[CVE-2026-89663][IMPORTANT] nfsd: revoke copy-notify stateids before dropping their reference

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

CVE: CVE-2026-89663
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: nfsd: revoke copy-notify stateids before dropping their reference
Commit: 6b8149448cdcb476fba4559d1ecc247efee9971b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6b8149448cdcb476fba4559d1ecc247efee9971b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89663
Analysis date: Sat, 12 Sep 2026 07:09:32 -0400
ActionableScore: 7
ActionableScore lower bound: 6
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-2026-89663 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89663

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: IMPORTANT
Manual review required: YES

A detailed explanation of the methodology and priority rules is included
at the end of this message.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-89663	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-89663 CHECK WITH IMPACT FROM ORIG NN MODERATE	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-672;Other CVSS 'AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7.5';DESCR 'A use-after-free can occur in nfsd COPY_NOTIFY stateid handling because revoke paths could drop the membership reference without first removing the object from the IDR and parent stateid list. A concurrent finder or OFFLOAD_CANCEL holder may still keep a pointer while the entry remains discoverable, and a second revoke or laundromat expiry can free the same copy-notify state before the holder releases it. For the CVSS the PR:L is selected because practical triggering normally requires an authenticated or otherwise permitted NFS client context with access to NFSv4 copy offload related operations. The issue is network reachable when an NFS server exposes affected NFSv4 COPY_NOTIFY functionality to the attacker. Impact is at least denial of service via kernel crash and in the paranoid case may include confidentiality or integrity impact because the bug class is kernel use-after-free in a network service.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate (with actual score 7)	YES	READ DANGER DISK INIT UAF NETWORK LINUS KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=6
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 6
* Conservative score: 6. No floor or cap changed the score.
* Raw paranoid score: 7
* Paranoid score: 7. No floor or cap changed the score.
* Final recommended bucket: **Strong Important candidate**::

2. Signal breakdown

Conservative signals:

* +2 Real UAF / active lifetime violation. The commit directly describes a KASAN-detectable use-after-free where a copy-notify state object remains discoverable after its membership reference has been consumed and can later be freed while a reader still holds it.
* +2 Remote reachable / network-triggerable. The affected code is in nfsd NFSv4 COPY_NOTIFY / OFFLOAD_CANCEL server-side state handling and can be reached by NFS protocol operations when the server exposes this functionality to a permitted client.
* +1 Reliable kernel crash / strong DoS. The described stale holder reaches nfs4_put_cpntf_state() after the object can already have been freed, which supports kernel crash / DoS.
* +1 Concrete repeated release / refcount operation. The vulnerable paths can drop the membership reference without unlinking the object, leaving the same object discoverable for a second revoke, laundromat expiry, or later holder put.

Paranoid additional signal:

* +1 Paranoid repeated-refcount leverage. The second lifetime/refcount operation can act through storage whose original object lifetime may already have ended, leaving unresolved post-lifetime refcount corruption leverage beyond an ordinary stale read.

Important non-awarded signals:

* +0 Strong exploitation leverage not awarded. Controlled reclaim, object replacement, arbitrary read/write, callback control, or demonstrated LPE is not established.
* +0 Structured stale-object leverage not separately awarded. The meaningful additional fact is already counted as concrete repeated release/refcount mutation.
* +0 Weak LPE not separately awarded. It would double-count the same repeated refcount mutation.
* +0 Extra availability not awarded. The crash/DoS is already counted once.
* +0 Confidentiality and integrity not awarded as independent ActionableScore points. The UAF supports manual review, but no disclosure path or unauthorized write primitive is demonstrated.
* +0 Race penalty not applied. The issue involves concurrency, but supplied evidence does not establish a narrow one-shot or externally uncontrollable race sufficient for a negative point.
* +0 Subsystem rarity penalty not applied. NFSv4 copy offload is feature-specific, but the evidence does not establish an unusually rare or unreachable configuration.

Repeated-release/refcount checklist:

* First release can end lifetime: yes. refcount_dec_and_test() can free the copy-notify state.
* Exact same reference survives: yes. The object remains in the IDR and parent list after the membership reference is consumed.
* Second operation mutates lifetime/refcount: yes. A second revoke, laundromat expiry, or holder put can perform another lifetime-affecting operation.
* Dedicated +1 awarded: yes.
* Paranoid repeated-refcount +1 awarded: yes, because the second operation is a concrete write-like refcount/lifetime mutation through potentially post-lifetime storage.
* FLOOR 5 applied: not needed. Raw conservative score is already 6.

3. Reachability analysis

The evaluated attacker is a remote NFS client that can access the affected NFSv4 server functionality, especially COPY_NOTIFY / OFFLOAD_CANCEL related paths. The attacker does not need host administrator privileges on the server, but practical triggering normally requires being an authenticated or otherwise permitted NFS client for an export or server configuration that exposes these operations.

This is not a namespace-scoped local capability issue. NamespaceScopedPrivilege is therefore NO.

There is prior server-side setup in the sense that nfsd and the relevant NFSv4 copy offload functionality must be enabled and reachable, but that is normal service exposure rather than a requirement for the attacker to hold host-admin privileges. No special hardware requirement is indicated.

The bug is concurrent: a holder from find_cpntf_state() / manage_cpntf_state() can race with revoke paths such as OFFLOAD_CANCEL, parent-stid drain, or laundromat expiry. I do not apply a hard-race penalty because the affected state is protocol-created and the revoke paths are repeatable enough that the patch does not support treating this as a rare one-shot timing condition.

4. Primitive and severity interpretation

The exact primitive is a server-side kernel lifetime bug in struct nfs4_cpntf_state. It is an active structured object stored in an IDR and a parent stateid list. The old code decremented the reference count before reliably removing the object from discoverable state, so another path could find or revoke it again after the membership reference had already been consumed.

This is not merely an ordinary stale cleanup dereference. It matches the stronger repeated-lifetime pattern:

A. Ordinary stale cleanup dereference: stale pointer is only read or passed into cleanup and immediately faults.

B. Second lifetime/refcount mutation through a potentially dangling object: an earlier revoke/put can consume the membership reference, the object remains discoverable, and a later revoke/laundromat/holder put can mutate the same lifetime state again.

This case is B. It remains CLASS_B rather than CLASS_A because controlled reclaim, arbitrary write, callback hijack, refcount takeover, and demonstrated privilege escalation are not shown. The demonstrated consequence is kernel UAF and likely crash/DoS. The speculative consequence is stronger memory-corruption impact from post-lifetime refcount mutation, which is plausible enough to require manual review but not enough to claim a proven exploit primitive.

ImportantCandidate is YES because this is a network-reachable kernel UAF with concrete repeated lifetime/refcount mutation and unresolved exploitation leverage. AutoDowngradeAllowed is NO because automatically treating it as ordinary crash-only Moderate would discard a meaningful unresolved post-lifetime corruption primitive.

Likely operational severity is at least Actionable Moderate and potentially Important, depending on exposure of NFSv4 COPY_NOTIFY/offload features and whether further analysis finds useful reclaim or refcount corruption behavior.

5. One-sentence report phrase

A network-reachable nfsd COPY_NOTIFY stateid lifetime bug can leave a revoked object discoverable after its membership reference is consumed, allowing a later revoke or laundromat path to free it while a holder still uses it and creating a UAF with DoS impact and unresolved refcount-corruption leverage.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a real kernel UAF in a network service with a concrete repeated lifetime/refcount mutation on the affected object, so Important-vs-Moderate exploitation leverage remains unresolved even though controlled reclaim or LPE is not demonstrated.

7. Call-site confidence

Call-site confidence: high

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

Patch: nfsd: revoke copy-notify stateids before dropping their reference
Commit: 6b8149448cdcb476fba4559d1ecc247efee9971b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6b8149448cdcb476fba4559d1ecc247efee9971b

Commit description:

Copy-notify stateids live in the s2s_cp_stateids IDR and on their parent
stid's sc_cp_list, pinned by a single membership reference.
_free_cpntf_state_locked() only unlinks an entry once its refcount reaches
zero, so any revoke path that runs while a concurrent
find_cpntf_state()/manage_cpntf_state() holder has elevated cs_count drops
the reference without unlinking, leaving the entry discoverable with its
membership reference already consumed. A second revoke or a laundromat tick
then frees it while the reader still holds the pointer -- a
KASAN-detectable use-after-free at the reader's nfs4_put_cpntf_state().

This affected all three revoke paths:

  - The parent-stid drain (nfs4_free_cpntf_statelist()) repeatedly called
    _free_cpntf_state_locked() on the first list entry; a holder that had
    bumped cs_count made it return early, so the next iteration
    re-decremented and burned the holder's reference.

  - OFFLOAD_CANCEL (manage_cpntf_state()) and laundromat expiry likewise
    used _free_cpntf_state_locked() and could drop 2->1 without unlinking.

Add revoke_cpntf_state_locked(), which unhashes the entry from the IDR and
sc_cp_list first (deferring the final free to any holder), and use it from
all three revoke paths. The drain now walks with list_for_each_entry_safe()
and revokes each entry unconditionally, so it terminates in one pass per
entry regardless of cs_count. The unhash is gated on
!list_empty(&cps->cp_list); the idr_remove() gate matters because
idr_alloc_cyclic() may have recycled the so_id by then. Keep
_free_cpntf_state_locked() for the reference-holder put path only, where a
concurrent revoke may already have unlinked the entry (its list_del_init()
then a no-op).

Fixes: 624322f ("NFSD add COPY_NOTIFY operation")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-7
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260710-nfsd-testing-v3-6-a0ff7db6aa3e@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=6b8149448cdcb476fba4559d1ecc247efee9971b

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

The original Linux kernel CVE announcement for CVE-2026-89663 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89663

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: 6
  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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).