[CVE-2026-72218][MODERATE 7.0] lockd: Plug nlm_file refcount leak on cached nlm_do_fopen() failure

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

CVE: CVE-2026-72218
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: lockd: Plug nlm_file refcount leak on cached nlm_do_fopen() failure
Commit: 6cd84cefd8b73e85b9eda17b319bd40a670f3a38
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6cd84cefd8b73e85b9eda17b319bd40a670f3a38
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72218
Analysis date: Sun, 16 Aug 2026 02:18:50 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: NO

Summary:
A network-reachable refcount leak in the NFS lockd cached file path can leave `nlm_file` objects unreapable after `nlm_do_fopen()` failures, allowing repeated NLM requests to cause resource exhaustion and denial of service.

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

The original Linux kernel CVE announcement for CVE-2026-72218 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72218

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: NO

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-2026-72218	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';*CWE-772;*CWE-401;**CWE-400;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.9';DESCR 'A reference leak in the NFS lockd cached file path can occur when nlm_do_fopen() fails for an already hashed nlm_file. The error path can increment file f_count but return before the caller receives a valid nlm_file pointer, so the normal cleanup path does not call nlm_release_file(). Repeated failures can keep nlm_file objects unreapable and may lead to resource exhaustion over time. For the CVSS the PR:N is used because an attacker may only need network reachability to the lockd/NLM service and an accepted NFS locking request path, not a local account on the target. The issue is network reachable only when NFS lockd is exposed to the attacker, which is usually limited to trusted storage or internal networks rather than the public Internet. Impact is denial of service through resource exhaustion, with no concrete memory corruption or privilege escalation primitive shown by the patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	DISK SIMPLEFIX LEAK  KPANIC INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS	YES	NO	checked

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**

## 2. Signal breakdown

Triggered signals:

* Remote reachable / network-triggerable: +2
  lockd/NLM is an RPC network service. If NFS locking is exposed to the attacker, malformed or failure-inducing lock requests can plausibly reach this path remotely.

* Real lifetime corruption: +1
  This is a refcount lifetime bug. `file->f_count` is incremented but the caller never receives the pointer needed to release it.

* Availability impact realistic: +1
  Repeated failures can keep `nlm_file` objects unreapable, leading to resource exhaustion over time.

* Broad deployed network service exposure: +1 in paranoid scoring only
  NFS/lockd is common in enterprise and storage environments, though usually internal rather than Internet exposed.

Subtracting signals:

* Hard or condition-dependent trigger: -1
  The bug requires the cached `nlm_file` path plus `nlm_do_fopen()` failure on an already hashed file. This is not a trivial single-packet crash path.

No signals awarded for:

* No generic memory corruption primitive.
  The patch shows a refcount leak, not UAF, double free, OOB write, type confusion, or arbitrary write.

* No privilege escalation signal.
  There is no supported reclaim, overwrite, callback control, stale object reuse, or sensitive-object access primitive.

## 3. Reachability analysis

The likely trigger is a remote NFS/NLM client if the target exports NFS locking services and the attacker can send accepted lock-related RPC requests. No local account on the target is required in the network-exposed case, so PR:N is reasonable for CVSS-style reasoning.

The path is not Internet-default in most deployments. NFS and lockd are normally exposed inside trusted storage, HPC, virtualization, or data-center networks. Namespaces do not materially reduce the privilege requirement for a remote client, but local containerized access to an internal NFS network could make practical reachability easier.

Realistic exploitation depends on repeatedly hitting the cached-file failure path. This makes the issue more serious than a local bookkeeping leak, but less serious than a direct remote kernel crash or memory corruption primitive.

## 4. Severity interpretation

This behaves like an actionable network-reachable resource exhaustion issue, not an Important-class memory corruption vulnerability.

Theoretical impact is remote DoS through accumulated unreleased `nlm_file` references. Realistic evidence supports resource exhaustion and service or host degradation over time. It does not support confidentiality impact, integrity impact, or privilege escalation.

Because the affected service is network reachable in real NFS deployments and the paranoid score reaches 5, this should not be auto-closed without at least a focused manual check.

## 5. One-sentence report phrase

A network-reachable refcount leak in the NFS lockd cached file path can leave `nlm_file` objects unreapable after `nlm_do_fopen()` failures, allowing repeated NLM requests to cause resource exhaustion and denial of service.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: network reachability depends on NFS lockd exposure, and practical severity depends on whether an attacker can reliably force repeated cached `nlm_do_fopen()` failures. This is not a strong Important candidate, but it is an Actionable Moderate issue.

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

Patch: lockd: Plug nlm_file refcount leak on cached nlm_do_fopen() failure
Commit: 6cd84cefd8b73e85b9eda17b319bd40a670f3a38
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6cd84cefd8b73e85b9eda17b319bd40a670f3a38

Commit description:

The cached-file path in nlm_lookup_file() reaches the found: label
unconditionally, even when nlm_do_fopen() fails. At that label
*result and file->f_count are updated before the error is returned.
The wrappers nlm3svc_lookup_file() and nlm4svc_lookup_file() then
bail out of their switch without copying *result back to their
caller, so the proc handler's local nlm_file pointer remains NULL
and the cleanup path skips nlm_release_file(). The f_count
increment is never released, and nlm_traverse_files() can no
longer reap the file because its refcount never returns to zero
between requests.

Short-circuit the cached path so neither *result nor f_count is
touched when nlm_do_fopen() fails on a hashed nlm_file.

Fixes: 7f024fc ("Keep read and write fds with each nlm_file")
Cc: stable@vger.kernel.org
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/lockd/svcsubs.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=6cd84cefd8b73e85b9eda17b319bd40a670f3a38

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

The original Linux kernel CVE announcement for CVE-2026-72218 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72218

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:N/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.9

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).