[CVE-2026-89705][MODERATE 7.0] nfsd: restore rq_status_counter to even on all nfsd_dispatch() exit paths

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

CVE: CVE-2026-89705
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: nfsd: restore rq_status_counter to even on all nfsd_dispatch() exit paths
Commit: b08c30f08d57f30699d36809d92f0b683dca261f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b08c30f08d57f30699d36809d92f0b683dca261f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89705
Analysis date: Fri, 11 Sep 2026 21:05:59 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate
Manual review required: YES

Summary:
An NFSD rpc status sequencing bug can leave rq_status_counter odd after certain nfsd_dispatch() exits, allowing a local netlink dump reader to observe mutating request fields and potentially read past the inline ops array, causing DoS or possible kernel data exposure.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-89705 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89705

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-89705	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89705 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:N/A:H';CWE-362;CWE-367;CWE-125;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:H';BEST CVSS score: '6.3';DESCR 'nfsd_dispatch can leave rq_status_counter in an odd state on cache hit, drop, and encode error exit paths after a request has already been decoded. This breaks the seq-lock style protocol used by nfsd_nl_rpc_status_get_dumpit() and can let the lockless status reader observe actively mutating request fields such as args ops and args opcnt. The risky primitive is an out-of-bounds read past the 8-element inline ops array during compound decode or release. For the CVSS the PR:L is used because a local process must access the nfsd rpc status dump interface, while remote NFS traffic alone is not enough to receive the exposed data. The issue is not directly network reachable as an attacker controlled packet processing bug, but a remote NFS client may help create the server side request states that race with a local status dump reader. Impact is at least local denial of service via kernel crash and in worst case may allow confidentiality impact through kernel memory exposure. YES REQUIRES MANUAL CHECK.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate (with actual score 5)	YES	DISK ERRORPATH NETWORK LINUS  KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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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
* Raw paranoid score: 5
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate**::

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

## 2. Signal breakdown

Conservative signals:

* +1: Limited read-only OOB primitive. The broken seq-lock-like protocol can let `nfsd_nl_rpc_status_get_dumpit()` observe mutating `rqstp` fields and read past the 8-element inline `ops` array.
* +1: Confidentiality plausible. The faulty reader is a netlink dump path, so OOB-read data may affect user-visible status output, though the exact disclosure quality is not fully established.
* +1: Reliable DoS plausible. Kernel OOB read from concurrently mutating request state can plausibly lead to fault or crash.
* +1: Below-host-admin local reachability. Supplied context indicates `PR:L`, because a local process can access the nfsd RPC status dump interface.

Paranoid additional signal:

* +1: Practical chain is strongly implied. The supplied description connects specific `nfsd_dispatch()` exit paths to a permanently odd counter, then to failed retry detection, then to OOB read in the status dump reader.

Non-awarded signals:

* +0: No UAF. The patch does not establish post-free object use.
* +0: No arbitrary write or directed kernel write.
* +0: No controlled reclaim or object replacement.
* +0: No privilege escalation demonstrated.
* +0: No direct remote reachability. Remote NFS traffic may help create server-side request states, but the security-relevant read is through a local netlink dump reader.
* +0: No race penalty. This is a race/consistency issue, but the odd counter can persist, so the evidence does not establish a narrow one-shot timing window.
* +0: No repeated release/refcount signal. There is no first put/free followed by a second lifetime mutation on the same dangling reference.
* +0: No extra availability point. The possible crash is already counted once.
* +0: No Class A disclosure. Possible kernel memory exposure is plausible, but substantial or controlled disclosure is not established.

Repeated-release/refcount checklist:

* First release can end lifetime: no.
* Exact same reference survives first release: not applicable.
* Second operation mutates lifetime/refcount: no.
* Concrete repeated release/refcount +1 awarded: no.
* Paranoid repeated-refcount leverage +1 awarded: no.
* FLOOR 5 for repeated release/refcount applied: no.

## 3. Reachability analysis

The vulnerable state is created in NFSD request processing after a request has been decoded and `rq_status_counter` is set odd. Several non-success exit paths failed to restore the counter to an even value.

The security-relevant reader is `nfsd_nl_rpc_status_get_dumpit()`, a local netlink status dump path. Supplied context supports a `PR:L` style model rather than host-admin-only access, so `HostAdminRequired=NO`.

The issue is not directly network-triggerable in the sense that an unauthenticated remote packet alone receives the leaked data or directly invokes the dump reader. A remote NFS client may influence server-side request states such as cache-hit, drop, or encode-error paths, but a local process reading the RPC status dump is still needed for the described OOB read.

No namespace-scoped capability chain is established. There is no evidence that a complete trigger depends on attacker-created namespaces or namespaced capabilities. `NamespaceScopedPrivilege=NO`.

No special hardware is required. NFSD must be enabled and the rpc status netlink feature must be present. The race is not treated as hard solely because concurrent mutation is involved.

## 4. Primitive and severity interpretation

The exact primitive is a race/consistency bug in a seq-lock-like status protocol. `nfsd_dispatch()` leaves `rq_status_counter` odd on cache-hit, drop, and encode-error exits. The lockless dump reader can then believe request fields are stable and read `args->ops` / `args->opcnt` while they are being changed.

This is not a UAF and not a repeated lifetime mutation. It is best interpreted as a limited OOB read / invalid concurrent structured read.

Primitive class: `CLASS_C`. The demonstrated behavior is primarily invalid-state/OOB-read behavior with possible crash or limited disclosure, but without a concrete write, lifetime, ownership, or authorization-bypass primitive.

Demonstrated consequence: local DoS risk and possible OOB read through the status dump path.

Speculative consequence: stronger kernel memory disclosure could be possible depending on the exact dump formatting and adjacent memory layout, but controlled arbitrary read or LPE is not established.

`ImportantCandidate=NO` because the patch does not establish Class A behavior or a strong Class B lifetime/corruption primitive.
`AutoDowngradeAllowed=NO` because the issue has a concrete OOB read in a user-visible netlink status path and should not be automatically closed without checking disclosure behavior.

Likely operational severity: actionable Moderate. It is not a strong Important candidate from the supplied evidence, but it deserves manual review because the OOB read occurs in a local dump interface and may expose kernel data.

## 5. One-sentence report phrase

An NFSD rpc status sequencing bug can leave rq_status_counter odd after certain nfsd_dispatch() exits, allowing a local netlink dump reader to observe mutating request fields and potentially read past the inline ops array, causing DoS or possible kernel data exposure.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the patch establishes a concrete OOB read condition in a user-visible status dump path, and the exact confidentiality impact depends on how the out-of-bounds data is serialized or otherwise consumed.

## 7. Call-site confidence

Call-site confidence: medium

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

Patch: nfsd: restore rq_status_counter to even on all nfsd_dispatch() exit paths
Commit: b08c30f08d57f30699d36809d92f0b683dca261f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b08c30f08d57f30699d36809d92f0b683dca261f

Commit description:

nfsd_dispatch() sets rq_status_counter to an odd value once a request has
been decoded, and back to an even value once it has been fully processed,
forming a seq-lock like protocol with the lockless reader in
nfsd_nl_rpc_status_get_dumpit().

Only the fully successful path restored the counter to even. The cache-hit
(RC_REPLY), drop (RC_DROPIT / RQ_DROPME) and encode-error paths all return
after the odd-valued store without ever bringing the counter back to even.
Once one of those paths is taken, rq_status_counter is left odd: the next
request's decode ORs in 1 (still odd) and only a subsequent successful
encode restores even. While stuck odd, the dumpit reader treats the rqstp
fields as stable and its retry check compares against the same unchanging
odd value, so it never detects concurrent mutation. This exposes actively
mutating fields (e.g. args->ops / args->opcnt during compound decode and
release) to the lockless reader, which can read past the end of the
8-element inline ops array.

Add a helper that advances the counter to the next even value and call it
on every return path that follows the odd-valued store. The decode-error
path is left untouched as it is reached before the counter is set odd.

Fixes: bd9d6a3 ("NFSD: add rpc_status netlink support")
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/20260611-nfsd-testing-v2-19-5b90e276f2d9@kernel.org
Signed-off-by: Chuck Lever <cel@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/nfsd/nfssvc.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=b08c30f08d57f30699d36809d92f0b683dca261f

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

The original Linux kernel CVE announcement for CVE-2026-89705 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89705

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:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:N/A:H
  The Best / paranoid CVSS score: 6.3

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).