[CVE-2026-53392][MODERATE 7.0] NFSv4/flexfiles: reject zero filehandle version count [ Upstream

[CVE-2026-53392][MODERATE 7.0] NFSv4/flexfiles: reject zero filehandle version count [ Upstream

[AL-KERNEL]

CVE: CVE-2026-53392
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: NFSv4/flexfiles: reject zero filehandle version count [ Upstream
Commit: 7779c85028a0676fb190cde4f0c540f4f8e97761
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7779c85028a0676fb190cde4f0c540f4f8e97761
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53392
Analysis date: Sun, 19 Jul 2026 09:47:18 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A malicious or compromised NFSv4 pNFS flexfiles server can send a malformed layout with zero filehandle versions, causing the client to store ZERO_SIZE_PTR and later crash the kernel during layoutreturn handling.

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

The original Linux kernel CVE announcement for CVE-2026-53392 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53392

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-2026-53392	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H';*CWE-20;**CWE-476;CWE-754;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7.5';DESCR 'A NULL-pointer dereference in the NFSv4/flexfiles client path can occur because ff_layout_alloc_lseg accepts a zero fh_count from the flexfiles layout body. A malicious or compromised NFS server can provide a malformed layout where the zero-count allocation produces ZERO_SIZE_PTR and dss_info fh_versions is later used by paths that assume at least one filehandle version exists. This can crash the client kernel during layoutreturn handling and causes denial of service. For the CVSS the PR:N is used because the attacker does not need local privileges on the client and only needs the ability to act as or influence the NFS server endpoint. The issue is network reachable in deployments where NFSv4 pNFS flexfiles is used, but it normally requires access to a storage network or a server already trusted by the client rather than arbitrary Internet exposure. Impact is denial of service only based on the observed NULL-pointer dereference.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	KASAN NULLPTR DISK INIT NETWORK LINUS KPANIC  KPANIC INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS	NO	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

Conservative signals:

* Remote reachable / network-triggerable: +2. A malicious or compromised NFSv4 pNFS flexfiles server can provide the malformed layout body to the client.
* Reliable kernel crash / strong DoS: +1. The commit shows KASAN null-ptr-deref followed by kernel panic in the layoutreturn path.
* Availability impact realistic: +1. The demonstrated outcome is a host kernel panic, not only a warning or recoverable error.
* Important filesystem/storage path: +1. The bug is in NFSv4 pNFS flexfiles layout handling.
* Rare AND difficult-to-reach subsystem/configuration: -1. Triggering requires NFSv4 pNFS flexfiles use and influence over a server already trusted by the client.

Paranoid adjustment:

* Do not apply the rare/configuration penalty when evaluating an environment where pNFS flexfiles is actually deployed. In that case, the malformed layout is a direct server-to-client protocol input and the score becomes 5.

Not counted:

* No generic memory corruption bonus. The demonstrated primitive is ZERO_SIZE_PTR / NULL-pointer dereference, not UAF, OOB write, type confusion, double free, or controlled overwrite.
* No privilege escalation bonus. The patch and crash path do not show a reclaim, write primitive, callback control, or object confusion.
* No confidentiality or integrity impact. The observed effect is DoS only.

## 3. Reachability analysis

The realistic attacker is a malicious NFS server, compromised NFS server, or an attacker able to tamper with NFSv4 pNFS flexfiles layout responses. No local privileges are needed on the client, so from the client perspective this is PR:N. It is network-triggerable, but normally only inside storage or data-center networks where the client already mounts an NFSv4 pNFS flexfiles export. It is not a generic Internet-exposed attack surface unless such NFS service is exposed across an untrusted network.

Namespaces and containers do not materially reduce the requirement on the attacker side, because the trigger is server-provided layout data consumed by the host kernel NFS client. The path is not universal default exposure because pNFS flexfiles must be in use.

## 4. Severity interpretation

This behaves like an actionable Moderate network/storage DoS issue, not an Important LPE. The theoretical concern is that malformed remote filesystem metadata reaches kernel layout handling, but the demonstrated primitive is an invalid pointer dereference near NULL caused by accepting fh_count == 0. There is no evidence of attacker-controlled memory corruption, data disclosure, or privilege escalation. Manual review is still warranted because the trigger is network-protocol driven and causes a reliable client kernel panic.

## 5. One-sentence report phrase

A malicious or compromised NFSv4 pNFS flexfiles server can send a malformed layout with zero filehandle versions, causing the client to store ZERO_SIZE_PTR and later crash the kernel during layoutreturn handling.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: network-triggerable storage-protocol input can cause a host kernel panic, and the issue should not be auto-closed even though the demonstrated impact is DoS only.

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

Patch: NFSv4/flexfiles: reject zero filehandle version count [ Upstream
Commit: 7779c85028a0676fb190cde4f0c540f4f8e97761
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7779c85028a0676fb190cde4f0c540f4f8e97761

Changed files:
  fs/nfs/flexfilelayout/flexfilelayout.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=7779c85028a0676fb190cde4f0c540f4f8e97761

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

The original Linux kernel CVE announcement for CVE-2026-53392 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53392

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:H
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 7.5

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).