[CVE-2026-89688][MODERATE 7.0] nfsd: drop the stateid, not the stateowner, on seqid_op replay retry

[CVE-2026-89688][MODERATE 7.0] nfsd: drop the stateid, not the stateowner, on seqid_op replay retry

[AL-KERNEL]

CVE: CVE-2026-89688
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: nfsd: drop the stateid, not the stateowner, on seqid_op replay retry
Commit: 69ed78b6b947c9257213164678b4edf17533093b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=69ed78b6b947c9257213164678b4edf17533093b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89688
Analysis date: Fri, 11 Sep 2026 23:30:05 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate
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-2026-89688 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89688

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-2026-89688	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89688 CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-911;CWE-362;Other CVSS 'AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7.5';DESCR 'An NFS server state handling bug can occur in nfs4_preprocess_seqid_op because the retry path drops a stateowner reference that was not acquired while leaking the stid reference that was actually held. This can cause stateowner refcount underflow and a possible use-after-free, and the leaked stid reference can also stall close processing that waits for sc_count to drop. For the CVSS the PR:L is used because a remote attacker typically needs NFS access sufficient to create or replay NFSv4 stateful operations, but does not need local code execution on the server. The issue is network reachable when the NFS server is exposed to the attacker, although NFS services are usually limited to trusted storage or internal networks. Impact is at least denial of service via kernel crash or stuck state cleanup, and in the paranoid case may include confidentiality and integrity impact due to the UAF class.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate (with actual score 6)	YES	OOB DEADLOCK DISK LEAK ERRORPATH UAF NETWORK LINUS KPANIC  DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH	YES	NO	checked

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

ActionableScore=7
ActionableScoreLower=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 5
* Conservative score: 5. FLOOR 5 applies but does not change the score.
* Raw paranoid score: 7
* Paranoid score: 7. No floor or cap changes the score.
* Final recommended bucket: **Strong Important candidate**::

2. Signal breakdown

Conservative signals:

* +2 Real UAF / lifetime violation. The retry path calls nfs4_put_stateowner() on a stateowner reference that was not acquired, which can underflow so_count and cause use-after-free.
* +2 Network reachable. The affected code is in NFS server NFSv4 state operation handling and can be reached by a remote NFS client with sufficient protocol access.
* +1 Reliable kernel crash / strong DoS. The described refcount underflow and UAF can crash the kernel, and the leaked stid reference can stall close cleanup.
* +1 Concrete persistent resource/reference leak. The sc_count reference acquired on the stid is leaked on the buggy retry path.
* -1 Hard timing / teardown condition. The vulnerable path requires the replay owner to be in teardown with RP_UNHASHED and nfsd4_cstate_assign_replay() returning -EAGAIN.

Paranoid additional signals:

* +1 Attacker-influenced active-object lifetime concern. A remote NFSv4 client can influence creation, replay, and teardown of stateful server objects, and the vulnerable operation mutates lifetime state of an active structured object.
* The conservative -1 timing penalty is not applied in the paranoid score because repeated NFS state operations may make the teardown window practically reachable under load.

Important non-awarded signals:

* Controlled reclaim is not demonstrated.
* Arbitrary kernel read or write is not demonstrated.
* Callback or function-pointer control is not demonstrated.
* Confidentiality and integrity bonuses are not awarded directly. The score preserves the unresolved UAF and refcount-corruption risk instead.
* Extra availability is not awarded because the crash and cleanup stall are already covered by the DoS and reference leak signals.
* Concrete repeated release/refcount on the exact same dangling reference is not fully established as a two-step same-reference double-put sequence. The bug is still a concrete wrong lifetime/refcount mutation.
* Weak LPE is not separately awarded because that would double count the same unresolved lifetime corruption concern.
* Strong LPE / Class A is not awarded because no controlled reclaim, arbitrary R/W, or demonstrated privilege escalation is supplied.

Repeated-release/refcount checklist:

* First release can end lifetime: yes, nfs4_put_stateowner() can decrement so_count and may destroy/free the stateowner.
* Exact same reference survives into a second concrete put: not established from the supplied patch.
* Second operation is a real lifetime/refcount mutation: not established as a separate second put on the same dangling reference.
* +1 Concrete repeated release/refcount: not awarded.
* Paranoid repeated-refcount leverage +1: not awarded under the strict repeated-release rule.
* FLOOR 5: applied because RealUAF=YES, host-admin privilege is not required, the object is active structured NFS server state, and the bug is a network-reachable lifetime/refcount corruption rather than fixed stale storage.

3. Reachability analysis

A remote NFSv4 client with access to the server can plausibly reach nfs4_preprocess_seqid_op() through stateful operations involving stateids and replay handling. This does not require local code execution or host administrator privileges on the NFS server, so HostAdminRequired=NO.

The attacker still likely needs NFS protocol access to the exported service and suitable NFSv4 state, so this is not anonymous Internet reachability in every deployment. NFS servers are commonly restricted to storage or internal networks, but that is a deployment restriction rather than a code-level host-admin requirement.

NamespaceScopedPrivilege=NO because this is not a namespace-local capability path. The relevant exposure is the host NFS server network service.

There is a timing/state condition: the replay owner must be in teardown and nfsd4_cstate_assign_replay() must return -EAGAIN. This supports AC:H-like reasoning for CVSS and the conservative -1 ActionableScore timing penalty, but it does not make the issue local-only or host-admin-only.

No special hardware is required. Prior setup is the normal existence and exposure of an NFS server.

4. Primitive and severity interpretation

The exact primitive is a wrong reference ownership operation in NFS server state handling. nfsd4_lookup_stateid() returns a stateid/stid reference, but the error path drops the associated stateowner instead of dropping the stid reference actually held.

This is an active structured object lifetime bug, not fixed stale storage. The affected stateowner and stid participate in NFSv4 state management, replay handling, close processing, and refcount-based lifetime control.

The demonstrated consequences are stateowner refcount underflow, possible use-after-free, leaked stid reference, kernel crash, and stalled close cleanup. The speculative consequence is stronger exploitation through post-lifetime corruption of active NFS server state, but no controlled reclaim or arbitrary write is shown.

PrimitiveClass=CLASS_B because a real kernel lifetime/refcount corruption primitive is established, but Class A exploitation leverage is not demonstrated.

ImportantCandidate=YES because this is a network-reachable active-object UAF/refcount corruption in kernel nfsd state, and the exploitation leverage beyond crash is materially unresolved.

AutoDowngradeAllowed=NO because automatically closing this as ordinary DoS would discard a concrete network-reachable kernel lifetime/refcount corruption primitive.

Likely operational severity is at least actionable Moderate. The paranoid interpretation reaches Strong Important candidate because of remote NFS reachability and unresolved active-object UAF/refcount corruption.

5. One-sentence report phrase

A network-reachable NFSv4 server state handling bug drops an unowned stateowner reference on a replay retry path, causing possible refcount underflow, use-after-free, leaked stid references, kernel crash, or stalled state cleanup.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the patch establishes a real network-reachable kernel lifetime/refcount corruption primitive with possible UAF in active NFS server state, and exploitation leverage beyond DoS is unresolved.

7. Call-site confidence

Call-site confidence: high

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

Patch: nfsd: drop the stateid, not the stateowner, on seqid_op replay retry
Commit: 69ed78b6b947c9257213164678b4edf17533093b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=69ed78b6b947c9257213164678b4edf17533093b

Commit description:

In nfs4_preprocess_seqid_op() the stateid is obtained from
nfsd4_lookup_stateid(), which holds a reference on the nfs4_stid
(sc_count) but takes no reference on the stateowner. openlockstateid()
merely casts that stid and likewise takes no reference.

When nfsd4_cstate_assign_replay() returns -EAGAIN (the replay owner is
being torn down, RP_UNHASHED) it has not taken a stateowner reference on
that path. The error handling nevertheless called
nfs4_put_stateowner(stp->st_stateowner), dropping an so_count reference
the function never acquired -- risking a stateowner refcount underflow and
use-after-free -- while leaking the sc_count reference held on the stid.
The leaked stid reference can also stall a concurrent
nfsd4_close_open_stateid() waiting for sc_count to drop.

Drop the reference actually held -- the stid -- before retrying. The
stateowner stays alive through the reference held by the stid. This mirrors
the open path in nfsd4_process_open1(), where the put balances a reference
that path explicitly holds on the stateowner.

Fixes: eec7620 ("nfsd: replace rp_mutex to avoid deadlock in move_to_close_lru()")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260611-nfsd-testing-v2-21-5b90e276f2d9@kernel.org
Signed-off-by: Chuck Lever <cel@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/nfsd/nfs4state.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=69ed78b6b947c9257213164678b4edf17533093b

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

The original Linux kernel CVE announcement for CVE-2026-89688 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89688

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:N/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.5

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: 5
  Paranoid ActionableScore: 7

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).