* [CVE-2026-89667][MODERATE REGULAR] nfsd: close shrinker/GC/fsnotify vs per-net shutdown race in filecache [ Upstream
@ 2026-09-12 4:18 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-12 4:18 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-89667
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: nfsd: close shrinker/GC/fsnotify vs per-net shutdown race in filecache [ Upstream
Commit: 6d6b9f6a75c3767250e9c23ace4e384ab8f7843e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6d6b9f6a75c3767250e9c23ace4e384ab8f7843e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89667
Analysis date: Sat, 12 Sep 2026 00:18:24 -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-2026-89667 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89667
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-2026-89667 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-89667 CHECK WITH IMPACT FROM ORIG NN LOW Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';CWE-362;CWE-667;*CWE-772;*CWE-416;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.8';DESCR 'A race in the nfsd filecache can occur when the shrinker, GC worker, or fsnotify lease callback unhashed an nfsd_file and then queues it to the per-net dispose list while nfsd per-net shutdown is running. The shutdown walk can miss an already unhashed file and the dispose list drain can complete before that file is queued, leaving the file and related state without a later drainer. For the CVSS the PR:L in the paranoid score reflects environments where a reduced capability root, container service, or delegated management process can trigger nfsd or net namespace shutdown paths. The issue is not directly network reachable because the decisive trigger is local nfsd lifecycle or net namespace teardown rather than NFS request traffic. Remote NFS clients may help create filecache pressure or cached state, but they do not by themselves provide reliable triggering of the shutdown race. Impact is at least local denial of service through resource leakage. In the worst still defensible case, limited confidentiality or integrity impact is kept for manual review because the patch context involves a race with per-net object lifetime and net_generic access.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3) YES LOCK DISK INIT RACE LEAK NETWORK HARDWARE DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 YES NO checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=3
ActionableScoreLower=2
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
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:
* +1 Concrete persistent kernel resource/reference leak. The commit directly states that an already unhashed nfsd_file can be missed by shutdown cleanup and then remain on the per-net dispose list with no later drainer, leaking the file and associated state.
* +1 Availability impact through persistent resource leakage. The leak can persist after the relevant per-net shutdown path has already drained the list, so the resource loss is not just transient bookkeeping.
* +0 Reliable kernel crash not awarded. The supplied evidence does not show panic, Oops, BUG, or a guaranteed fatal fault.
* +0 Real UAF not awarded. The patch establishes a race and leak, but not a concrete post-free dereference, double release, or stale refcount mutation.
* +0 Concrete repeated release/refcount not awarded. No first release that may end lifetime followed by a second lifetime mutation on the exact same dangling reference is shown.
* +0 C/I impact not awarded. No disclosure, unauthorized modification, controlled corruption, or security-boundary bypass is established.
* +0 Remote reachability not awarded. Remote NFS clients may create filecache state, but the decisive race requires local nfsd lifecycle or per-net shutdown activity.
* +0 Host-admin penalty not applied. Host-admin-equivalent control is plausible for nfsd shutdown, but the complete privilege boundary is not fully established by the supplied patch alone.
* +0 Race penalty not applied. The bug is a race, but the evidence does not prove that it is so narrow or unreliably triggerable that a separate timing penalty is justified.
Paranoid additional signal:
* +1 Weak per-net lifetime synchronization concern. The patch comments explicitly state that the lock guarantees nf_net is still live when net_generic() is called, so manual review should verify that the issue is limited to leakage and does not hide a stronger per-net lifetime hazard.
Important non-awarded signals:
* Controlled reclaim not demonstrated.
* Arbitrary read/write not demonstrated.
* Privilege escalation not demonstrated.
* Authorization bypass not demonstrated.
* Security-boundary bypass not demonstrated.
* RealUAF remains NO because the supplied evidence describes leaked objects, not use of freed objects.
* AutoDowngradeAllowed remains YES because the established primitive is constrained to resource leakage and unresolved lifetime synchronization, without concrete memory corruption leverage.
3. Reachability analysis
The vulnerable sequence requires concurrency between nfsd filecache disposal paths and nfsd per-net shutdown. The disposal side can be driven by shrinker, GC worker, or fsnotify/lease callbacks, while the decisive cleanup side is nfsd_file_cache_shutdown_net().
Remote NFS clients may help create cached file state or filecache pressure, but they do not by themselves control the per-net shutdown path. The issue is therefore not directly network-triggerable from ordinary NFS request traffic.
Host-admin-equivalent privilege is plausible because starting, configuring, or shutting down nfsd commonly requires administrative control. However, the supplied input does not fully resolve whether every possible trigger path requires host admin, or whether some namespace or delegated service-management environment can reach it below host-admin equivalence. Therefore HostAdminRequired=UNKNOWN.
NamespaceScopedPrivilege=UNKNOWN because the patch involves per-net shutdown, but the input does not establish that the complete required chain, including nfsd setup, cached state creation, and shutdown, is attacker-reachable inside namespaces without host-admin-equivalent setup.
No special hardware is required. Prior setup requires nfsd to be active and to have filecache state. Race repeatability is unclear and depends on nfsd lifecycle churn, GC/shrinker/fsnotify timing, and available cached files.
4. Primitive and severity interpretation
The exact primitive is a synchronization race causing a persistent nfsd_file and associated state leak. The object is an active structured kernel object, but the demonstrated consequence is that it is lost from normal cleanup rather than reused after free.
This is not a concrete UAF based on the supplied evidence. It is also not a concrete repeated release/refcount case. There is no visible first put/free that can destroy the object followed by a second put/free on the same dangling reference.
PrimitiveClass=CLASS_C because the demonstrated behavior is primarily resource leak and invalid lifecycle ordering without an independently useful corruption primitive.
Demonstrated consequence: persistent kernel resource/state leak after per-net shutdown ordering loses the object.
Speculative consequence: a stronger per-net lifetime issue is worth checking because the patch comments mention ensuring nf_net remains live for net_generic(). The supplied evidence is not strong enough to score this as RealUAF or Class B.
ImportantCandidate=NO because no controlled corruption, UAF, repeated lifetime mutation, authorization bypass, COW/ownership violation, or demonstrated privilege escalation is established.
AutoDowngradeAllowed=YES because the established issue can be treated as constrained resource leakage unless further evidence shows post-lifetime use, repeated refcount mutation, or a security-boundary consequence.
Likely operational severity: ordinary to borderline Moderate. It is worth manual review due to the per-net lifetime synchronization language, but it is not an Important-class candidate on the supplied evidence.
5. One-sentence report phrase
A race between nfsd filecache GC, shrinker, fsnotify or lease callbacks and per-net shutdown can leave an unhashed nfsd_file queued after the shutdown drain has already completed, causing a persistent filecache state leak rather than a demonstrated UAF or privilege escalation.
6. Manual review recommendation
MANUAL CHECK RECOMMENDED
Reason: the directly supported impact is resource leakage, but the per-net lifetime synchronization and net_generic() comment justify a focused check that no stronger stale per-net object use exists.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: nfsd: close shrinker/GC/fsnotify vs per-net shutdown race in filecache [ Upstream
Commit: 6d6b9f6a75c3767250e9c23ace4e384ab8f7843e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6d6b9f6a75c3767250e9c23ace4e384ab8f7843e
Changed files:
fs/nfsd/filecache.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=6d6b9f6a75c3767250e9c23ace4e384ab8f7843e
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89667 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89667
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:H/PR:H/UI:N/S:U/C:N/I:N/A:L
The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
The Best / paranoid CVSS score: 5.8
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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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