[CVE-2026-53026][MODERATE 7.0] NFSD: fix nfs4_file access extra count in nfsd4_add_rdaccess_to_wrdeleg [ Upstream

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

CVE: CVE-2026-53026
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: NFSD: fix nfs4_file access extra count in nfsd4_add_rdaccess_to_wrdeleg [ Upstream
Commit: 4584229395d0d65bd517780afe97ffea07cb2c3d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4584229395d0d65bd517780afe97ffea07cb2c3d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53026
Analysis date: Thu, 25 Jun 2026 04:32:49 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: NO

Summary:
A race in NFSD can add an extra nfs4_file READ access reference when another thread already installed the read file, leaking nfsd_file objects and potentially causing a kernel BUG during nfs-server shutdown.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-53026 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53026

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-2026-53026	MODERATE	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';*CWE-772;*CWE-401;CWE-362;**CWE-400;Other CVSS 'AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.3';DESCR 'NFSD can leak nfsd_file related references when nfsd4_add_rdaccess_to_wrdeleg increments the nfs4_file READ access count even though another thread already installed the O_RDONLY file for the same state object. The leaked access count can keep the corresponding nfsd_file object alive and later trigger a BUG during nfs-server shutdown when the nfsd_file cache is destroyed. For the CVSS the PR:L is used because a remote NFS client or workload needs access to exported NFS files, but does not require administrator privileges on the server. The issue is network reachable in deployments where NFS service is exposed to the client, but reliable triggering depends on concurrent NFSv4 delegation related activity and is therefore AC:H. Impact is denial of service through resource leakage and kernel crash during service teardown, with no clear confidentiality or integrity impact from the patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5) 	MAYBE	DISK INIT SIMPLEFIX LEAK NETWORK KPANIC 	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

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

## 2. Signal breakdown

* Remote reachable / network-triggerable: +2. The affected code is in NFSD and can be exercised by NFSv4 client workloads against an exported filesystem.
* Real lifetime corruption: +1. This is a refcount lifetime bug where an extra `nfs4_file` access count can keep `nfsd_file` objects alive.
* Reliable kernel crash / strong DoS: +1. The leak can later trigger a BUG in `kmem_cache_destroy()` when stopping `nfs-server`.
* Broad/default/common subsystem: +1 in paranoid scoring. NFSD is a common server subsystem in enterprise and storage deployments, though not enabled by default on all systems.
* Hard or unreliable race / special timing required: -1. The issue depends on concurrent access where another thread already installed `fi_fds[O_RDONLY]`.

No generic memory corruption score is added. The patch supports a reference leak/resource lifetime bug, not UAF, OOB write, arbitrary write, or attacker-controlled corruption.

## 3. Reachability analysis

A remote NFS client with access to an exported filesystem can plausibly trigger the affected path through NFSv4 delegation and file access activity. The reproducer mentioned in the commit is the Git test suite over NFS, which suggests practical workload reachability but not a trivial single-packet trigger.

The attacker does not need administrator privileges on the NFS server, but normally needs permission to access the exported NFS share. Namespace/container behavior is not the main factor here. The vulnerable service must be running and NFSv4 exports must be reachable.

## 4. Severity interpretation

This behaves as an actionable Moderate issue rather than an Important-class vulnerability. The main impact is resource lifetime leakage that can cause a kernel BUG during NFSD shutdown. There is no clear privilege escalation, confidentiality impact, integrity impact, UAF reclaim primitive, or controlled memory corruption supported by the patch.

The paranoid score reaches 5 because the bug is network reachable in an important kernel server path and involves a race-related lifetime/reference bug. It should not be auto-closed without awareness of deployment exposure.

## 5. One-sentence report phrase

A race in NFSD can add an extra nfs4_file READ access reference when another thread already installed the read file, leaking nfsd_file objects and potentially causing a kernel BUG during nfs-server shutdown.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: network-reachable NFSD path plus race-related refcount lifetime leak. The apparent impact is DoS only, but exposed NFS server deployments should be reviewed for practical reachability and operational impact.

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UPSTREAM PATCH SUMMARY
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Patch: NFSD: fix nfs4_file access extra count in nfsd4_add_rdaccess_to_wrdeleg [ Upstream
Commit: 4584229395d0d65bd517780afe97ffea07cb2c3d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4584229395d0d65bd517780afe97ffea07cb2c3d

Changed files:
  fs/nfsd/nfs4state.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=4584229395d0d65bd517780afe97ffea07cb2c3d

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

The original Linux kernel CVE announcement for CVE-2026-53026 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53026

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:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.3

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).