[CVE-2026-90039][MODERATE 7.0] NFSD: Guard admin state-revocation walks with NFSD_NET_UP [ Upstream

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

CVE: CVE-2026-90039
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: NFSD: Guard admin state-revocation walks with NFSD_NET_UP [ Upstream
Commit: 104a51265042b4424085741c963cb858ac29ec0b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=104a51265042b4424085741c963cb858ac29ec0b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90039
Analysis date: Wed, 16 Sep 2026 07:15:46 -0400
ActionableScore: 4
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-90039 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90039

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-90039	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-90039 CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H';**CWE-476;*CWE-416;*CWE-664;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '6.6';DESCR 'NFSD admin state revocation paths can walk NFSv4 client state tables when the server object exists but the per net state tables are not live yet. A local actor with NFSD administration capability can trigger this through unlock_filesystem or unlock_export style control plane operations after a listener is created but before the server is fully started, causing a NULL pointer dereference and kernel crash. The same lifetime boundary also references a possible dangling pointer condition outside the NFSD_NET_UP window, so the worst case should be treated as a stale pointer or UAF candidate for manual triage. For the CVSS the PR:L is used for the paranoid score because delegated CAP_SYS_ADMIN, container root, or a service account managing NFSD may be weaker than full host administrator control in practical deployments. The issue is not directly network reachable and is triggered through local NFSD administrative interfaces rather than remote NFS client traffic. Impact is at least local denial of service via kernel crash and in worst case may allow limited confidentiality or integrity impact due to stale pointer access and should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4)	YES	NULLPTR DISK INIT UAF NETWORK KERNEL_PANIC_PLUS_UAF LINUS KPANIC 	YES	NO	checked

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

ActionableScore=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 4
* Raw paranoid score: 3
* Paranoid score: 4
* Applied conservative floor/cap: Real UAF floor 4 applied because a stale freed table walk is directly implied and host-admin-equivalent privilege is not fully resolved from the supplied diff.
* Applied paranoid floor/cap: Real UAF floor 4 applied for the same reason.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

* +2 Real UAF / lifetime violation: the comments state that outside the NFSD_NET_UP window nn->conf_id_hashtbl can be unallocated or freed, and the walkers would dereference a NULL or dangling pointer. The prior wording explicitly says shutdown can free conf_id_hashtbl without clearing the pointer, causing walks over freed slab memory.
* +1 Reliable kernel crash / strong DoS: the commit states that a local CAP_SYS_ADMIN administrator can crash the kernel before the server is fully started via the startup-time NULL table dereference.
* +0 Remote reachability: not awarded. The trigger is through local NFSD administrative procfs or generic netlink control paths, not remote NFS client traffic.
* +0 Local unprivileged trigger: not awarded. The supplied evidence says CAP_SYS_ADMIN is required.
* +0 Concrete repeated release/refcount: not awarded. The patch does not show a first put/free followed by a second lifetime-mutating put/free on the same exact dangling reference.
* +0 Structured stale-object leverage: not awarded separately. Although the stale object is an active NFSv4 state table, the supplied context does not establish controlled reclaim, attacker-shaped replacement contents, callback control, arbitrary read/write, or another useful post-free primitive.
* +0 Weak LPE concern: not awarded. A UAF candidate exists, but no exploitation mechanism beyond stale traversal/crash is established.
* +0 Confidentiality / integrity: not awarded. The evidence supports NULL dereference and possible dangling-pointer traversal, but not disclosure, protected-object access, arbitrary write, or privilege escalation.
* +0 Extra availability: not awarded. The same crash is already counted once.
* Negative signals: no -2 host-admin penalty applied because the exact host-admin-equivalent boundary of the complete procfs/netlink trigger is not fully visible in the supplied patch. CAP_SYS_ADMIN is stated, but namespace scope and complete reachability are not resolved here.

Repeated release/refcount checklist:

* First release can end lifetime: not established for an exact object reference in the shown control flow.
* Exact same reference survives: not established.
* Second lifetime/refcount mutation: not established.
* Concrete repeated release/refcount +1 awarded: no.
* Paranoid repeated-refcount leverage +1 awarded: no.
* FLOOR 5 applied: no.

3. Reachability analysis

The demonstrated trigger is local NFSD administration. The inputs are writing to /proc/fs/nfsd/unlock_filesystem or sending NFSD_CMD_UNLOCK_FILESYSTEM / NFSD_CMD_UNLOCK_EXPORT netlink commands after an NFSD listener has created nn->nfsd_serv but before nfsd_startup_net has allocated nn->conf_id_hashtbl.

The commit states that CAP_SYS_ADMIN is needed. The supplied patch does not fully prove whether the complete operation is host init_user_ns CAP_SYS_ADMIN only, or whether any namespace-scoped administrative path could reach it. Therefore HostAdminRequired is UNKNOWN rather than NO, and NamespaceScopedPrivilege is UNKNOWN.

No prior remote NFS client action is required. No direct network packet trigger is established. The vulnerable path is a local control-plane operation. No special hardware is involved. This is not primarily a race in the shown patch. It is a lifetime/readiness-state mismatch: nn->nfsd_serv becomes true before the NFSv4 state tables are live, and may remain an insufficient liveness indicator after shutdown.

4. Primitive and severity interpretation

The exact primitive is an invalid state-table walk guarded by the wrong liveness predicate. During startup this is a NULL pointer dereference because conf_id_hashtbl has not been allocated. During shutdown or outside the NFSD_NET_UP lifetime window, the comments directly imply a dangling pointer / stale freed table walk, so RealUAF=YES and PrimitiveClass=CLASS_B.

This is an active structured kernel object path, because the walkers traverse NFSv4 client state tables to revoke open state and cancel async COPY operations. However, the patch does not establish controlled reclaim, attacker-controlled replacement object contents, directed kernel memory access, callback use, credential manipulation, or a concrete second refcount/lifetime mutation. The demonstrated consequence is kernel crash / DoS.

ImportantCandidate=NO because the currently established primitive is constrained to local administrative NFSD control paths and lacks meaningful exploitation leverage beyond crash. ManualReview=RECOMMENDED because the patch directly references a dangling-pointer/freed-slab walk in active NFSD state management, so it should not be treated as a pure NULL-deref without a quick lifetime review. AutoDowngradeAllowed=YES because no unresolved Class A primitive, concrete repeated release/refcount mutation, authorization bypass, or demonstrated LPE path is established.

Likely operational severity: ordinary to borderline Moderate for most environments, with manual triage recommended due to the UAF/lifetime wording.

5. One-sentence report phrase

NFSD admin state revocation can walk NFSv4 state tables when they are not live, causing a local administrative NULL dereference crash and leaving a bounded stale table walk concern that warrants manual lifetime review.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: the primary demonstrated impact is local administrative DoS, but the patch context explicitly mentions freed conf_id_hashtbl traversal and dangling pointer risk, so a short manual check should confirm whether any below-host-admin or stronger post-free path exists.

7. Call-site confidence

Call-site confidence: high

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

Patch: NFSD: Guard admin state-revocation walks with NFSD_NET_UP [ Upstream
Commit: 104a51265042b4424085741c963cb858ac29ec0b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=104a51265042b4424085741c963cb858ac29ec0b

Changed files:
  fs/nfsd/nfsctl.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=104a51265042b4424085741c963cb858ac29ec0b

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

The original Linux kernel CVE announcement for CVE-2026-90039 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90039

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:L/PR:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 6.6

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: unknown
  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: 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).