[CVE-2026-89662][MODERATE 7.0] NFSD: Prevent lock owner use-after-free during client teardown

[CVE-2026-89662][MODERATE 7.0] NFSD: Prevent lock owner use-after-free during client teardown

[AL-KERNEL]

CVE: CVE-2026-89662
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: NFSD: Prevent lock owner use-after-free during client teardown
Commit: a6ead6fff3a7d03e49845f048f7000a2e0d34431
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a6ead6fff3a7d03e49845f048f7000a2e0d34431
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89662
Analysis date: Sat, 12 Sep 2026 05:12:02 -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-2026-89662 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89662

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-2026-89662	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89662 CHECK WITH IMPACT FROM ORIG NN LOW	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-362;CWE-667;**CWE-476;Other CVSS 'AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7.5';DESCR 'A use-after-free can occur in the NFSD NFSv4 client teardown path when __destroy_client handles lock owners that are still referenced only by blocked locks. The laundromat can reap blocked locks independently while __destroy_client walks the owner hash table without holding a stable stateowner reference, so the owner lifetime can be lost while it is still being dereferenced. A remote NFSv4 client may be able to reach this path by creating blocked lock state and then triggering client teardown with DESTROY_CLIENTID or related cleanup paths. For the CVSS the PR:L is used because triggering normally requires a client context able to establish NFSv4 state and exercise locking operations on the target server. The issue is network reachable when NFSD is exposed to the attacker, but exploitation is timing dependent and the race window makes AC:H appropriate. Impact is at least denial of service via kernel crash. For the paranoid score, the bug is treated as a kernel use-after-free candidate with possible confidentiality and integrity impact and should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	YES	LOCK DANGER NULLPTR DISK UAF NETWORK LINUS KPANIC  KPANIC INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS	NO	NO	checked

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

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

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 4
* Raw paranoid score: 5
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

No floor or cap changed the conservative score. No floor or cap changed the paranoid score.

2. Signal breakdown

Conservative positive signals:

* +2 Real UAF / active lifetime violation. The commit directly states that the laundromat can free the lock owner while `__destroy_client()` still dereferences it.
* +2 Remote reachable / network-triggerable path. The vulnerable code is in NFSD NFSv4 client teardown and can be reached by a remote NFSv4 client able to create lock state and trigger client teardown.
* +1 Reliable kernel crash / strong DoS. The described consequence is a NULL dereference in `remove_blocked_locks()` in kernel context.

Conservative negative signals:

* -1 Hard or unreliable race / special timing. The UAF requires concurrency between client teardown and the per-net laundromat reaping blocked locks, with lifetime loss occurring in the race window.

Paranoid difference:

* The paranoid score withholds the race penalty because the attacker can plausibly repeat NFSv4 state creation and teardown attempts, and the race is in an ordinary server lifecycle path rather than a one-shot physical or hardware-only condition.

Important non-awarded signals:

* Controlled reclaim is not demonstrated.
* Arbitrary or directed kernel write is not demonstrated.
* Confidentiality and integrity bonuses are not awarded because no disclosure, write primitive, credential change, or protected-object modification is established.
* Structured stale-object exploitation leverage is not separately awarded. The stale object is active and structured, but the shown stale use is primarily teardown cleanup through `remove_blocked_locks()`.
* Weak LPE is not awarded merely because the bug is a UAF.
* Extra availability is not awarded because the kernel crash already accounts for the demonstrated DoS.
* Concrete repeated release/refcount signal is not awarded. A blocked lock release can drop the owner reference and end lifetime, and the stale `so` reference survives in `__destroy_client()`, but the supplied patch does not clearly show a second lifetime/refcount mutation on the exact same dangling stateowner reference. The concrete operation shown is stale dereference/use during cleanup.
* Paranoid repeated-refcount leverage is withheld for the same reason.
* FLOOR 5 is not applied as a floor. The paranoid score reaches 5 by arithmetic, not by the repeated-release floor.

3. Reachability analysis

The evaluated attacker is a remote NFSv4 client with enough access to establish NFSv4 state and exercise locking operations against the NFSD server. Host administrator privilege is not required to send NFS protocol operations once the service is exposed and usable by the client.

The complete trigger appears network reachable when NFSD is configured and reachable. This is not a packet-processing bug reachable by arbitrary Internet traffic in all deployments, because practical reachability depends on NFS export policy, authentication mode, firewalling, and whether the attacker can create relevant lock state. However, those are deployment controls, not evidence that host-admin privilege is required by the vulnerable code path.

No namespace-scoped privilege chain is involved in the supplied evidence. No special hardware or boot mode is required. Prior server configuration is required in the ordinary sense that NFSD must be running and exposed, but that is not scored as a restrictive prerequisite. Race repeatability is plausible because a client can attempt to create blocked lock state and trigger teardown repeatedly, but the exact laundromat interleaving remains timing-dependent.

4. Primitive and severity interpretation

The exact primitive is a real use-after-free / stale lifetime violation of an NFSv4 lock owner object. A lock owner whose only reference is held by a blocked lock remains on `cl_ownerstr_hashtbl`. During client teardown, `__destroy_client()` walks that table without first taking a stable stateowner reference. Independently, the laundromat can reap blocked locks from `nn->blocked_locks_lru`, dropping the owner reference via `flc_owner` and freeing the owner while `__destroy_client()` still uses it.

This is an active structured kernel object, not fixed stale storage. The demonstrated stale use occurs in teardown cleanup, and the documented terminal symptom is a NULL dereference in `remove_blocked_locks()`. The patch fixes the lifetime invariant by taking a stateowner reference under `cl_lock`, unhashing the lock owner, dropping the lock, calling `remove_blocked_locks()`, and then releasing the reference.

This is not classified as CLASS_A because there is no controlled reclaim, controlled replacement object, callback control, arbitrary read/write, or demonstrated privilege escalation. It is CLASS_B because a real kernel lifetime violation exists in a remotely reachable server subsystem and exploitation leverage beyond crash is not fully resolved.

The demonstrated consequence is remote denial of service via kernel crash. The speculative consequence is stronger memory corruption or privilege impact if reclaimed stateowner storage can be usefully reused, but that is not established by the supplied evidence. ImportantCandidate is YES because a remote NFSD UAF in active state/lifetime handling should not be automatically downgraded from a small patch alone. AutoDowngradeAllowed is NO because automatic closure would discard an unresolved kernel lifetime primitive in a network server path, even though Strong Important severity is not proven.

5. One-sentence report phrase

A remote NFSv4 client that can create blocked lock state may trigger a race during NFSD client teardown where a lock owner is freed by the laundromat while `__destroy_client()` still dereferences it, causing at least a kernel crash and requiring manual review for UAF exploitation potential.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: real UAF in a network-exposed kernel server subsystem with active stateowner lifetime loss. The currently demonstrated effect is DoS, but the lifetime primitive is concrete enough that automatic downgrade is not appropriate.

7. Call-site confidence

Call-site confidence: high

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

Patch: NFSD: Prevent lock owner use-after-free during client teardown
Commit: a6ead6fff3a7d03e49845f048f7000a2e0d34431
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a6ead6fff3a7d03e49845f048f7000a2e0d34431

Commit description:

__destroy_client() releases a client's open owners, but a lock owner
whose only reference is a blocked lock (nbl) stays on
cl_ownerstr_hashtbl.  client_has_state() does not count a bare owner,
so DESTROY_CLIENTID can reach __destroy_client() with such owners
present.

__destroy_client() then walks the table, calling remove_blocked_locks()
on each owner without a reference.  Freeing a blocked lock drops the
owner reference held via flc_owner.  The per-net laundromat reaps
blocked locks from nn->blocked_locks_lru independently of client state.
The two paths share blocked_locks_lock only for the list splice, not
the owner's lifetime.  The laundromat therefore frees the owner as
__destroy_client() dereferences it, a NULL dereference in
remove_blocked_locks().

nfsd4_release_lockowner() holds a reference across the same call;
__destroy_client() does not.  Hold cl_lock across the walk, taking a
reference and unhashing each owner, then drop it before
remove_blocked_locks() and nfs4_put_stateowner(), which take
blocked_locks_lock and cl_lock.

Reported-by: Wolfgang Walter <linux@stwm.de>
Closes: https://lore.kernel.org/linux-nfs/6eccafaaaa60651ef091257c3439c46b@stwm.de/
Fixes: 68ef3bc ("nfsd: remove blocked locks on client teardown")
Cc: stable@vger.kernel.org
Reviewed-by: NeilBrown <neil@brown.name>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260709-cel-v4-1-1d519d9be0cb@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=a6ead6fff3a7d03e49845f048f7000a2e0d34431

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

The original Linux kernel CVE announcement for CVE-2026-89662 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89662

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:N/I:N/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: 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).