536][MODERATE REGULAR] writeback: report a Tasks-RCU quiescent state per cgwb drain pass

[CVE-2026-98536][MODERATE REGULAR] writeback: report a Tasks-RCU quiescent state per cgwb drain pass

[AL-KERNEL]

CVE: CVE-2026-98536
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: writeback: report a Tasks-RCU quiescent state per cgwb drain pass
Commit: 863049b4018237d1590cd13b16e9fe9c0faad2c6
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=863049b4018237d1590cd13b16e9fe9c0faad2c6
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98536
Analysis date: Sat, 24 Oct 2026 05:14:03 -0400
ActionableScore: 1
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
A long cgroup-writeback drain can starve Tasks-RCU grace periods and, when overlapping BPF/ftrace teardown, block tasks long enough to trigger a host panic.

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

The original Linux kernel CVE announcement for CVE-2026-98536 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98536

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-2026-98536	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98536 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 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-400;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 long cleanup_offline_cgwb drain can run lacking a Tasks RCU quiescent state because cond_resched may be ineffective under full or lazy preemption. A cgwb containing millions of attached inodes can therefore delay synchronize_rcu_tasks for minutes, block BPF or ftrace detach while locks are held, leave dependent tasks in D state, and eventually cause a hung task panic. For the CVSS the PR:H base value reflects that reliable triggering normally requires administrative control over cgroup offlining or privileged tracing operations. For the CVSS the PR:L paranoid value covers delegated environments where an untrusted workload can create the inode population and normal orchestration performs the remaining cleanup. AC:H reflects the large inode population and required overlap with a Tasks RCU waiter. The demonstrated impact is availability loss only and there is no supported UAF, OOB access, information leak, or write 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 1 with recommendation Ordinary Moderate. 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 4.7 and ActionableScore 1 with primitive class CLASS_C and manual review REQUIRED. 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 Ordinary Moderate (with actual score 1)	MAYBE	INIT BPF 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=1
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 base score: 1.
* Conservative score: 1; no floor or cap applied.
* Raw paranoid score: 1.
* Paranoid score: 1; no additional technically supported primitive.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Reliable kernel crash/strong DoS: an observed 11-minute cgwb drain prevented a Tasks-RCU grace period; a waiter held the trampoline mutex, tasks entered D state, and the hung-task policy panicked the host.
* +1 Broad, plausibly exercised subsystem: cgroup writeback cleanup and Tasks-RCU users such as BPF/ftrace detach are deployed kernel paths.
* -1 Hard/high-effort trigger conditions: the demonstrated case required millions of attached inodes, cgroup writeback offlining, and overlap with a Tasks-RCU waiter.
* No separate availability point: blocked tasks and panic are consequences of the same grace-period stall.
* No UAF, OOB write, stale-object corruption, authorization bypass, disclosure, or integrity primitive is shown. B5-R does not apply.
* No local-unprivileged bonus: an untrusted workload may populate a delegated cgroup, but the supplied evidence does not show that it can independently perform the complete offlining-plus-waiter trigger.

3. Reachability analysis

The complete attacker role is unresolved. Host administration or orchestration normally controls cgroup offlining and privileged BPF/ftrace detach, while a delegated workload may influence inode population. That is a compound trigger rather than proof that one below-host-admin principal controls every stage. The governing capability namespaces and any required prior delegation are not established, so both host-admin and namespace-scoped status remain unknown. No special hardware is required.

4. Primitive and severity reading

The defect is a scheduler/Tasks-RCU liveness failure: `cond_resched()` can fail to report a quiescent state during a long drain, delaying `synchronize_rcu_tasks()`. This is CLASS_C availability behavior, not memory or active structured-state corruption. Host panic is demonstrated under a configured hung-task panic policy, but non-crash exploit leverage is absent. ImportantCandidate is therefore NO, and auto-downgrade is allowed.

5. One-sentence report phrase

A long cgroup-writeback drain can starve Tasks-RCU grace periods and, when overlapping BPF/ftrace teardown, block tasks long enough to trigger a host panic.

6. Manual review advice

MANUAL CHECK RECOMMENDED

Confirm product hung-task panic settings, delegated cgroup population, and whether orchestration can supply the remaining trigger stages.

7. Call-site confidence

Call-site confidence: high

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

Patch: writeback: report a Tasks-RCU quiescent state per cgwb drain pass
Commit: 863049b4018237d1590cd13b16e9fe9c0faad2c6
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=863049b4018237d1590cd13b16e9fe9c0faad2c6

Commit description:

cleanup_offline_cgwbs_workfn() drains a dying cgwb by calling
cleanup_offline_cgwb() until it returns false, with a cond_resched()
between passes.  On a CONFIG_PREEMPTION kernel that cond_resched() does
nothing: _cond_resched() is a plain "return 0", and under PREEMPT_DYNAMIC
the full and lazy modes disable it.  Since commit 7dadeaa ("sched:
Further restrict the preemption modes") those are the only two models on
the architectures with PREEMPT_LAZY support, arm64 and x86 among them, so
the drain loop never reports a Tasks-RCU quiescent state.

A worker draining a cgwb with millions of attached inodes runs for
minutes.  On a 6.18 arm64 host in lazy mode the cgwb worker drained one
dying cgroup's writeback domain for over 11 minutes.  A BPF program unlink
(bpf_trampoline_unlink_prog -> bpf_trampoline_update ->
unregister_ftrace_direct -> ftrace_shutdown -> synchronize_rcu_tasks())
waited on that grace period while holding the trampoline mutex, 42 tasks
queued behind it in D state, and the hung task detector fired at 614 s and
panicked the host.  Any BPF or ftrace detach during a long drain inherits
the drain's length.

Fix this by calling cond_resched_tasks_rcu_qs() so we do not stall out
anybody who calls sycnrhonize_rcu_tasks().  We put this in a do { } while
loop because if we have many small cgroups cleanup_offline_cgwb() will
return false and we will never call cond_resched_tasks_rcu_qs(), creating
the same problem.

Link: https://lore.kernel.org/20260909-cgwb-tasks-rcu-qs-v1-1-967a7754771f@toxicpanda.com
Fixes: c22d70a ("writeback, cgroup: release dying cgwbs by switching attached inodes")
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Link: https://lore.kernel.org/bpf/9d444098-7c03-4163-af12-bd0a79a51443@paulmck-laptop/
Assisted-by: LLM
Acked-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>
Reviewed-by: Jan Kara <jack@suse.cz>
Acked-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Dennis Zhou <dennis@kernel.org>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: "Paul E . McKenney" <paulmck@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  mm/backing-dev.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=863049b4018237d1590cd13b16e9fe9c0faad2c6

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

The original Linux kernel CVE announcement for CVE-2026-98536 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98536

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: 1

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).