[CVE-2026-53394][MODERATE REGULAR] nfsd: avoid leaking pre-allocated openowner on unconfirmed retry race

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

CVE: CVE-2026-53394
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: nfsd: avoid leaking pre-allocated openowner on unconfirmed retry race
Commit: c9aefb2b5f11337c9202c5bd0c45d71198449718
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c9aefb2b5f11337c9202c5bd0c45d71198449718
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53394
Analysis date: Sun, 19 Jul 2026 10:06:58 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline manual review recommended
Manual review required: YES

Summary:
A race in the nfsd NFSv4.0 OPEN owner retry path can leak pre-allocated openowner objects when concurrent OPEN requests reuse the same owner string, allowing a network-reachable NFS client with service access to cause sustained kernel memory exhaustion and DoS.

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

The original Linux kernel CVE announcement for CVE-2026-53394 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53394

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 REGULAR
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-2026-53394	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	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-401;**CWE-400;CWE-362;*CWE-772;Other CVSS 'AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.3';DESCR 'An NFS server memory leak can occur in the NFSv4.0 OPEN stateowner lookup path when two concurrent OPEN operations use the same owner string and an unconfirmed owner is inserted between retry iterations. The pre-allocated openowner can be overwritten without being freed, leaking the slab object and owner name buffer on each successful race. For the CVSS the PR:L is used because a remote client normally needs access to the NFS service and export context in order to send the relevant NFSv4.0 OPEN operations. The issue is network reachable from clients that can talk to nfsd, but the race window makes reliable triggering high complexity. Impact is denial of service through sustained kernel memory exhaustion, with no supported evidence of UAF, arbitrary write, information disclosure, or privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review recommended (with actual score 4)	YES	DISK RACE LEAK UAF NETWORK  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline manual review recommended**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2
  The affected path is in nfsd NFSv4.0 OPEN handling and can be reached by a remote NFS client that can talk to the server.
* Availability impact realistic: +1
  The bug leaks a slab object plus owner name buffer per successful race, so sustained triggering can consume kernel memory.
* Important filesystem/storage path: +1
  This is server-side NFS state management for OPEN owners.
* Hard or unreliable race / special timing required: -1
  The leak requires a retry race involving concurrent OPEN operations with the same owner string and an inserted unconfirmed owner.

Paranoid additional interpretation:

* Reliable kernel resource exhaustion / strong DoS: +1
  The commit says the narrow race is repeatable under adversarial conditions, so sustained memory exhaustion is plausible if the attacker has NFS access.

Not awarded:

* No generic memory corruption bonus. The patch shows a lost allocation pointer and memory leak, not UAF, double-free, OOB write, stale callback, or type confusion.
* No privilege escalation bonus. There is no demonstrated reclaim, overwrite, object replacement, or write primitive.
* No confidentiality or integrity impact. The primitive is resource leakage only.

## 3. Reachability analysis

A remote NFSv4.0 client can potentially trigger the bug by issuing concurrent OPEN operations with the same owner string against nfsd. In practice, this normally requires network reachability to the NFS server and enough export or service access to exercise NFSv4.0 OPEN handling. It is not an Internet-default exposure, but it is realistic in internal storage networks, virtualization clusters, and enterprise NFS deployments. Namespaces do not materially reduce the victim-side requirement because the vulnerable code runs in the host nfsd server path.

Call-site confidence: high. The affected function and allocation retry behavior are described directly in the commit message and the patch changes the retry control flow.

## 4. Severity interpretation

This behaves like an actionable resource-exhaustion Moderate issue rather than an Important memory-corruption vulnerability. The theoretical impact is kernel memory exhaustion and DoS if the race can be won repeatedly. Realistic exploitation depends on NFS service exposure, client access, and race reliability. There is no evidence that the leak can be transformed into LPE, information disclosure, or arbitrary corruption.

## 5. One-sentence report phrase

A race in the nfsd NFSv4.0 OPEN owner retry path can leak pre-allocated openowner objects when concurrent OPEN requests reuse the same owner string, allowing a network-reachable NFS client with service access to cause sustained kernel memory exhaustion and DoS.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the bug is network reachable in a kernel server path and the commit states the race is repeatable under adversarial conditions, although the primitive remains a memory leak DoS and not proven memory corruption or privilege escalation.

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

Patch: nfsd: avoid leaking pre-allocated openowner on unconfirmed retry race
Commit: c9aefb2b5f11337c9202c5bd0c45d71198449718
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c9aefb2b5f11337c9202c5bd0c45d71198449718

Commit description:

When find_or_alloc_open_stateowner() encounters an unconfirmed owner, it
calls release_openowner() and sets oo = NULL. Control then falls through
past the `if (oo)` guard -- which would have freed any pre-allocated
`new` -- and unconditionally executes `new = alloc_stateowner(...)`. If
`new` was already allocated on a prior iteration, the pointer is
silently overwritten and the previous allocation (slab object + owner
name buffer) is leaked.

This requires a race: two NFSv4.0 OPEN threads with the same owner
string, where a concurrent thread inserts a new unconfirmed owner into
the hash between retry iterations. The window is narrow but repeatable
under adversarial conditions.

Fix by adding `goto retry` after `oo = NULL` so the already-allocated
`new` is reused on the next iteration rather than overwritten.

Reported-by: Chris Mason <clm@meta.com>
Fixes: 23df177 ("nfsd: perform all find_openstateowner_str calls in the one place.")
Cc: stable@vger.kernel.org
Assisted-by: kres:claude-opus-4-6
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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=c9aefb2b5f11337c9202c5bd0c45d71198449718

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

The original Linux kernel CVE announcement for CVE-2026-53394 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53394

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: 3
  Paranoid ActionableScore: 4

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: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

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).