[CVE-2026-89691][MODERATE 7.0] nfsd: clear opcnt on compound arg release to prevent OOB read

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

CVE: CVE-2026-89691
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: nfsd: clear opcnt on compound arg release to prevent OOB read
Commit: 58bcdfb2b2e412088839ae740b1a95154dc0b8d0
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=58bcdfb2b2e412088839ae740b1a95154dc0b8d0
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89691
Analysis date: Fri, 11 Sep 2026 22:46:45 -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.

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

The original Linux kernel CVE announcement for CVE-2026-89691 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89691

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-2026-89691	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89691 CHECK WITH IMPACT FROM ORIG NN IMPORTANT	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:N';CWE-125;*CWE-200;CWE-665;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N';BEST CVSS score: '4.7';DESCR 'An out-of-bounds read in the NFSD rpc_status reporting path can expose adjacent slab memory because nfsd4_release_compoundargs resets args ops to the inline iops array but leaves args opcnt stale. A remote NFS client can influence compound metadata and error paths, while a local user that can query the RPC status netlink interface may receive leaked kernel memory when the status counter remains in the in progress state. For the CVSS the PR:L is used for the paranoid score because practical data retrieval requires a local process or service account with access to the netlink status interface, even though stale metadata can be influenced by NFS traffic. The issue is network adjacent for the trigger and local for the leak sink. Impact is primarily confidentiality loss from kernel memory disclosure, with no direct write primitive or kernel crash shown by the patch context. NFS servers are typically deployed inside trusted internal or data-center networks rather than exposed to the public Internet.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6)	YES	OOB DISK SIMPLEFIX LEAK ERRORPATH NETWORK LINUS KPANIC  KPANIC LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHERTHAN5 DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	YES	NO	checked

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

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

1. ActionableScore

* Raw conservative score: 5
* Conservative score: 5. No floor or cap applied.
* Raw paranoid score: 6
* Paranoid score: 6. No floor or cap applied.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Conservative positive signals:

* +2: Concrete user-visible kernel information disclosure. The bug is an OOB read from args->ops after it has been reset to inline iops[8] while opcnt remains stale, and the supplied context states that adjacent slab memory is leaked to userspace through the RPC status netlink dump path.
* +1: Below-host-admin local retrieval path. The supplied analysis states that a local user or service account able to query the RPC status netlink interface can receive the leaked data, so host administrator privilege is not positively required for the leak sink.
* +1: Confidentiality impact is plausible and concrete. This is not merely an internal invalid read, because the out-of-bounds bytes are exposed to userspace.
* +1: Practical trigger-to-disclosure chain is established at a high level. Remote NFS compound metadata and error paths can influence the stale opcnt state, and the local status dump path can disclose the stale out-of-bounds data.

Paranoid additional signal:

* +1: Higher disclosure sensitivity. Adjacent slab memory disclosure from kernel objects may expose sensitive kernel data or pointers, so a higher manual-review score is defensible even though arbitrary read, write, or privilege escalation is not shown.

Non-awarded or withheld signals:

* RealUAF not awarded. The issue is stale metadata and OOB read, not a post-free object dereference.
* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Integrity impact not awarded. The patch and supplied context show read disclosure, not unauthorized modification.
* Reliable kernel crash not awarded. The described consequence is information leak via netlink, not panic or host-wide DoS.
* Remote reachable +2 not awarded. A remote NFS client may influence the stale compound state, but the demonstrated disclosure sink is local netlink status retrieval, so the complete leak path is not purely remote.
* Race penalty not applied. The rq_status_counter odd state is an error-path state condition, not evidence of a narrow externally uncontrollable timing race.
* No privilege penalty applied. Host-admin-equivalent privilege is not positively established for the complete disclosure path.

3. Reachability analysis

The trigger has two relevant stages. A remote NFS client can influence NFSv4 compound metadata and may help create the stale opcnt condition through an error path after rq_status_counter is set odd. The actual disclosure is through the RPC status netlink dumpit handler, which is a local userspace-facing sink.

Based on the supplied hint, the evaluated attacker for the actionable score is a below-host-admin local user or service account that can query the RPC status netlink interface. Therefore HostAdminRequired=NO. If a target distribution restricts this netlink interface to host administrators, the practical score could be lower, but that restriction is not established by the supplied input.

NamespaceScopedPrivilege is UNKNOWN. The supplied material does not establish that the complete trigger and disclosure path is reachable through attacker-created namespaces.

Prior NFS server setup is required in the sense that nfsd and rpc_status support must be present and active, but this is not scored as a rare configuration penalty because the supplied issue is specifically in an NFS server status interface and no unusual hardware or boot-only feature is required.

4. Primitive and severity interpretation

The exact primitive is a bounded out-of-bounds read from the inline iops[8] array after args->ops is reset while args->opcnt remains stale. Because iops is the last field in struct nfsd4_compoundargs, indices 8 through 15 can read adjacent slab memory. The supplied context states that this data is leaked to userspace via netlink, making this a concrete information disclosure primitive rather than a harmless internal OOB read.

This is not an active structured UAF and not fixed stale storage after free. It is stale compound metadata causing a later read beyond the intended inline array. There is no concrete repeated release or refcount mutation.

PrimitiveClass=CLASS_A is used because a kernel memory disclosure to userspace is directly established. It is a low-end Class A case rather than a high-end Important case because the read appears bounded, local retrieval is required, and no arbitrary read, write, LPE, or crash primitive is shown.

ImportantCandidate=YES because kernel slab disclosure can expose sensitive data and should not be automatically downgraded without confirming access restrictions and leaked object content. AutoDowngradeAllowed=NO because the issue has a concrete confidentiality primitive, not merely a warning, crash, or bookkeeping defect.

Likely operational severity is actionable Moderate, with Strong Important review warranted if the netlink status interface is broadly accessible or the leaked adjacent slab contents include kernel pointers, credentials, secrets, or other sensitive state.

5. One-sentence report phrase

An OOB read in the NFSD rpc_status path can leak adjacent slab memory to a local netlink reader when stale NFSv4 compound opcnt metadata survives argument release.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the patch and supplied context establish an actual kernel memory disclosure path to userspace, and the practical severity depends on rpc_status netlink access controls and the sensitivity of adjacent slab contents.

7. Call-site confidence

Call-site confidence: high

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

Patch: nfsd: clear opcnt on compound arg release to prevent OOB read
Commit: 58bcdfb2b2e412088839ae740b1a95154dc0b8d0
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=58bcdfb2b2e412088839ae740b1a95154dc0b8d0

Commit description:

nfsd4_release_compoundargs() resets args->ops to the inline iops[8]
array when the dynamically-allocated ops buffer is freed, but leaves
args->opcnt at its original value (which can be up to 200 for NFSv4.1+
compounds).

If rq_status_counter is stuck at an odd value (which can happen when
nfsd_dispatch() hits an error path after setting it odd), the RPC
status dumpit handler reads min(opcnt, 16) entries from args->ops[].
Since iops only has 8 elements and is the last field in struct
nfsd4_compoundargs, reading indices 8-15 accesses adjacent slab memory
and leaks it to userspace via netlink.

Zero opcnt unconditionally in nfsd4_release_compoundargs() so stale
compound metadata is never exposed through the status interface.

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>
[ cel: Remove the kvfree_rcu_mightsleep() sleep from the exposure window ]
Link: https://patch.msgid.link/20260611-nfsd-testing-v2-1-5b90e276f2d9@kernel.org
Signed-off-by: Chuck Lever <cel@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/nfsd/nfs4xdr.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=58bcdfb2b2e412088839ae740b1a95154dc0b8d0

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

The original Linux kernel CVE announcement for CVE-2026-89691 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89691

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:N
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:N/A:N
  The Best / paranoid CVSS score: 4.7

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).