[CVE-2026-89670][MODERATE 7.0] nfsd: hold rcu across localio cmpxchg retry

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

CVE: CVE-2026-89670
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: nfsd: hold rcu across localio cmpxchg retry
Commit: 763c0bad872368304db718f79af7e93048885c67
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=763c0bad872368304db718f79af7e93048885c67
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89670
Analysis date: Sat, 12 Sep 2026 04:56:52 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89670 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89670

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-2026-89670	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89670 CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-362;*CWE-672;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A use after free race exists in the nfsd localio path because nfsd_open_local_fh can load an nfsd_file pointer with cmpxchg and then call nfsd_file_get without holding an RCU read side critical section. The object is freed through call_rcu and the cache is not SLAB_TYPESAFE_BY_RCU, so after the RCU grace period the backing memory can be recycled before the refcount operation and later dereferences of nf_net and nf_file. For the CVSS the PR:L is used because a local user or local process normally needs access to a local NFS mount or localio capable NFS workflow, while administrative setup of NFS localio is a deployment prerequisite rather than the direct attacker privilege. The issue is not directly network reachable for ordinary remote NFS clients because the affected path is localio. Impact is at least local denial of service via kernel crash and in the paranoid case may include confidentiality and integrity impact or privilege escalation due to use after free on recycled kernel memory.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6)  SKIP CVE-2026-89670 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	LOCK DISK UAF NETWORK HARDWARE KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH	-	-	checked

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

ActionableScore=6
ActionableScoreLower=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 5
* Raw paranoid score: 6
* Paranoid score: 6
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

Conservative floor applied: 4 -> 5 because this is a real active-object RCU UAF with a concrete post-lifetime refcount mutation through a stale nfsd_file pointer.

2. Signal breakdown

Positive signals:

* +2 Real UAF / lifetime violation: nfsd_open_local_fh() can load an nfsd_file pointer without rcu_read_lock(), while another CPU can drop the last reference and free the object through call_rcu().
* +1 Concrete post-lifetime refcount mutation: nfsd_file_get(new) uses refcount_inc_not_zero() on storage that may already have been freed and recycled.
* +1 Local below-host-admin reachability: the affected path is localio and is plausibly reachable by a local process using a local NFS workflow after deployment setup.
* +1 Reliable kernel crash / strong DoS: after a false successful refcount increment, the caller dereferences new->nf_net and new->nf_file from freed or recycled memory.
* +1 Paranoid repeated-refcount leverage: the stale operation itself performs a write-like refcount mutation on recycled kernel memory, leaving exploitation leverage unresolved beyond a simple stale read.

Negative signals:

* -1 Conservative hard timing: the conservative path requires a race where cmpxchg observes the pointer, another CPU removes and drops the final reference, an RCU grace period elapses, and the slab is recycled before nfsd_file_get().

Material non-awarded signals:

* Controlled reclaim not demonstrated.
* Arbitrary kernel read/write not demonstrated.
* Remote network reachability not established for ordinary remote NFS clients.
* Structured stale-object leverage not separately awarded because no attacker-shaped replacement object is shown.
* Weak LPE not separately awarded because the refcount mutation is already counted.
* Extra Availability not awarded because the same crash is already counted.
* C/I impact not awarded as direct ActionableScore points because confidentiality or integrity effects are not concretely demonstrated.

Repeated/refcount checklist:

* First release can end lifetime: yes, nfsd_file_put(nf) can drop the last reference and queue call_rcu().
* Exact same stale reference survives: yes, CPU0 has the pointer value loaded from cmpxchg before the object lifetime is protected.
* Second operation mutates lifetime/refcount: yes, nfsd_file_get(new) performs refcount_inc_not_zero().
* Dedicated +1 awarded: yes.
* Paranoid repeated-refcount leverage +1 awarded: yes, because the stale refcount increment can operate on recycled memory.
* FLOOR 5 applied: yes.

3. Reachability analysis

The evaluated attacker is a local user or local process able to exercise the NFS localio path, not an ordinary remote NFS client. The vulnerable path is localio, so the issue is not directly network reachable through standard remote NFS traffic.

Host administrator setup may be needed to configure the local NFS and localio environment, but the direct vulnerable operation does not appear to require host-admin-equivalent privilege once that environment exists. Therefore HostAdminRequired=NO for the practical trigger model. No namespace-scoped capability chain is established, so NamespaceScopedPrivilege=NO.

The race is nontrivial because it requires losing RCU protection around cmpxchg and refcount acquisition, with object removal and RCU reclamation occurring before the stale refcount operation. Conservative scoring treats this as hard timing. Paranoid scoring avoids over-penalizing the race because the local workload may be repeatable and the primitive is a real post-lifetime refcount mutation.

4. Primitive and severity interpretation

The exact primitive is an RCU lifetime bug leading to use-after-free of an active structured nfsd_file object. This is not fixed stale storage. The stale path performs a refcount operation and then dereferences structured fields nf_net and nf_file.

This is stronger than an ordinary stale cleanup dereference because the stale operation is not only reading a freed pointer. It can actively modify the refcount word of memory whose original object lifetime has ended. This does not prove CLASS_A because controlled reclaim, refcount takeover, arbitrary write, callback control, or demonstrated LPE is not established.

PrimitiveClass=CLASS_B. Demonstrated consequence is local kernel crash / DoS. Speculative consequence is privilege escalation or confidentiality/integrity impact through recycled-object manipulation, but that requires further exploitability analysis. ImportantCandidate=YES because meaningful post-lifetime refcount corruption leverage remains unresolved. AutoDowngradeAllowed=NO because automatically closing this as ordinary crash-only would lose a concrete UAF plus stale refcount mutation primitive.

Likely operational severity is Actionable Moderate at minimum, with Strong Important candidacy depending on whether manual review finds practical reclaim or object-shaping leverage.

5. One-sentence report phrase

A real RCU use-after-free in nfsd localio allows a stale nfsd_file pointer to be refcounted and later dereferenced after call_rcu reclamation, causing at least a local kernel crash and requiring manual review for possible post-lifetime refcount corruption leverage.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the bug establishes a real active-object UAF and a concrete refcount mutation through potentially recycled kernel memory, which is meaningfully stronger than a simple NULL dereference or crash-only stale read.

7. Call-site confidence

Call-site confidence: high

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

Patch: nfsd: hold rcu across localio cmpxchg retry
Commit: 763c0bad872368304db718f79af7e93048885c67
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=763c0bad872368304db718f79af7e93048885c67

Commit description:

nfsd_file objects are freed via call_rcu (filecache.c:296), and
nfsd_file_slab is created without SLAB_TYPESAFE_BY_RCU
(KMEM_CACHE(nfsd_file, 0) at filecache.c:789), so the slab page
backing a freed nfsd_file becomes freely reclaimable once the RCU
grace period elapses.

The again: retry block in nfsd_open_local_fh() loads a pointer with
cmpxchg and then calls nfsd_file_get(new) (which is
refcount_inc_not_zero) without holding rcu_read_lock. The sole caller
nfs_open_local_fh() drops rcu_read_lock before invoking this helper,
so no outer reader-side critical section covers the load.

    CPU 0 (nfsd_open_local_fh)        CPU 1 (nfsd_file_put_local)
    -----                             -----
    new = cmpxchg(pnf, NULL, ...)
                                      nf = xchg(pnf, NULL)
                                      nfsd_file_put(nf)
                                        last ref -> call_rcu()
                                      /* grace period elapses;
                                         slab page recycled */
    nfsd_file_get(new)
      refcount_inc_not_zero(&new->nf_ref)
      /* operates on recycled memory */

A non-zero word at the nf_ref offset of the recycled object makes the
refcount bump appear to succeed, and the caller then dereferences
new->nf_net and new->nf_file out of freed memory.

Fix by taking rcu_read_lock() immediately before the cmpxchg and
releasing it on all three exits of the if (new) block: the goto-again
retry, the lost-race cleanup path, and the install-succeeded path.
nfsd_file_put() and nfsd_net_put() stay outside the RCU section so
they remain free to block.

Fixes: e6f7e14 ("nfs_localio: simplify interface to nfsd for getting nfsd_file")
Cc: stable@vger.kernel.org
Assisted-by: kres:claude-opus-4-7
Signed-off-by: Chris Mason <clm@meta.com>
Link: https://patch.msgid.link/20260602-nfsd-testing-v2-2-e4ea62e3cd5c@kernel.org
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/nfsd/localio.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=763c0bad872368304db718f79af7e93048885c67

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

The original Linux kernel CVE announcement for CVE-2026-89670 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89670

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:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7

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: 6

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).