[CVE-2026-89595][MODERATE REGULAR] fsnotify: Fix stale object mask after concurrent mark updates [ Upstream

[CVE-2026-89595][MODERATE REGULAR] fsnotify: Fix stale object mask after concurrent mark updates [ Upstream

[AL-KERNEL]

CVE: CVE-2026-89595
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: fsnotify: Fix stale object mask after concurrent mark updates [ Upstream
Commit: 947400af98b9e63841929d079a2c3ea0a8ba227b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=947400af98b9e63841929d079a2c3ea0a8ba227b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89595
Analysis date: Sat, 12 Sep 2026 03:29:03 -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-2026-89595 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89595

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-2026-89595	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89595 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:L';CWE-362;CWE-667;CWE-693;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:N';BEST CVSS score: '4.5';DESCR 'A race in fsnotify mark updates can leave the cached object mask stale after concurrent fanotify or inotify updates on the same connector. This can cause FAN_MODIFY or IN_MODIFY events to be missed even after the relevant mark update has returned successfully. For the CVSS the PR:L is used because a local user or local process context is needed to create or update watches and to participate in the racing control path. The issue is not directly network reachable and does not involve packet processing. Impact is mainly a local notification or monitoring bypass, not a kernel crash or memory corruption. In practical terms this may matter for audit tools, file monitoring agents, or security software that relies on fanotify or inotify event delivery.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3)	YES	DISK LINUS  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK 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 positive signals:

* +1 Local unprivileged trigger. The inotify path can be reached by an ordinary local process that creates or updates watches on accessible filesystem objects.
* +1 Concrete stale active notification state. The patch establishes that concurrent mark updates can leave the cached object mask stale after both operations complete, causing expected modify events to be missed.

Paranoid additional signal:

* +1 Security-relevant monitoring impact. Missed fanotify or inotify events can affect audit tools, file monitoring agents, or security software that relies on event delivery, although this is not a kernel authorization bypass.

Non-awarded signals:

* RealUAF not awarded. The patch shows stale aggregate mask state, not post-free object use.
* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Kernel crash or strong DoS not demonstrated.
* Confidentiality impact not awarded because there is no disclosure path.
* Integrity impact is limited to missed notification semantics, not unauthorized kernel or file modification.
* Authorization bypass not awarded because the patch does not show a DENY to ALLOW access-control decision change.
* Remote reachability not awarded because the issue is triggered through local fanotify or inotify watch update operations.
* Race penalty not applied because the race is reproducible with concurrent local watch updates and is not shown to be a one-shot or externally uncontrollable timing window.

Repeated release or refcount analysis:

* First release can end lifetime: no evidence.
* Exact same reference survives: not applicable.
* Second lifetime or refcount mutation: not established.
* Concrete repeated release/refcount +1 awarded: no.
* Paranoid repeated-refcount leverage +1 awarded: no.
* FLOOR 5 applied: no.

3. Reachability analysis

The practical trigger is local. An attacker or local process must create or update fanotify or inotify marks on the same connector and race concurrent updates so that one path recalculates the aggregate mask while another path changes a mark and skips recalculation.

For inotify, ordinary local users can generally create watches on filesystem objects they can access, so host administrator privilege is not required for that path. Fanotify capabilities depend on the fanotify mode and requested features, but the supplied evidence also identifies an equivalent inotify interleaving that loses IN_MODIFY events. Therefore HostAdminRequired is NO for the complete inotify-based trigger.

No network packet path is involved. Namespace-scoped privilege is not relevant to the complete trigger. No special hardware, boot option, or rare subsystem prerequisite is shown beyond fsnotify support and normal filesystem watch usage. The race is concurrent but appears repeatable through ordinary local watch update operations.

4. Primitive and severity interpretation

The exact primitive is stale cached fsnotify object mask state after concurrent fanotify or inotify mark updates. This is not a heap UAF, not an RCU lifetime violation, not a refcount bug, and not an OOB access. The affected state is active kernel notification state, but the demonstrated consequence is missed events rather than memory corruption or kernel crash.

This is best classified as CLASS_C because it is an invalid-state or stale-state behavior without an independently useful corruption primitive. The demonstrated consequence is that FAN_MODIFY or IN_MODIFY may not be delivered even though the final mark state says the event should be reported. The speculative security consequence is that monitoring, audit, or security tooling relying on these events may miss file modifications.

ImportantCandidate is NO because there is no memory corruption, no authorization decision bypass, no privileged resource theft, no demonstrated LPE, and no strong exploitation primitive beyond missed notifications. AutoDowngradeAllowed is YES because the issue is suitable for non-urgent handling if the product does not rely on fanotify or inotify for security enforcement. Manual review is still recommended when such event delivery is part of a security monitoring or audit policy.

Likely operational severity is low to borderline moderate, with higher practical importance only in environments where file modification notifications are treated as a security control.

5. One-sentence report phrase

A race in fsnotify mark updates can leave the cached object mask stale, causing fanotify or inotify modify events to be missed after concurrent watch changes, which may affect local monitoring or audit tools but does not show kernel memory corruption or privilege escalation.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: the issue is low-scoring from a kernel corruption perspective, but missed fanotify or inotify events can be security relevant for products that depend on filesystem event monitoring.

7. Call-site confidence

Call-site confidence: high

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

Patch: fsnotify: Fix stale object mask after concurrent mark updates [ Upstream
Commit: 947400af98b9e63841929d079a2c3ea0a8ba227b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=947400af98b9e63841929d079a2c3ea0a8ba227b

Changed files:
  fs/notify/fanotify/fanotify_user.c
  fs/notify/inotify/inotify_user.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=947400af98b9e63841929d079a2c3ea0a8ba227b

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

The original Linux kernel CVE announcement for CVE-2026-89595 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89595

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:L/UI:N/S:U/C:N/I:L/A:N
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:L
  The Best / paranoid CVSS score: 4.5

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: 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).