[CVE-2026-89676][IMPORTANT] nfsd: fix stale s2s_cp_stateids IDR entry for async COPY

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

CVE: CVE-2026-89676
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: nfsd: fix stale s2s_cp_stateids IDR entry for async COPY
Commit: 9b4e5e9ba5ae13808b8a6d229d87c54611ba0e7a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9b4e5e9ba5ae13808b8a6d229d87c54611ba0e7a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89676
Analysis date: Fri, 11 Sep 2026 23:12:10 -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
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The original Linux kernel CVE announcement for CVE-2026-89676 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89676

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-2026-89676	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-89676 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-825;*CWE-664;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 async COPY lifetime bug can leave the s2s_cp_stateids IDR pointing at cp_stateid memory from a transient per request COMPOUND buffer. That buffer can be reused by later requests while the background copy is still active, so IDR walkers may dereference stale or reused request memory as a copy state object. The laundromat path can read cs_type from this stale memory and may later reach refcount_dec(), idr_remove(), or kfree() on invalid data if the bytes match an expired NFS4_COPYNOTIFY_STID. For the CVSS the PR:L is selected because practical triggering requires network access to an NFS export and the ability to issue relevant NFSv4.2 COPY related operations, but does not require local code execution on the server. The issue is network reachable on systems exposing vulnerable nfsd functionality. Impact is at least denial of service via kernel crash and in the worst case may include confidentiality or integrity impact due to a UAF style stale pointer and possible memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate (with actual score 7)	YES	DANGER DISK INIT SIMPLEFIX NETWORK LINUS KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	NO	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 applied.
* Raw paranoid score: 7
* Paranoid score: 7. No floor or cap applied.
* Final recommended bucket: **Strong Important candidate**::

2. Signal breakdown

Positive signals:

* +2 Real UAF / lifetime violation. The IDR stores a pointer to cp_stateid inside the per request COMPOUND buffer, but that buffer is transient and reused by later requests while the async COPY remains active.
* +2 Remote reachable / network-triggerable. The affected code is in nfsd async COPY processing and can be reached through NFSv4.2 COPY related operations against a vulnerable NFS server.
* +1 Reliable kernel crash / strong DoS. IDR walkers can dereference reused request memory, and the described paths can reach refcount_dec(), idr_remove(), or kfree() on invalid interpreted state.
* +1 Structured stale-object exploitation leverage. The stale IDR entry is not only read as a raw pointer. It is interpreted as an active copy state object, including cs_type and lifetime operations.
* Paranoid additional +1 Attacker-influenced reused state. Later NFS requests can reuse the same per request buffer while the background copy is active, so the stale object contents may be influenced by attacker-driven request traffic.

Non-awarded signals:

* Strong exploitation leverage +2 not awarded. Controlled reclaim, arbitrary kernel read/write, callback control, refcount takeover, or demonstrated LPE is not established.
* Concrete repeated release/refcount +1 not awarded as a separate rule. The patch shows refcount_dec()/kfree() on garbage through stale structured interpretation, but it does not clearly establish first release then exact same reference survives then second release on that same reference.
* Extra Availability +1 not awarded. The kernel crash/DoS is already counted once.
* Confidentiality and integrity bonuses not awarded separately. They remain plausible worst-case concerns from memory corruption, but no concrete disclosure or unauthorized modification primitive is demonstrated.
* Race/timing penalty not applied. The issue depends on async COPY lifetime and buffer reuse, but the evidence does not prove a narrow one-shot race that is externally uncontrollable.

3. Reachability analysis

The evaluated attacker is a remote NFS client with access to the vulnerable NFS service and the ability to issue relevant NFSv4.2 COPY related operations. Host administrator privilege on the server is not required by the supplied evidence. This is not a namespace-scoped local privilege issue.

The complete trigger depends on nfsd async COPY support, an exposed export that permits the relevant operation, a background async COPY lifetime, and later IDR walking through paths such as the laundromat or manage_cpntf_state(). The vulnerable condition is network reachable when such NFS server functionality is exposed to the attacker. It is more realistic in environments where NFSv4.2 server-side copy is enabled and accessible from a storage or internal network, rather than arbitrary Internet exposure.

No special hardware requirement is indicated. No host-admin-equivalent local setup by the attacker is established. The async lifetime and buffer reuse requirement increases practical complexity, but the supplied evidence describes a concrete stale IDR pointer and concrete later walkers rather than a purely speculative race.

4. Primitive and severity interpretation

The exact primitive is a stale lifetime/ownership bug in nfsd async COPY state registration. nfs4_init_copy_state() registered a stateid from the transient request-local COMPOUND buffer instead of the stable async_copy object. As a result, s2s_cp_stateids can contain a dangling pointer to memory whose request lifetime has ended and whose storage may be reused by later requests.

This is an active structured stale-object case, not fixed stale storage. The stale memory is later interpreted as copy state and can drive cs_type checks and lifetime operations such as refcount_dec(), idr_remove(), and kfree(). That is stronger than an ordinary stale cleanup dereference, but not enough by itself to prove CLASS_A exploitation such as arbitrary write or controlled reclaim.

PrimitiveClass is CLASS_B. Demonstrated consequence is at least kernel crash or host DoS. Speculative consequence includes broader memory corruption and possible confidentiality or integrity impact if reused request memory can be shaped into useful state, but this requires manual exploitation analysis.

ImportantCandidate is YES because this is a network-reachable active structured stale pointer with possible lifetime/free operations on invalid data. AutoDowngradeAllowed is NO because automatic closure would lose a meaningful unresolved kernel lifetime-corruption primitive.

Likely operational severity is high Actionable Moderate to Strong Important candidate, depending on deployment exposure and whether further analysis confirms attacker control over reused COMPOUND buffer contents and downstream lifetime effects.

5. One-sentence report phrase

An nfsd async COPY lifetime bug can leave s2s_cp_stateids pointing at transient COMPOUND request memory that is later reused and interpreted as copy state, allowing a remote NFS client with COPY access to trigger stale-object dereference and possible refcount or free operations on invalid data, causing at least kernel DoS and requiring manual review for memory-corruption impact.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the bug is network reachable in nfsd and establishes a real stale structured object/lifetime primitive with possible refcount_dec(), idr_remove(), and kfree() on reused request memory.

7. Call-site confidence

Call-site confidence: high

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

Patch: nfsd: fix stale s2s_cp_stateids IDR entry for async COPY
Commit: 9b4e5e9ba5ae13808b8a6d229d87c54611ba0e7a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9b4e5e9ba5ae13808b8a6d229d87c54611ba0e7a

Commit description:

For an async COPY, nfsd4_copy() called nfs4_init_copy_state() before
dup_copy_fields(), so the s2s_cp_stateids IDR was pointed at
&u->copy->cp_stateid -- memory in the per-rqstp COMPOUND buffer that is
reused by the next request. dup_copy_fields() copies only the value into
async_copy, so the IDR slot dangled at the transient buffer for the whole
background copy. Any IDR walker then dereferences reused request memory:
the laundromat reads cs_type from it and, if the bytes look like an
expired NFS4_COPYNOTIFY_STID, follows into
refcount_dec()/idr_remove()/kfree() on garbage; manage_cpntf_state() has
the same exposure via idr_find().

Duplicate the fields first, then register the stateid on the stable
async_copy. result->cb_stateid is unchanged.

Fixes: e0639dc ("NFSD introduce async copy feature")
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/20260710-nfsd-testing-v3-3-a0ff7db6aa3e@kernel.org
Signed-off-by: Chuck Lever <cel@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/nfsd/nfs4proc.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=9b4e5e9ba5ae13808b8a6d229d87c54611ba0e7a

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

The original Linux kernel CVE announcement for CVE-2026-89676 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89676

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).