[CVE-2026-53395][MODERATE 7.0] nfsd: fix dead ACL conflict guard in nfsd4_create

[CVE-2026-53395][MODERATE 7.0] nfsd: fix dead ACL conflict guard in nfsd4_create

[AL-KERNEL]

CVE: CVE-2026-53395
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: nfsd: fix dead ACL conflict guard in nfsd4_create
Commit: 8371cc5c0a2cc2a71b3dcfd47ff1f7fcfc526a5e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8371cc5c0a2cc2a71b3dcfd47ff1f7fcfc526a5e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53395
Analysis date: Sun, 19 Jul 2026 10:15:26 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A remote NFSv4 client with write access to an export can send conflicting ACL attributes in CREATE requests and leak posix_acl slab objects repeatedly, potentially exhausting kernel memory and causing server-side denial of service.

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

The original Linux kernel CVE announcement for CVE-2026-53395 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53395

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-53395	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:L/UI:N/S:U/C:N/I:N/A:H';*CWE-401;*CWE-772;**CWE-400;Other CVSS 'AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '6.5';DESCR 'An NFS server memory leak can be triggered when an NFSv4 client sends a CREATE request that combines FATTR4_WORD0_ACL with FATTR4_WORD2_POSIX_DEFAULT_ACL or FATTR4_WORD2_POSIX_ACCESS_ACL. The conflict check looked at source ACL pointers after they had already been cleared, so the invalid combination was not rejected and existing posix_acl objects could be overwritten without being released. A remote client can repeat such CREATE requests against a writable export and gradually exhaust kernel slab memory, causing a server-side denial of service. For the CVSS the PR:L is used because reliable triggering normally requires permission to access an NFS export and create files on it. The issue is network reachable when nfsd is exposed to the attacker, although NFS servers are commonly limited to internal storage or data center networks rather than the public Internet. Impact is denial of service via memory exhaustion and no direct privilege escalation primitive is visible from the patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)  SKIP CVE-2026-53395 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: d2ca50606f5f0235d7780c1cd73b6614a5d07620 YES  NO NO unknown 	MAYBE	DISK 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
  NFSv4 CREATE is handled by nfsd over the network when an export is reachable.

* Reliable kernel crash / strong DoS: +1
  The bug leaks posix_acl slab objects per malformed CREATE request. Repeated requests can cause unbounded slab exhaustion.

* Important filesystem/storage path: +1
  The affected code is in NFS server file creation and ACL handling.

* Availability impact realistic: +1, paranoid only
  Added in the paranoid score because sustained remote requests can realistically exhaust kernel slab memory and affect the whole server.

* No generic memory corruption: +0
  The patch indicates a memory leak from overwritten ACL pointers, not UAF, OOB write, type confusion, or arbitrary write.

* No privilege escalation signal: +0
  No LPE primitive is visible from the patch. The impact is resource exhaustion.

## 3. Reachability analysis

A remote NFSv4 client can trigger the issue by sending CREATE with both FATTR4_WORD0_ACL and FATTR4_WORD2_POSIX_DEFAULT_ACL or FATTR4_WORD2_POSIX_ACCESS_ACL. Reliable triggering normally requires access to a writable NFS export, so this is not PR:N in the practical sense. It is best treated as PR:L because the attacker needs export access and create permission, but not local shell access on the server.

Namespaces do not materially reduce the network exposure if nfsd is reachable. The path is not Internet-default in most deployments, but NFS servers are common in storage and data-center networks. Realistic exploitation requires network reachability to nfsd and permission to create files in an affected export.

## 4. Severity interpretation

This behaves more like an actionable Moderate than an ordinary Moderate. The bug is not a memory-corruption or privilege-escalation issue, but it is remotely reachable in a common storage service and can cause unbounded kernel memory exhaustion through repeated requests. The theoretical impact remains DoS-only based on the patch, but the operational risk is high enough that it should not be auto-closed without review.

## 5. One-sentence report phrase

A remote NFSv4 client with write access to an export can send conflicting ACL attributes in CREATE requests and leak posix_acl slab objects repeatedly, potentially exhausting kernel memory and causing server-side denial of service.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Network reachability plus unbounded kernel slab exhaustion makes this actionable even without memory corruption or privilege escalation.

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

Patch: nfsd: fix dead ACL conflict guard in nfsd4_create
Commit: 8371cc5c0a2cc2a71b3dcfd47ff1f7fcfc526a5e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8371cc5c0a2cc2a71b3dcfd47ff1f7fcfc526a5e

Commit description:

nfsd4_create() steals create->cr_dpacl/cr_pacl into the local
nfsd_attrs via the designated initializer, then immediately sets the
source pointers to NULL. The subsequent conflict guard tests the
already-nilled source fields, making it permanently dead code:

    if (create->cr_acl) {
        if (create->cr_dpacl || create->cr_pacl)  /* always false */

When a client encodes both FATTR4_WORD0_ACL and
FATTR4_WORD2_POSIX_{DEFAULT,ACCESS}_ACL in the same CREATE fattr
bitmap, nfsd4_acl_to_attr() overwrites attrs.na_pacl/na_dpacl without
releasing the originals, leaking two posix_acl slab objects per
request. Repeated requests cause unbounded slab exhaustion.

Fix by checking attrs.na_dpacl/na_pacl (the stolen values) instead of
the nilled create->cr_dpacl/cr_pacl, matching the correct pattern
already used in nfsd4_setattr().

Reported-by: Chris Mason <clm@meta.com>
Assisted-by: kres:claude-opus-4-6
Fixes: d2ca506 ("NFSD: Add support for POSIX draft ACLs for file creation")
Cc: stable@vger.kernel.org
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=8371cc5c0a2cc2a71b3dcfd47ff1f7fcfc526a5e

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

The original Linux kernel CVE announcement for CVE-2026-53395 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53395

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:N/A:L
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 6.5

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).