[CVE-2026-89685][MODERATE 7.0] nfsd: fix clock domain mismatch in clients_still_reclaiming() [ Upstream

[CVE-2026-89685][MODERATE 7.0] nfsd: fix clock domain mismatch in clients_still_reclaiming() [ Upstream

[AL-KERNEL]

CVE: CVE-2026-89685
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: nfsd: fix clock domain mismatch in clients_still_reclaiming() [ Upstream
Commit: f9cec313efb2fd18d962aad738a4892bce806016
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f9cec313efb2fd18d962aad738a4892bce806016
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89685
Analysis date: Fri, 11 Sep 2026 19:05:16 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89685 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89685

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-2026-89685	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89685 CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H';CWE-682;**CWE-400;CWE-754;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7.5';DESCR 'An nfsd grace period logic bug occurs because clients_still_reclaiming computes a reclaim deadline from CLOCK_REALTIME boot_time but compares it with CLOCK_BOOTTIME. This clock domain mismatch can make the reclaim deadline effectively unreachable and can allow an NFSv4 client to keep the server in grace by sending CLAIM_PREVIOUS OPEN requests. While the server remains in grace, normal non-reclaim operations for other clients can be blocked, causing a remote denial of service for the exported NFS service. For the CVSS the PR:N is used because no privileges on the vulnerable server are required beyond network access to the NFS service. The base score uses AC:H because the trigger depends on the server being in the grace or reclaim phase, while the paranoid score uses AC:L because an attacker on a storage network may be able to observe or repeatedly target server restarts and then keep the condition active. Impact is availability loss only, with no evidence of memory corruption, information disclosure, or privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate (with actual score 5)	YES	LOCK DISK SIMPLEFIX NETWORK LINUS KPANIC  KPANIC INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS	NO	NO	checked

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

ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 4. No floor or cap applied.
* Raw paranoid score: 5
* Paranoid score: 5. No floor or cap applied.
* Final recommended bucket: **Actionable Moderate**::

2. Signal breakdown

Conservative signals:

* +2 Remote reachable / directly network-triggerable. The vulnerable behavior is reached by NFSv4 client protocol traffic to nfsd, specifically CLAIM_PREVIOUS OPEN requests.
* +1 Strong DoS. A client can keep the server in grace indefinitely, blocking non-reclaim operations for other clients.
* +1 Broad/common deployed service path. NFSv4 nfsd is a common enterprise storage service when enabled, and grace/reclaim behavior is a normal server lifecycle path.

Paranoid additional signal:

* +1 Sustained cross-client service impact. The commit states that any client can hold grace indefinitely, making the availability effect broader than a single failed request or transient local stall.

Non-awarded signals:

* No UAF, double free, refcount corruption, OOB access, or memory corruption is established.
* No confidentiality or integrity impact is awarded.
* No privilege escalation or authorization bypass is established.
* No race/timing penalty is applied. This is a deterministic clock-domain logic bug, not a narrow race.
* No host-admin penalty is applied because the attacker only needs network reachability to the NFS service, not host administrative control.
* No resource-exhaustion primitive is awarded because the issue is state blocking, not a leaked resource or accumulating allocation.

3. Reachability analysis

A remote NFSv4 client that can reach the vulnerable nfsd service can send CLAIM_PREVIOUS OPEN requests during the server grace/reclaim window. No local code execution on the server and no host-admin-equivalent privilege are required. The complete trigger does require the NFS server to be running and reachable, and the server must enter or be in the grace period, typically after nfsd start or restart. The supplied text says any client can keep the server in grace indefinitely once this state is reached.

This is not namespace-scoped in the relevant sense. The attack is against the exported NFS service over the network, not against a namespace-local privileged API. No special hardware, firmware, or unusual kernel memory-management condition is required.

4. Primitive and severity interpretation

The primitive is a logic/state bug caused by comparing a deadline derived from CLOCK_REALTIME against CLOCK_BOOTTIME. This makes the double-grace deadline effectively unreachable for normal uptimes and allows reclaim state to persist incorrectly.

PrimitiveClass is CLASS_C because the demonstrated behavior is service denial through invalid server state, without a useful kernel corruption, ownership, lifetime, or authorization primitive. RealUAF is NO. The demonstrated consequence is remote NFS service availability loss for other clients. The speculative consequence remains limited to broader operational outage of the NFS service, not kernel memory corruption or LPE.

ImportantCandidate is NO because there is no Class A primitive and no unresolved Class B corruption/lifetime primitive. AutoDowngradeAllowed is NO because this is a directly network-reachable service-wide DoS with manual triage value, even though it is not an Important-class corruption candidate.

5. One-sentence report phrase

An NFSv4 client can exploit a clock-domain mismatch in nfsd grace-period handling to keep the server in reclaim grace indefinitely, blocking normal operations for other clients and causing a remote denial of service.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the issue is network reachable and can block NFS service availability for other clients, even though it is a logic DoS and not a memory-corruption or privilege-escalation primitive.

7. Call-site confidence

Call-site confidence: high

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

Patch: nfsd: fix clock domain mismatch in clients_still_reclaiming() [ Upstream
Commit: f9cec313efb2fd18d962aad738a4892bce806016
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f9cec313efb2fd18d962aad738a4892bce806016

Changed files:
  fs/nfsd/netns.h
  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=f9cec313efb2fd18d962aad738a4892bce806016

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

The original Linux kernel CVE announcement for CVE-2026-89685 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89685

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