[CVE-2026-89711][MODERATE REGULAR] NFSD: remove flawed WARN_ON_ONCE from nfsd_mode_check

[CVE-2026-89711][MODERATE REGULAR] NFSD: remove flawed WARN_ON_ONCE from nfsd_mode_check

[AL-KERNEL]

CVE: CVE-2026-89711
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: NFSD: remove flawed WARN_ON_ONCE from nfsd_mode_check
Commit: b55b4d880bb080fa10eb08ba21a5d8679b8102fe
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b55b4d880bb080fa10eb08ba21a5d8679b8102fe
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89711
Analysis date: Sat, 12 Sep 2026 02:50:42 -0400
ActionableScore: 3
ActionableScore lower bound: 2
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-2026-89711 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89711

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.

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

CVE-2026-89711	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89711 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:N/UI:N/S:U/C:N/I:N/A:H';CWE-617;CWE-703;CWE-755;Other CVSS 'AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.9';DESCR 'NFSD can hit a reachable WARN_ON_ONCE in nfsd_mode_check when an NFS server reexports an NFS filesystem and a LOOKUP reaches a directory like dentry that does not support d_can_lookup. This is a valid reexport case rather than a kernel invariant violation, so a remote NFS client can trigger a warning in the server side lookup path under specific export conditions. For the CVSS the PR:N is used for the paranoid score because a remote NFS client may only need network access to an exported namespace in deployments where the export is reachable without per user authentication. The issue is network reachable through NFSD rather than local only. Impact is mainly denial of service, especially on systems configured with panic_on_warn where the reachable warning can cause a kernel panic. No memory corruption primitive is introduced by this change because the nfserr_notdir return remains in place and still blocks the later NULL lookup method call path.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3)	YES	DISK SIMPLEFIX NETWORK LINUS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4	NO	NO	checked

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

ActionableScore=3
ActionableScoreLower=2
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 2
* Conservative score: 2. No floor or cap applied.
* Raw paranoid score: 3
* Paranoid score: 3. No floor or cap applied.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

Conservative signals:

* +2 Remote reachable / directly network-triggerable: a remote NFS client can reach the NFSD LOOKUP path when the server exports an affected NFS reexport namespace.
* +0 WARN_ON only: the demonstrated behavior is a reachable WARN_ON_ONCE, not a guaranteed crash on default systems.
* +0 No C/I signal: the nfserr_notdir return remains and prevents the later lookup_one_unlocked -> __lookup_slow NULL method call path.
* +0 No memory corruption: no UAF, OOB, arbitrary write, refcount corruption, or stale object reuse is established.
* +0 No reexport penalty applied: NFS reexport is a required condition, but the supplied evidence does not justify treating it as rare enough for a scoring penalty.

Paranoid additional signal:

* +1 Reliable kernel crash / strong DoS under supported configuration: on systems with panic_on_warn enabled, the reachable WARN can become a kernel panic and host DoS.

Non-awarded signals:

* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* UAF not demonstrated.
* NULL method pointer dereference is not scored because the branch still returns nfserr_notdir and the patch explicitly keeps that protection.
* Confidentiality and integrity are not awarded because there is no disclosure path and no unauthorized modification primitive.
* Extra Availability is not awarded because panic_on_warn DoS is the same consequence already counted in the paranoid score.
* ImportantCandidate is not set because this is a reachable warning / DoS condition, not an unresolved corruption or boundary-bypass primitive.

3. Reachability analysis

The evaluated attacker is a remote NFS client that can issue LOOKUP operations against an exported namespace. Host administrator privilege is not required for the remote client side trigger once the export is reachable, so HostAdminRequired=NO.

The vulnerable condition depends on NFSD exporting an NFS filesystem, i.e. NFS reexport, where a dentry may appear directory-like but lacks d_can_lookup. This is a deployment prerequisite configured by an administrator, but it does not make the client trigger host-admin-only.

NamespaceScopedPrivilege=NO because the issue is not about attacker-created Linux namespaces or namespace-scoped capabilities. It is a server-side NFSD path reached through NFS protocol operations.

No special hardware is required. No race or timing condition is established. The trigger is a protocol operation reaching a valid reexport corner case.

4. Primitive and severity interpretation

Exact primitive: reachable kernel WARN_ON_ONCE in nfsd_mode_check for a valid NFS reexport case.

Primitive class: CLASS_C. This is primarily warning / assertion / DoS behavior.

RealUAF=NO. There is no lifetime violation, stale reference, double release, or post-free access in the patch.

Active structured vs fixed stale storage: not applicable. No stale storage primitive exists.

Cleanup/destructor-only vs broader active use: not applicable. The affected path is active NFSD lookup validation, but the vulnerable operation is only the WARN.

Concrete repeated release/refcount operation: no.

Demonstrated consequence: remote client can trigger a warning in the server-side lookup path under specific reexport conditions.

Speculative / conditional consequence: host DoS if panic_on_warn is enabled.

Leverage beyond crash: none established. The preserved nfserr_notdir return blocks the later NULL i_op->lookup call path, so the patch does not establish a usable memory corruption primitive.

ImportantCandidate=NO because there is no Class A primitive and no meaningful Class B corruption or lifetime leverage requiring Important-vs-Moderate review.

AutoDowngradeAllowed=YES because this is a constrained CLASS_C issue with no unresolved memory corruption, authorization bypass, ownership violation, or repeated lifetime mutation. Manual review is still recommended because it is network reachable and deployment impact depends on reexport exposure and panic_on_warn policy.

Likely operational severity: Borderline / Moderate-like remote DoS condition for affected NFS reexport deployments, higher operational concern on hardened systems that panic on WARN.

5. One-sentence report phrase

NFSD can expose a remotely reachable WARN_ON_ONCE during NFS reexport LOOKUP, which is mainly a warning but can become host denial of service on systems configured with panic_on_warn.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: the issue is network reachable through NFSD and impact depends on NFS reexport exposure and panic_on_warn configuration, but the patch does not establish memory corruption or privilege escalation.

7. Call-site confidence

Call-site confidence: high

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

Patch: NFSD: remove flawed WARN_ON_ONCE from nfsd_mode_check
Commit: b55b4d880bb080fa10eb08ba21a5d8679b8102fe
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b55b4d880bb080fa10eb08ba21a5d8679b8102fe

Commit description:

The header for commit e75b23f ("nfsd: check d_can_lookup in
fh_verify of directories") details the assumption that justified
adding the WARN_ON_ONCE to nfsd_mode_check(), that assumption is
invalid (in the case of NFS reexport).

When NFSD exports an NFS filesystem it is very possible for
nfsd_mode_check() to encounter a @dentRy that doesn't have
i_op->lookup (see nfs_fhget()'s NFS_ATTR_FATTR_MOUNTPOINT and
NFS_ATTR_FATTR_V4_REFERRAL handling, and d_flags_for_inode()).

So remove nfsd_mode_check()'s WARN_ON_ONCE(). The nfserr_notdir
return on that branch must stay. It guards the subsequent
lookup_one_unlocked() -> __lookup_slow() path, which calls
inode->i_op->lookup() with no NULL check, so returning nfserr_notdir
is what keeps a client LOOKUP into such a @dentRy from dereferencing
a NULL method pointer.

Fixes: e75b23f ("nfsd: check d_can_lookup in fh_verify of directories")
Cc: stable@vger.kernel.org
Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Link: https://patch.msgid.link/20260612191410.50177-1-snitzer@kernel.org
Signed-off-by: Chuck Lever <cel@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/nfsd/nfsfh.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=b55b4d880bb080fa10eb08ba21a5d8679b8102fe

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

The original Linux kernel CVE announcement for CVE-2026-89711 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89711

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:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.9

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: 2
  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).