[CVE-2026-89707][MODERATE 7.0] nfsd: release path refs on follow_down() error

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

CVE: CVE-2026-89707
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: nfsd: release path refs on follow_down() error
Commit: 194316df81263519156ebe714c4a286bee00e5be
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=194316df81263519156ebe714c4a286bee00e5be
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89707
Analysis date: Fri, 11 Sep 2026 20:26:05 -0400
ActionableScore: 4
ActionableScore lower bound: 3
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-2026-89707 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89707

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-2026-89707	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89707 CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';*CWE-772;CWE-404;**CWE-400;Other CVSS 'AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.3';DESCR 'nfsd_cross_mnt can leak VFS path references when follow_down returns an error after mntget and dget have already taken entry time references. An authenticated remote NFS client may be able to trigger the affected path through lookup or NFSv4 READDIR processing when an exported tree uses cross mount or automount behavior and follow_down fails. For the CVSS the PR:L is used because the attacker needs authenticated client access to the NFS export rather than local administrator privileges. The issue is network reachable through NFS service operations, but it is configuration dependent and requires a failing cross mount traversal path. Impact is denial of service through resource exhaustion, pinned dentries, increased mount reference counts, and blocked umount operations. No confidentiality or integrity impact is assigned because the primitive is a reference leak rather than memory corruption or information disclosure.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4)	YES	LOCK DISK LEAK NETWORK KPANIC 	NO	NO	checked

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

ActionableScore=4
ActionableScoreLower=3
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 3. No floor or cap applied.
* Raw paranoid score: 4
* Paranoid score: 4. No floor or cap applied.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

Conservative signals:

* +2 Remote reachable / directly network-triggerable: the input states that the path is reachable by any authenticated NFS client through `nfsd_lookup_dentry` or the NFSv4 `READDIR` encode path.
* +1 Concrete persistent kernel resource/reference leak: `mntget()` and `dget()` take references before `follow_down()`, and the negative error path failed to call `path_put()`, leaking mount and dentry references.
* 0 Reliable kernel crash / strong DoS: no panic, UAF, invalid dereference, or immediate host crash is established.
* 0 Confidentiality and integrity: no information disclosure, unauthorized write, memory corruption, UAF, or authorization bypass is established.

Paranoid additional signal:

* +1 Persistent availability impact: repeated failed cross-mount traversal can inflate `mnt_count` and `d_count`, block `umount`, and pin dentries against the shrinker. This is still resource-leak DoS, not memory corruption.

Negative signals:

* No privilege penalty: host-admin-equivalent privilege is not required by the supplied evidence because an authenticated remote NFS client can reach the path.
* No race penalty: this is an ordered error-path leak, not a timing race.
* No rare-subsystem penalty: NFS server exposure is deployment-dependent, but the supplied evidence already establishes authenticated client reachability.

Important non-awarded signals:

* Real UAF not awarded: the bug leaks references but does not use an object after its lifetime ends.
* Concrete repeated release/refcount not awarded: there is no first release that may free the object followed by a second release on the same dangling reference.
* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Structured stale-object leverage not awarded because there is no stale post-free object use.
* Weak LPE not awarded because the issue is a reference leak/resource exhaustion primitive only.
* Extra availability was not double-counted as a crash because no crash signal was awarded.
* C/I not awarded merely because leaked VFS references affect kernel lifetime accounting.

3. Reachability analysis

An authenticated remote NFS client can trigger the affected path through server-side lookup or NFSv4 `READDIR` processing when the exported tree involves cross-mount or automount behavior and `follow_down()` returns an error.

The attacker does not need host administrator privileges according to the supplied input, but does need authenticated access to the NFS export. This is therefore below-host-admin reachability, not local root or host `CAP_SYS_ADMIN`.

Namespace-scoped privilege is not relevant to the established trigger. The complete trigger is through the NFS service, not through attacker-created user or network namespaces.

There is prior server-side setup in the sense that NFS must be running and an affected export topology must exist, but the prompt does not establish that the attacker must create that topology. The issue is configuration-dependent because the leak requires a failed cross-mount traversal path.

No special hardware, boot mode, or race timing is required. Repeated attempts appear feasible once the authenticated client can reach the failing path.

4. Primitive and severity interpretation

The exact primitive is a persistent VFS reference leak: references acquired by `mntget()` and `dget()` are not released on the `follow_down()` error path.

This is not active structured UAF and not fixed stale storage. No object is shown to be freed and then accessed. The references are instead kept alive too long, which can pin dentries and mounts.

There is no concrete repeated release/refcount operation. The bug is the absence of `path_put()`, not a second `put()` on a dangling object.

PrimitiveClass is `CLASS_C` because the demonstrated behavior is resource-leak DoS without an independently useful corruption, ownership, authorization, or memory-safety primitive.

The demonstrated consequence is persistent resource retention that can block unmount and keep dentries from being reclaimed. The speculative consequence is broader resource exhaustion if an authenticated client can repeat the failed traversal many times.

ImportantCandidate is `NO` because there is no UAF, memory corruption, COW violation, authorization bypass, controlled reclaim, arbitrary read/write, or privilege-escalation primitive.

AutoDowngradeAllowed is `YES` because the issue is a resource/reference leak without unresolved corruption or boundary-bypass leverage. Manual review is still recommended because it is network reachable through NFS and practical impact depends on export topology.

Likely operational severity is Moderate-like. It is actionable enough to avoid blind dismissal in exposed NFS environments, but it is not an Important-class kernel corruption candidate from the supplied evidence.

5. One-sentence report phrase

An authenticated NFS client can trigger a missing `path_put()` error path in `nfsd_cross_mnt()`, leaking mount and dentry references and potentially causing persistent resource exhaustion or blocked unmount operations.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: the bug is network reachable by authenticated NFS clients and can cause persistent VFS reference leaks, but no memory corruption, UAF, privilege escalation, or authorization bypass is established.

7. Call-site confidence

Call-site confidence: high

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

Patch: nfsd: release path refs on follow_down() error
Commit: 194316df81263519156ebe714c4a286bee00e5be
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=194316df81263519156ebe714c4a286bee00e5be

Commit description:

nfsd_cross_mnt() initializes a local struct path with mntget() and
dget() before calling follow_down(). On a negative return the error
arm jumps to out without releasing those references:

    err = follow_down(&path, follow_flags);
    if (err < 0)
            goto out;

follow_down() never drops the caller's entry-time refs on any error
sub-case; for example a pre-cross d_manage() failure leaves path
untouched, so the mntget()/dget() taken on entry survive the call.

Every other early-exit arm in nfsd_cross_mnt() (other-namespace
return, IS_ERR(exp2), and the success tail after the swap) already
calls path_put(&path); the err < 0 arm is the lone omission. The
leak inflates mnt_count and d_count on each failed cross-mount,
blocking umount and pinning dentries against the shrinker, and is
reachable by any authenticated NFS client through nfsd_lookup_dentry
or the NFSv4 READDIR encode path.

Fix by calling path_put(&path) before the goto out in the err < 0
arm so the entry-time refs are released on all follow_down() error
returns.

Fixes: cc53ce5 ("Add a dentry op to allow processes to be held during pathwalk transit")
Cc: stable@vger.kernel.org
Assisted-by: kres:claude-opus-4-7
Signed-off-by: Chris Mason <clm@meta.com>
Link: https://patch.msgid.link/20260531-nfsd-testing-v1-2-7bfa481b0540@kernel.org
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/nfsd/vfs.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=194316df81263519156ebe714c4a286bee00e5be

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

The original Linux kernel CVE announcement for CVE-2026-89707 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89707

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:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.3

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: 3
  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).