[CVE-2026-53399][IMPORTANT] nfsd: release layout stid on setlease failure

[CVE-2026-53399][IMPORTANT] nfsd: release layout stid on setlease failure

[AL-KERNEL]

CVE: CVE-2026-53399
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: nfsd: release layout stid on setlease failure
Commit: d369e5edfaaf83a448016e2f1da392b2174be801
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d369e5edfaaf83a448016e2f1da392b2174be801
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53399
Analysis date: Sun, 19 Jul 2026 09:26:30 -0400
ActionableScore: 8
ActionableScore lower bound: 7
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
An NFSv4 pNFS layout stateid can remain published in the nfsd client IDR after being freed on a setlease failure path, allowing later IDR traversal to dereference freed slab memory and causing at least a remote denial of service with broader UAF impact requiring review.

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

The original Linux kernel CVE announcement for CVE-2026-53399 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53399

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-2026-53399	IMPORTANT	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:H/I:H/A:H';*CWE-416;CWE-825;CWE-459;Other CVSS 'AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7.5';DESCR 'An nfsd layout stateid can be published in the client IDR before nfsd4_layout_setlease fails. The old error path freed the layout stateid directly, leaving the IDR entry pointing to freed slab memory. A later IDR traversal such as states_show or client teardown can dereference the stale pointer and trigger a kernel use after free. For the CVSS the PR:L because an attacker generally needs NFS client access to an exported filesystem and the ability to issue NFSv4 pNFS layout operations, but full local administrator privileges on the server are not required. The issue is network reachable when an affected NFS server exposes pNFS functionality to the attacker. Impact is at least denial of service via kernel crash and in the paranoid case may include confidentiality and integrity impact due to kernel memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8)	YES	DANGER DISK INIT ERRORPATH UAF NETWORK KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=8
ActionableScoreLower=7

## 1. ActionableScore

* Conservative score: 7
* Paranoid score: 8
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2
  A reachable NFS server exposing NFSv4 pNFS layout operations can receive the triggering client operation over the network.

* Generic memory corruption, strong corruption primitive: +2
  The old error path leaves an IDR entry pointing to freed slab memory. This is a concrete stale pointer / use-after-free condition, not just a warning or leak.

* Real lifetime corruption: +1
  The bug is an object lifetime teardown mismatch. A stateid is published in the IDR, then freed directly without removing the IDR reference.

* Reliable kernel crash / strong DoS: +1
  The commit explicitly states that later IDR walkers such as states_show or client teardown dereference the dangling pointer.

* Important filesystem/storage path: +1
  The affected code is in nfsd pNFS layout state management, a server-side filesystem/storage protocol path.

* Hard or conditional trigger: -1
  Triggering requires the setlease failure path after IDR publication, then a later IDR traversal. This is more constrained than a simple malformed packet crash.

Conservative total: 7

Paranoid additional signal:

* Weak LPE / broader memory-corruption concern: +1
  The primitive is a freed slab object reachable through a persistent IDR entry. No controlled reclaim or overwrite is shown, so this is not strong LPE evidence, but it is enough to require manual review.

Paranoid total: 8

## 3. Reachability analysis

The likely trigger is a remote NFS client with access to an exported filesystem and ability to exercise NFSv4 pNFS layout operations. This is not Internet-wide by default, but it is network reachable in deployments that expose affected nfsd pNFS functionality to tenants or internal clients.

Privileges are not equivalent to host root. The attacker generally needs NFS client access and permissions sufficient to reach the layout path, so this is closer to PR:L in CVSS terms than PR:H. Namespaces or containers do not directly grant the server-side capability, but containerized workloads may still act as NFS clients if they can reach the export.

The path is configuration dependent because pNFS must be in use and the setlease failure path must be reachable. However, once the stale IDR entry exists, later normal server operations such as client teardown or debug/state walking can dereference freed memory.

Call-site confidence: high. The commit message describes the publication point, the incorrect free path, the correct nfs4_put_stid teardown path, and concrete later IDR walkers.

## 4. Severity interpretation

This behaves above ordinary Moderate. The realistic impact is at least a network-reachable kernel DoS against affected NFS servers. The theoretical risk is higher because the bug leaves a persistent IDR reference to freed slab memory, which is a real use-after-free lifetime corruption primitive.

There is not enough evidence to claim reliable privilege escalation or arbitrary code execution. No attacker-controlled reclaim, overwrite, callback hijack, or type confusion is demonstrated. Still, because this is server-side network-reachable UAF in kernel nfsd state management, it should be handled as a Strong Important candidate or at least Actionable Moderate and should not be auto-closed.

## 5. One-sentence report phrase

An NFSv4 pNFS layout stateid can remain published in the nfsd client IDR after being freed on a setlease failure path, allowing later IDR traversal to dereference freed slab memory and causing at least a remote denial of service with broader UAF impact requiring review.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a concrete network-reachable kernel use-after-free involving a stale IDR pointer in nfsd state management. Even though exploitation beyond DoS is not demonstrated, the lifetime primitive and server-side exposure make manual analysis necessary.

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

Patch: nfsd: release layout stid on setlease failure
Commit: d369e5edfaaf83a448016e2f1da392b2174be801
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d369e5edfaaf83a448016e2f1da392b2174be801

Commit description:

nfs4_alloc_stid() publishes the new stid into cl->cl_stateids via
idr_alloc_cyclic() under cl_lock before returning to
nfsd4_alloc_layout_stateid(). When nfsd4_layout_setlease() then
fails, the error path frees the layout stateid directly with
kmem_cache_free() without ever calling idr_remove(), leaving the
IDR slot pointing at freed slab memory. Any subsequent IDR walker
(states_show, client teardown) dereferences the dangling pointer.

The correct teardown for an IDR-published stid is nfs4_put_stid(),
which removes the IDR slot under cl_lock, dispatches sc_free
(nfsd4_free_layout_stateid) to release ls->ls_file via
nfsd4_close_layout(), and drops the nfs4_file reference in its
tail.

Replace the manual nfsd_file_put + put_nfs4_file + kmem_cache_free
cleanup with a single nfs4_put_stid(stp).

Fixes: c5c707f ("nfsd: implement pNFS layout recalls")
Cc: stable@vger.kernel.org
Signed-off-by: Chris Mason <clm@meta.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Chuck Lever <cel@kernel.org>
[ cel: no ls_fence_work in 6.12.y; dropped INIT_DELAYED_WORK hunk ]
Signed-off-by: Chuck Lever <cel@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Changed files:
  fs/nfsd/nfs4layouts.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=d369e5edfaaf83a448016e2f1da392b2174be801

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

The original Linux kernel CVE announcement for CVE-2026-53399 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53399

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:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/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: 7
  Paranoid ActionableScore: 8

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).