[CVE-2026-89679][MODERATE REGULAR] nfsd: fix null dereference in nfsd4_setattr for deleg timestamp attrs

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

CVE: CVE-2026-89679
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: nfsd: fix null dereference in nfsd4_setattr for deleg timestamp attrs
Commit: 3c5119b799a7f57db332b2c31e806de16c869f0f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3c5119b799a7f57db332b2c31e806de16c869f0f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89679
Analysis date: Fri, 11 Sep 2026 17:29:55 -0400
ActionableScore: 3
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89679 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89679

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 REGULAR
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-2026-89679	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89679 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:N/UI:N/S:U/C:N/I:N/A:H';**CWE-476;*CWE-20;CWE-754;BEST CVSS score: '7.5';DESCR 'A NULL-pointer dereference in the NFSv4 server SETATTR path occurs when delegated timestamp attributes are processed with the special one stateid. The special stateid is accepted without creating an nfs4_stid object, but nfsd4_setattr later dereferences the resulting NULL st pointer while checking the delegation type. A remote NFSv4 client can send a crafted COMPOUND request containing PUTROOTFH and SETATTR with delegated timestamp attributes to crash the kernel. For the CVSS the PR:N rating is used because the vulnerable request path does not require prior delegation state or authenticated local access. Network reachability depends on an exposed NFSv4 export and its network access controls. Impact is denial of service through a kernel oops or crash. No memory corruption or privilege escalation primitive is demonstrated by the patch.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3)  SKIP CVE-2026-89679 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	DANGER NULLPTR DISK NETWORK LINUS  INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4	-	-	checked

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

ActionableScore=3
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 3
* Raw paranoid score: 3
* Paranoid score: 3
* Final recommended bucket: **Borderline / manual review recommended**::

No floor or cap was applied. Conservative and paranoid scores are identical because the patch establishes a direct NULL-pointer dereference and remote kernel DoS, but no memory corruption, lifetime violation, disclosure, integrity impact, or privilege-escalation primitive.

2. Signal breakdown

* +2 Remote reachable / directly network-triggerable. The supplied context establishes that an NFSv4 client can send a crafted COMPOUND request containing PUTROOTFH and SETATTR with ONE_STATEID and delegated timestamp attributes. No prior delegation, authenticated session state, or local access is required by the vulnerable path.
* +1 Reliable kernel crash / strong DoS. The NULL dereference of st->sc_type occurs in the nfsd server kernel path and can cause an Oops or kernel crash.

Non-awarded material signals:

* +0 Memory corruption. The pointer is NULL, not stale or attacker-controlled. The patch adds a NULL check and does not show UAF, OOB access, double free, refcount misuse, or an attacker-controlled write.
* +0 Confidentiality and integrity. There is no demonstrated read-back, information disclosure, unauthorized modification, or security-boundary bypass.
* +0 Privilege escalation. No privilege-escalation path is established.
* +0 Local unprivileged trigger. The established attack vector is remote network traffic, not a local interface.
* +0 Broad/default subsystem exposure. NFSv4 server deployment is a configuration decision. The supplied evidence establishes reachability when nfsd is exposed, but does not establish broad default exposure.
* +0 Additional Availability impact. The same kernel crash is already counted once as reliable DoS.
* No privilege penalty. The evidence positively establishes a remote client trigger without host-admin-equivalent attacker privilege.
* No rare-subsystem penalty. NFS server exposure may be deployment-dependent, but this is not evidence of a rare or difficult-to-reach execution mode once an NFSv4 export is reachable.

3. Reachability analysis

The evaluated attacker is a remote NFSv4 client. The supplied description establishes that the attacker can trigger the vulnerable path by sending a malformed but protocol-parseable NFSv4 COMPOUND request after reaching the NFSv4 server.

The attacker does not need local code execution, host root, CAP_SYS_ADMIN, CAP_NET_ADMIN, an existing delegation, or a prior stateid. Therefore HostAdminRequired is NO.

The NFS server itself must have been enabled and exposed by an administrator. That is a deployment prerequisite, but it is not an attacker privilege requirement and does not justify a host-admin penalty. Practical reachability depends on exported filesystems, firewall policy, RPC/NFS exposure, and network ACLs. NFS services are commonly limited to trusted storage networks, but an exposed export can be attacked by any network client that can reach the service.

NamespaceScopedPrivilege is UNKNOWN because namespaces are not relevant to the supplied remote NFSv4 trigger and no namespace-specific capability chain is established.

No race, special hardware, boot option, or unusual device configuration is required by the demonstrated trigger.

4. Primitive and severity interpretation

The exact primitive is an invalid NULL-pointer dereference. A special all-FF one stateid is accepted by the special-stateid validation path without populating the output nfs4_stid pointer. The later delegated-timestamp handling path assumes st is valid and reads st->sc_type.

This is not a UAF. No object is freed, no stale pointer survives object lifetime, and no second release or refcount mutation occurs. This is also not an active structured-object reuse scenario. The dereference is a shallow invalid access near address zero and the demonstrated consequence is a kernel Oops or crash.

PrimitiveClass is CLASS_C because the established effect is crash/DoS from invalid state handling without an independently useful corruption primitive. ImportantCandidate is NO because no meaningful exploitation leverage beyond the crash is supported. AutoDowngradeAllowed is YES because the issue is a direct crash-only NULL dereference with no unresolved memory-corruption, authorization-bypass, ownership, or lifetime primitive.

Operationally, this is a remotely triggerable high-CVSS availability issue for exposed NFSv4 servers, but it is not an Important-class kernel corruption issue based on the supplied evidence.

5. One-sentence report phrase

A remote NFSv4 client can send a crafted SETATTR request with a special one stateid and delegated timestamp attributes to trigger a NULL-pointer dereference in nfsd4_setattr, causing kernel denial of service.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

The issue is directly network-triggerable and can crash an exposed NFSv4 server, so deployment exposure and affected nfsd configuration should be verified, although the patch supports crash-only impact rather than memory corruption or privilege escalation.

7. Call-site confidence

Call-site confidence: medium

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

Patch: nfsd: fix null dereference in nfsd4_setattr for deleg timestamp attrs
Commit: 3c5119b799a7f57db332b2c31e806de16c869f0f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3c5119b799a7f57db332b2c31e806de16c869f0f

Commit description:

When a SETATTR request includes FATTR4_WORD2_TIME_DELEG_ACCESS or
FATTR4_WORD2_TIME_DELEG_MODIFY in the attribute bitmap, nfsd4_setattr()
sets deleg_attrs=true and calls nfs4_preprocess_stateid_op() to validate
the stateid.

If the client supplies the NFSv4 "one stateid" (all-0xFF bytes),
check_special_stateids() returns nfs_ok without populating the output
nfs4_stid pointer, because the special-stateid path in
nfs4_preprocess_stateid_op() jumps to done: with s==NULL, and the
"if (s)" block that would set *cstid is skipped. The local variable `st`
remains NULL.

Back in nfsd4_setattr(), the if (deleg_attrs) block then unconditionally
dereferences st->sc_type (at offset 4 from NULL), causing a kernel oops.

This is remotely triggerable by any NFSv4 client: send COMPOUND [PUTROOTFH,
SETATTR(ONE_STATEID, {bmval2=FATTR4_WORD2_TIME_DELEG_ACCESS, ...})].
No authentication, delegation, or prior state is required.

Fix by adding a NULL check before the dereference. A special stateid is
not a delegation stateid, so the existing nfserr_bad_stateid return value
is already correct; we only need to guard the pointer dereference itself.

Fixes: 7e13f4f ("nfsd: handle delegated timestamps in SETATTR")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-sonnet-4-6
Signed-off-by: Nikol Kuklev <nikolk202@gmail.com>
Signed-off-by: Chuck Lever <cel@kernel.org>
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=3c5119b799a7f57db332b2c31e806de16c869f0f

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

The original Linux kernel CVE announcement for CVE-2026-89679 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89679

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: Not available
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/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: unknown
  Paranoid ActionableScore: 3

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: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

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).