[CVE-2026-89682][MODERATE 7.0] nfsd: fix fcache_disposal UAF by inlining dispose state into nfsd_net [ Upstream

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-89682
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: nfsd: fix fcache_disposal UAF by inlining dispose state into nfsd_net [ Upstream
Commit: ba0ee9e04b7a0356c28d5a2fe445db675d24e572
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ba0ee9e04b7a0356c28d5a2fe445db675d24e572
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89682
Analysis date: Fri, 11 Sep 2026 23:37:55 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89682 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89682

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 7.0
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-2026-89682	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89682 CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-362;*CWE-672;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A use-after-free exists in the NFSD file cache disposal path because a separately allocated per-net disposal object can be freed during net namespace teardown while global shrinker, laundrette, or fsnotify callbacks may still be inside nfsd_file_dispose_list_delayed() and dereference it. The bug is a race condition with a real UAF primitive against a spinlock and list head used for deferred file disposal, so the realistic impact is at least a kernel crash or local denial of service. For the CVSS the PR:L is used for the paranoid score because reduced-capability root, container service context, or delegated nfsd namespace management may be enough to reach the affected teardown and disposal paths. The issue is not directly network reachable as a standalone packet-triggered bug. Remote NFS traffic may help populate or age the file cache, but reliable triggering depends on local nfsd or net namespace lifecycle control. Impact is at least local denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to use-after-free memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	YES	LOCK DISK INIT NOMEMCHK UAF NETWORK HARDWARE KPANIC 	YES	NO	checked

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

ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 4
* Applied conservative floor/cap: Real UAF floor 4 applied because a stale freed object is dereferenced and modified, and host-admin-equivalent reachability is not positively established.
* Raw paranoid score: 5
* Paranoid score: 5
* Applied paranoid floor/cap: none.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Conservative signals:

* +2 Real UAF / lifetime violation: `nn->fcache_disposal` points to a separately allocated `struct nfsd_fcache_disposal` that can be freed during per-net teardown while shrinker, laundrette, or fsnotify paths can still dereference it in `nfsd_file_dispose_list_delayed()`.
* +1 Reliable kernel crash / strong DoS: stale use of a freed spinlock/list object can realistically fault or corrupt list state enough to crash the kernel.
* +0 No host-admin penalty: host-admin-equivalent privilege is not positively established from the supplied patch.
* +0 No race penalty: the issue is a race, but the supplied evidence does not prove a narrow, externally uncontrollable, one-shot timing condition sufficient for a negative point.

Paranoid additional signals:

* +1 Weak exploitation/corruption concern: the stale object contains a `spinlock_t` and `list_head`, and the stale path performs write-like operations such as `spin_lock()` and `list_move_tail()` through freed storage. This is more than a pure read-only stale dereference.
* +1 Attacker-influenced active-object/lifetime concern: in a delegated nfsd or namespace-management scenario, the same actor may be able to influence file-cache population and teardown cycles, leaving unresolved reuse/list-corruption leverage. This is not treated as controlled reclaim or proven LPE.

Important non-awarded signals:

* Controlled reclaim is not demonstrated.
* Arbitrary or directed kernel write is not demonstrated.
* Concrete repeated release/refcount mutation on the exact same dangling reference is not established.
* Structured stale-object leverage is not separately awarded beyond the write-like freed lock/list operation.
* Weak LPE is not upgraded to Strong LPE because no controlled replacement, callback control, credential corruption, or arbitrary R/W primitive is shown.
* Extra Availability is not awarded because the same crash/DoS is already counted once.
* Confidentiality and Integrity are not awarded as direct impacts in the conservative score because the patch does not establish disclosure or unauthorized modification of protected data.
* Remote reachability is not awarded. Remote NFS clients may help populate or age file-cache state, but the complete trigger depends on local nfsd or net namespace lifecycle activity.
* Concrete repeated release/refcount check: first release can end lifetime for `struct nfsd_fcache_disposal`, but the later stale operation is not a second `put` or refcount decrement on the exact same object. Dedicated repeated release/refcount +1 is not awarded. FLOOR 5 is not applied via the repeated-release rule.

3. Reachability analysis

The directly affected code is in NFSD file-cache disposal. The vulnerable object is per-net NFSD disposal state that is freed during network namespace teardown, while global shrinker, laundrette worker, or fsnotify callbacks may still be operating on file-cache disposal paths.

A remote NFS client is not enough by itself to trigger the full bug as a packet-only network attack. Remote activity can plausibly influence which files are cached, aged, or closed, but reliable exploitation requires the teardown/lifecycle side as well.

The complete privilege boundary is unresolved. Starting, stopping, or tearing down NFSD instances and relevant network namespaces often involves privileged service or container-management operations, but the supplied patch does not prove that full host-admin-equivalent privilege is required in all deployments. Therefore `HostAdminRequired=UNKNOWN`.

The namespace angle is also unresolved. The bug is per-net, but the mere presence of a network namespace does not prove that an ordinary unprivileged user can create all required NFSD state, run the service, populate the file cache, and tear it down. Therefore `NamespaceScopedPrivilege=UNKNOWN`.

No special hardware is required. No direct public-Internet network trigger is established. Race repeatability depends on the ability to drive NFSD file-cache activity and namespace/service teardown repeatedly.

4. Primitive and severity interpretation

The exact primitive is a real use-after-free of `struct nfsd_fcache_disposal`. The freed object contains active synchronization and list state. Stale users can dereference and modify the freed spinlock and list head while moving `nfsd_file` entries to the deferred disposal queue.

This is active structured stale storage, not a fixed async buffer. It is also not a clean repeated-release/refcount bug because the later stale operation is lock/list manipulation rather than a second lifetime put on the exact same reference.

The demonstrated consequence is kernel crash or local denial of service. The speculative consequence is memory corruption through stale lock/list operations if the freed object is reused in a meaningful way, but controlled reclaim or a practical privilege-escalation chain is not established.

PrimitiveClass is CLASS_B because a concrete kernel lifetime primitive exists, but no Class A primitive such as controlled reclaim, arbitrary write, callback control, or demonstrated LPE is shown.

ImportantCandidate is YES because the UAF touches active lock/list state and leaves meaningful exploitation leverage unresolved. AutoDowngradeAllowed is NO because automatically closing this as ordinary crash-only cleanup would discard a real active-object UAF with write-like stale operations.

Likely operational severity is Actionable Moderate, with Strong Important review needed if a product environment allows below-host-admin nfsd namespace lifecycle control or if further analysis shows controlled reuse of the freed disposal object.

5. One-sentence report phrase

A real UAF in NFSD file-cache disposal can occur when per-net disposal state is freed during namespace teardown while shrinker, laundrette, or fsnotify paths still dereference and modify its lock/list fields, causing at least kernel DoS and requiring manual review for unresolved memory-corruption leverage.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a real active-object UAF involving freed synchronization and list state, with no proven controlled exploit primitive but enough unresolved corruption leverage to prevent automatic downgrade.

7. Call-site confidence

Call-site confidence: high

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

Patch: nfsd: fix fcache_disposal UAF by inlining dispose state into nfsd_net [ Upstream
Commit: ba0ee9e04b7a0356c28d5a2fe445db675d24e572
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ba0ee9e04b7a0356c28d5a2fe445db675d24e572

Changed files:
  fs/nfsd/filecache.c
  fs/nfsd/netns.h

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=ba0ee9e04b7a0356c28d5a2fe445db675d24e572

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

The original Linux kernel CVE announcement for CVE-2026-89682 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89682

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:H/I:H/A:H
  The Best / paranoid CVSS score: 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: 4
  Paranoid ActionableScore: 5

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: YES
Published priority for this report (same as in Subject): MODERATE 7.0

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).