[CVE-2026-89697][MODERATE 7.0] nfsd: add fh_want_write() for early-verified SETATTR in nfsd_proc_setattr()

[CVE-2026-89697][MODERATE 7.0] nfsd: add fh_want_write() for early-verified SETATTR in nfsd_proc_setattr()

[AL-KERNEL]

CVE: CVE-2026-89697
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: nfsd: add fh_want_write() for early-verified SETATTR in nfsd_proc_setattr()
Commit: 15ca3b64a69caa6194bed8f98a20ed90a11c11dd
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=15ca3b64a69caa6194bed8f98a20ed90a11c11dd
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89697
Analysis date: Sat, 12 Sep 2026 05:50:58 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89697 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89697

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-2026-89697	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89697 CHECK WITH IMPACT FROM ORIG NN IMPORTANT	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:L/A:L';CWE-284;CWE-667;CWE-703;Other CVSS 'AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N';BEST CVSS score: '6.5';DESCR 'An NFS server SETATTR path can run notify_change() without taking a mount write reference because the BOTH_TIME_SET path calls fh_verify() early and then skips fh_want_write() later. This can bypass mount level write gating such as read-only or frozen mount protection and may allow remote metadata changes when the server should reject the operation. For the CVSS the PR:N is used in the paranoid score because some NFS exports can be reachable with anonymous or weak AUTH_SYS style access, while the base score uses PR:L for deployments where valid export access and setattr permission are required. The issue is network reachable through nfsd when the affected NFS SETATTR path is exposed. Impact is mainly limited integrity policy bypass and possible availability impact from violating mount write barriers, not memory corruption or privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6)	YES	WRITE OOB DISK SIMPLEFIX NETWORK LINUS KPANIC  DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH	NO	NO	checked

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

ActionableScore=6
ActionableScoreLower=5
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 5
* Conservative score: 5. No floor or cap applied.
* Raw paranoid score: 6
* Paranoid score: 6. No floor or cap applied.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Positive signals:

* +2 Concrete authorization or policy gate bypass. The missing `fh_want_write()` means `notify_change()` can run without the normal mount write reference, bypassing mount-level write gating such as read-only or frozen mount protection.
* +2 Network reachable nfsd operation. The affected path is reached via NFS `SETATTR` requests to an exposed nfsd service.
* +1 Integrity plausible. The bug can allow metadata changes when the mount write barrier should have rejected the operation.
* Paranoid only: +1 Availability plausible. Running metadata modification without the mount write reference can violate freeze/write-barrier assumptions and may cause operational availability impact, but this is weaker than a demonstrated crash.

Negative or non-awarded signals:

* RealUAF not awarded. No stale pointer, object lifetime violation, or post-free access is shown.
* Memory corruption not awarded. The patch does not establish OOB access, UAF, double free, refcount corruption, arbitrary write, or controlled kernel memory corruption.
* Strong LPE not awarded. No credential overwrite, controlled corruption, privileged FD theft, or root escalation chain is established.
* Confidentiality not awarded. No disclosure path is shown.
* Reliable kernel crash not awarded. The patch describes missing mount write reference, not a panic path.
* No privilege penalty applied. Host-admin privilege is not required by the attacker on an already exposed NFS export, although valid export access or weak anonymous access may be needed depending on deployment.
* No rare-subsystem penalty applied. nfsd is optional but common enough that the vulnerable path should not be treated as obscure solely from configuration dependence.

3. Reachability analysis

The evaluated attacker is a remote NFS client that can reach an affected nfsd export and send a `SETATTR` request hitting the `BOTH_TIME_SET` path.

Conservative reachability assumes the attacker has valid NFS export access and sufficient file-level permission to perform metadata changes, so this is below host-admin but not necessarily anonymous. Paranoid reachability allows deployments with anonymous access, weak `AUTH_SYS`, or broadly trusted internal NFS clients, where CVSS-like PR:N can be defensible.

Host administrator setup is required to run nfsd and export the filesystem, but that is a deployment condition rather than an attacker privilege requirement. The attacker does not need host root or host CAP_SYS_ADMIN to send the network request once the export is reachable.

Namespace-scoped privilege is not relevant here. The vulnerable operation is in the host nfsd server path, not an attacker-created namespace-local operation.

No race is required. The bug is an ordered logic/control-flow issue caused by early `fh_verify()` making the later setattr path skip `fh_want_write()`.

4. Primitive and severity interpretation

The exact primitive is a mount write-reference bypass in the NFS server `SETATTR` path. The security-relevant operation is `notify_change()` executing without the normal `fh_want_write()` protection.

This is not active structured UAF and not fixed stale storage. It is a policy and synchronization gate omission around a filesystem metadata update.

PrimitiveClass is CLASS_A because the patch restores a mandatory write-gating condition whose absence can change a kernel decision from reject/block to allow/proceed for externally reachable metadata modification.

The demonstrated consequence is limited integrity policy bypass: metadata may be changed through NFS when mount-level write protection should prevent it. The paranoid consequence adds limited availability concern due to possible freeze or write-barrier violation. Privilege escalation is not established.

ImportantCandidate is YES because this is a network-reachable filesystem policy bypass in nfsd, not a simple local bookkeeping bug. AutoDowngradeAllowed is NO because automatically closing it as ordinary low/moderate could miss a real write-policy boundary bypass.

Likely operational severity is Actionable Moderate, with manual review needed to decide whether product-specific NFS export defaults, anonymous access, read-only mounts, or frozen filesystem usage make it closer to Important.

5. One-sentence report phrase

An NFS server SETATTR path can run notify_change() without fh_want_write(), allowing a reachable NFS client to bypass mount level write gating and potentially modify file metadata when read-only or frozen mount protection should reject the operation.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a network-reachable nfsd path with a concrete mount write-policy bypass, even though no memory corruption or privilege escalation primitive is shown.

7. Call-site confidence

Call-site confidence: high

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

Patch: nfsd: add fh_want_write() for early-verified SETATTR in nfsd_proc_setattr()
Commit: 15ca3b64a69caa6194bed8f98a20ed90a11c11dd
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=15ca3b64a69caa6194bed8f98a20ed90a11c11dd

Commit description:

The BOTH_TIME_SET branch calls fh_verify() early so setattr_prepare()
can inspect the dentry. This causes nfsd_setattr() to skip
fh_want_write(), so notify_change() runs without a mount write
reference.

Add the missing fh_want_write() call after the early fh_verify().

Fixes: cc26508 ("nfsd: Disable NFSv2 timestamp workaround for NFSv3+")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260611-nfsd-testing-v2-11-5b90e276f2d9@kernel.org
Signed-off-by: Chuck Lever <cel@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/nfsd/nfsproc.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=15ca3b64a69caa6194bed8f98a20ed90a11c11dd

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

The original Linux kernel CVE announcement for CVE-2026-89697 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89697

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:L/A:N
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:L
  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: 5
  Paranoid ActionableScore: 6

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).