[CVE-2026-74730][MODERATE 7.0] NFS: Pin the 'struct nfs_server' during a FREE_STATEID call [ Upstream

[CVE-2026-74730][MODERATE 7.0] NFS: Pin the 'struct nfs_server' during a FREE_STATEID call [ Upstream

[AL-KERNEL]

CVE: CVE-2026-74730
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: NFS: Pin the 'struct nfs_server' during a FREE_STATEID call [ Upstream
Commit: ed2f92ce2fc48463c41e0e540b9a3454889e8af8
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ed2f92ce2fc48463c41e0e540b9a3454889e8af8
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74730
Analysis date: Sat, 22 Aug 2026 13:19:01 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum**:: Actionable Moderate. Manual review required due to real kernel UAF lifetime bug.
Manual review required: YES

Summary:
An asynchronous NFSv4 FREE_STATEID operation may outlive the referenced `struct nfs_server`, causing a kernel use-after-free if the RPC is delayed while the NFS mount or server state is being torn down.

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

The original Linux kernel CVE announcement for CVE-2026-74730 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74730

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
======================================================================

CVE-2026-74730	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-362;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'An asynchronous NFSv4 FREE_STATEID call could outlive the referenced struct nfs_server because the operation did not pin the NFS superblock for the lifetime of the in flight RPC. If the RPC is delayed while the mount or server state is being torn down, nfs41_free_stateid_release can dereference data server after it has been freed, creating a kernel use after free condition. For the CVSS the PR:L is used for the paranoid score because a local user with access to an existing NFSv4 mount may be able to generate stateid create and release activity, while reliable triggering still depends on timing and mount teardown behavior. The issue is not directly network reachable by arbitrary inbound packets, but a malicious or compromised NFS server can increase practical risk by delaying FREE_STATEID replies. Impact is at least local denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to kernel memory corruption, so manual review is recommended.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 5)	YES	DANGER DISK UAF NETWORK LINUS KPANIC 	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
======================================================================

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**:: Actionable Moderate. Manual review required due to real kernel UAF lifetime bug.

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1
  A local user with access to an existing NFSv4 mount may be able to generate stateid activity, although reliable triggering may depend on mount teardown or server behavior.

* Memory corruption, weak primitive: +1
  The commit explicitly describes a use-after-free, but the patch shows a stale object lifetime issue without demonstrated attacker-controlled reclaim, overwrite, callback control, or type confusion.

* Real lifetime corruption: +1
  The asynchronous FREE_STATEID RPC can outlive the referenced `struct nfs_server`.

* Reliable kernel crash / strong DoS: +1
  A stale `struct nfs_server` dereference can plausibly crash the kernel.

* Important filesystem/storage path: +1
  This affects NFSv4 client state management in a filesystem/storage path.

* Hard or unreliable race / special timing required: -1
  Triggering depends on a delayed FREE_STATEID operation racing with NFS mount/server teardown.

Conservative total: 4

Paranoid adjustment:

* Broad/common deployment relevance: +1
  NFSv4 is a common enterprise storage/client deployment, and a malicious or delayed NFS server can make the timing condition more realistic.

Paranoid total: 5

Not awarded:

* Remote reachable / network-triggerable: +0
  This is not directly triggerable by arbitrary inbound network packets against the victim.

* Strong LPE plausibility: +0
  No controlled reclaim, arbitrary write, type confusion, callback hijack, or refcount takeover is shown.

* Confidentiality / Integrity impact: +0 in the ActionableScore
  The UAF justifies manual review, but the patch does not show a concrete read/write primitive beyond stale dereference.

## 3. Reachability analysis

The most realistic attacker is a local user who can access an already mounted NFSv4 filesystem and cause NFSv4 state creation and release. Reliable triggering likely also depends on timing with mount teardown, automount expiry, server cleanup, or delayed RPC completion.

A malicious or compromised NFS server can increase exploitability by delaying the FREE_STATEID reply, but this is better treated as external influence rather than a fully remote unauthenticated network attack.

Containers or namespaces may matter if an untrusted container workload can access a host-provided NFS mount. However, creating or tearing down the mount is usually privileged, so the conservative interpretation remains local with timing constraints.

## 4. Severity interpretation

This is above an ordinary Moderate because it is a real kernel use-after-free in an asynchronous filesystem RPC lifetime path. It should not be auto-closed as a simple DoS-only bug.

Realistically, the demonstrated impact is kernel crash / DoS. Theoretical privilege escalation cannot be ruled out for kernel UAFs, but the supplied patch does not show the primitives needed for a strong LPE claim. Therefore, the practical bucket is Actionable Moderate, with Important-candidate review sensitivity.

## 5. One-sentence report phrase

An asynchronous NFSv4 FREE_STATEID operation may outlive the referenced `struct nfs_server`, causing a kernel use-after-free if the RPC is delayed while the NFS mount or server state is being torn down.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the commit explicitly fixes a kernel UAF lifetime bug in an asynchronous NFSv4 path. Even though practical exploitation appears timing-dependent and currently looks DoS-first, UAFs in kernel object lifetime management should be reviewed manually before downgrade or closure.

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

Patch: NFS: Pin the 'struct nfs_server' during a FREE_STATEID call [ Upstream
Commit: ed2f92ce2fc48463c41e0e540b9a3454889e8af8
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ed2f92ce2fc48463c41e0e540b9a3454889e8af8

Changed files:
  fs/nfs/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=ed2f92ce2fc48463c41e0e540b9a3454889e8af8

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

The original Linux kernel CVE announcement for CVE-2026-74730 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74730

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

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).