[CVE-2026-53396][IMPORTANT] nfsd: fix posix_acl leak and ignored error in nfsd4_create_file

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

CVE: CVE-2026-53396
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: nfsd: fix posix_acl leak and ignored error in nfsd4_create_file
Commit: 18cf006a08babec0bbac2a3784f8f28e56e47490
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=18cf006a08babec0bbac2a3784f8f28e56e47490
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53396
Analysis date: Sun, 19 Jul 2026 09:48:50 -0400
ActionableScore: 7
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
nfsd4_create_file incorrectly ignored NFSv4 ACL conversion failures and leaked POSIX ACL objects on early CREATE failures, allowing an authorized remote NFS client to cause ACL policy misapplication and potentially exhaust kernel memory through repeated malformed CREATE requests.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-53396 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53396

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: IMPORTANT
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-2026-53396	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H';CWE-252;*CWE-401;CWE-284;Other CVSS 'AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '7.6';DESCR 'nfsd4_create_file had two ACL handling flaws in the NFSv4 CREATE path. The server ignored failures from nfsd4_acl_to_attr, so an unsupported ACL could result in a file being created without the requested ACL while the client still receives success. The same path could also leak POSIX ACL allocations when start_creating failed after ACL attributes were populated, for example with an invalid filename longer than NAME_MAX. For the CVSS the PR:L because a remote NFS client normally needs access to an exported filesystem and permission to issue CREATE, but does not need local privileges on the server. The issue is network reachable through NFS service access. Impact is integrity weakening from incorrect ACL handling and denial of service through repeated memory leaks. In the paranoid score, availability is raised to high because sustained remote abuse may exhaust kernel memory, while confidentiality and integrity are kept low because the patch supports policy bypass and resource leak rather than a direct arbitrary write primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)  SKIP CVE-2026-53396 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 7ab96df840e60eb933abfe65fc5fe44e72f16dc0 YES  NO NO unknown 	MAYBE	DISK LEAK ERRORPATH NETWORK  KPANIC INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: 6
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2. The affected path is NFSv4 OPEN(CREATE) handled by nfsd, reachable by a remote NFS client with access to the export.
* Availability impact realistic: +1. The POSIX ACL leak is explicitly described as repeatable with OPEN(CREATE), ACL attributes, and an invalid filename.
* Integrity impact plausible: +1. Ignoring nfsd4_acl_to_attr() errors can create a file without the requested ACL while returning success, weakening expected ACL semantics.
* Important filesystem/storage path: +1. This is in NFS server create and ACL handling, a storage access-control path.
* Broad/common deployed exposure: +1. nfsd is not enabled on every host, but it is a common production server role and is commonly network exposed inside trusted networks.

Paranoid additional signal:

* Reliable kernel memory exhaustion / strong DoS concern: +1. Sustained remote abuse can plausibly accumulate unreleased posix_acl allocations and exhaust kernel memory.

Not counted:

* No generic memory corruption bonus. The patch shows a memory leak and ignored error handling, not UAF, OOB write, double free, type confusion, or arbitrary write.
* No LPE bonus. The realistic attacker is a remote NFS client, not a local privilege escalation actor.
* No sensitive file theft primitive. The ACL issue is an integrity and policy correctness problem, but the patch does not demonstrate direct access to privileged files or credentials.

## 3. Reachability analysis

A remote NFSv4 client can trigger the affected code path if it has access to an exported filesystem and permission to issue CREATE operations. For the CVSS style interpretation, PR:L is appropriate because the attacker needs NFS export access and create permission, but does not need local privileges on the server.

Namespaces and containers do not materially reduce the server-side exposure unless nfsd is isolated or not reachable from the attacker network. The path is not Internet-default, but NFS servers are common in datacenter, storage, build, and internal infrastructure networks.

Realistic exploitation conditions exist for DoS because the commit explicitly describes a repeatable leak pattern using ACL attributes and an invalid filename such as longer than NAME_MAX. The ACL error handling issue is also practically relevant because unsupported ACL attributes can be accepted incorrectly as success.

## 4. Severity interpretation

This behaves stronger than an ordinary Moderate because it is remotely reachable through a real storage protocol and combines two impacts: repeated kernel memory leakage and incorrect ACL enforcement on file creation.

It does not behave like a classic memory-corruption Important vulnerability because the patch does not show UAF, overwrite, object confusion, or arbitrary write. The conservative result is therefore Actionable Moderate. The paranoid result reaches Strong Important candidate because a reachable NFS client can repeatedly trigger kernel memory leakage and the same path weakens ACL integrity semantics.

## 5. One-sentence report phrase

nfsd4_create_file incorrectly ignored NFSv4 ACL conversion failures and leaked POSIX ACL objects on early CREATE failures, allowing an authorized remote NFS client to cause ACL policy misapplication and potentially exhaust kernel memory through repeated malformed CREATE requests.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Manual review is required because the issue is network reachable through nfsd, affects ACL enforcement in a filesystem access-control path, and includes a repeatable kernel memory leak that may become a practical remote DoS in exposed NFS deployments.

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UPSTREAM PATCH SUMMARY
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Patch: nfsd: fix posix_acl leak and ignored error in nfsd4_create_file
Commit: 18cf006a08babec0bbac2a3784f8f28e56e47490
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=18cf006a08babec0bbac2a3784f8f28e56e47490

Commit description:

nfsd4_create_file() has two bugs in its ACL handling:

The return value of nfsd4_acl_to_attr() is silently discarded.  When
the NFSv4-to-POSIX ACL conversion fails (e.g., -EINVAL for
unsupported ACE types), the file is created without any ACL and the
client receives NFS4_OK.  This violates RFC 7530/8881 which require
the server to reject unsupported attributes on CREATE.

When start_creating() fails after ACL attributes have been populated
in attrs (either via nfsd4_acl_to_attr or via ownership transfer from
open->op_dpacl/op_pacl), the function jumps to out_write which skips
nfsd_attrs_free().  The posix_acl allocations are leaked.  A client
can trigger this repeatedly with OPEN(CREATE), ACL attributes, and an
invalid filename (e.g., longer than NAME_MAX).

Fix both by capturing the nfsd4_acl_to_attr() return value and by
changing the early error paths to jump to out instead of out_write.
Initialize child to ERR_PTR(-EINVAL) so that end_creating() is safe
to call even if start_creating() was never reached.

Reported-by: Chris Mason <clm@meta.com>
Fixes: 7ab96df ("VFS/nfsd/cachefiles/ovl: add start_creating() and end_creating()")
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/nfs4proc.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=18cf006a08babec0bbac2a3784f8f28e56e47490

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

The original Linux kernel CVE announcement for CVE-2026-53396 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53396

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:L/PR:L/UI:N/S:U/C:N/I:L/A:L
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 7.6

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: 6
  Paranoid ActionableScore: 7

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).