[CVE-2026-89643][MODERATE 7.0] audit: avoid dropping live tree ref on fsnotify rule autoremove

[CVE-2026-89643][MODERATE 7.0] audit: avoid dropping live tree ref on fsnotify rule autoremove

[AL-KERNEL]

CVE: CVE-2026-89643
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: audit: avoid dropping live tree ref on fsnotify rule autoremove
Commit: cf37b61b5420fd4600fe78cf8c24da6e4d9bfe28
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=cf37b61b5420fd4600fe78cf8c24da6e4d9bfe28
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89643
Analysis date: Sat, 12 Sep 2026 01:55:44 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89643 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89643

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-2026-89643	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89643 CHECK WITH IMPACT FROM ORIG NN IMPORTANT	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-416;CWE-911;*CWE-672;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.8';DESCR 'An audit tree lifetime bug can occur because audit_del_rule is used both for netlink deletion templates and for internal fsnotify autoremove of installed rules. The old code unconditionally dropped an audit_tree reference even when the caller passed the installed rule itself, which can free a shared tree while another audit rule still references it. A later autoremove may dereference the freed pathname while comparing rules, resulting in a use-after-free condition and at least a local denial of service through a kernel crash. For the CVSS the PR:L value is used for the paranoid score because a local user may be able to trigger fsnotify autoremove when vulnerable audit rules already exist, while creating the rules normally requires audit administration privileges. The issue is not network reachable. Impact is at least availability loss and in the worst defensible case may include limited confidentiality or integrity impact due to the UAF primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	YES	UAF LINUS KPANIC  KPANIC LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS 	NO	NO	checked

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

ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 4
* Raw paranoid score: 4
* Paranoid score: 5
* Conservative floor/cap: Real UAF floor applied, 3 -> 4.
* Paranoid floor/cap: Real UAF floor 5 applied for an active shared audit_tree lifetime bug with post-lifetime structured pathname/rule comparison, 4 -> 5.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Positive signals:

* +2 Real UAF / lifetime violation. The patch establishes that audit_del_rule() can incorrectly drop the live audit_tree reference of an installed rule during fsnotify autoremove, allowing the tree to be freed while another rule still references it.
* +1 Reliable kernel crash / strong DoS. The commit states that a later autoremove can dereference the freed pathname while comparing rules, which supports at least a kernel fault or crash.
* +1 Paranoid reachability signal. If vulnerable audit rules already exist, a local user may be able to trigger fsnotify autoremove events through filesystem activity, but the complete privilege boundary is not fully established from the supplied patch alone.

Non-awarded or withheld signals:

* Concrete repeated release/refcount +1 was not separately awarded under the exact same dangling reference rule. The bug involves repeated wrong puts against a shared audit_tree lifetime, but the supplied control flow does not prove a second lifetime mutation through the exact same already dangling local reference.
* Structured stale-object leverage +1 was not separately awarded. The stale pathname is consumed during rule comparison, but controlled reclaim, attacker-shaped replacement contents, callback control, or arbitrary read/write is not demonstrated.
* Weak LPE +1 was not awarded solely because the bug is a UAF.
* Strong exploitation leverage +2 was not awarded. No controlled reclaim, directed corruption, callback hijack, refcount takeover, or demonstrated LPE is supplied.
* Extra Availability +1 was not awarded because the crash/DoS consequence is already counted once.
* Confidentiality and Integrity bonuses were not awarded. The UAF could theoretically have broader consequences, but the supplied evidence does not establish disclosure, unauthorized modification, arbitrary write, or credential/security-state manipulation.
* Remote reachability was not awarded. This is not a network-triggered vulnerability.

Repeated-release/refcount checklist:

* First release can end lifetime: yes, audit_put_tree() can drop a lifetime reference and may free audit_tree when the reference count reaches zero.
* Exact same reference survives to a second lifetime mutation: not proven from the supplied diff.
* Second operation is a real lifetime/refcount mutation on the exact same dangling reference: not proven.
* Dedicated +1 Concrete repeated release/refcount awarded: no.
* Paranoid repeated-refcount leverage +1 awarded: no.
* FLOOR 5 applied: yes, but due active shared lifetime object plus unresolved post-lifetime structured use, not due the exact-same-reference repeated-release rule.

3. Reachability analysis

The normal rule creation and deletion path through audit netlink requires audit administration privileges, typically CAP_AUDIT_CONTROL or host-admin-equivalent control. However, the vulnerable path also involves internal fsnotify autoremove of already installed rules, and the commit specifically distinguishes this from netlink deletion templates.

The complete attacker model is therefore unresolved. Creating the vulnerable mixed AUDIT_DIR plus AUDIT_EXE rules is privileged, but a local non-admin user may be able to cause fsnotify autoremove events if such rules already exist and cover paths the user can affect. Because the supplied evidence does not prove that host-admin privilege is required for the complete trigger, HostAdminRequired is UNKNOWN rather than YES.

No namespace-scoped capability path is established. Audit rule management is not shown to be fully available inside attacker-created namespaces, so NamespaceScopedPrivilege is NO.

No prior special hardware, boot mode, or rare device dependency is involved. No direct network vector is established. The issue is event/lifetime driven rather than a narrow timing race, so no race penalty is applied.

4. Primitive and severity interpretation

The exact primitive is an audit_tree reference ownership/lifetime bug. audit_del_rule() was shared by two ownership models: netlink deletion templates that own a temporary tree reference, and fsnotify autoremove paths that pass an installed rule entry. The old unconditional audit_put_tree() treated both cases as if they owned a temporary reference, which could prematurely drop a live tree reference from an installed rule.

This is an active structured object UAF rather than fixed stale storage. The stale object is part of audit rule state, and the later stale use dereferences pathname state while comparing rules during autoremove. The demonstrated consequence is local DoS through a kernel crash or fault. The speculative consequence is limited unresolved exploitation leverage from a real kernel lifetime bug in security/audit state, but no controlled reclaim or arbitrary kernel R/W is demonstrated.

This is CLASS_B because a real active-object lifetime/UAF primitive is established, but Class A exploitation primitives are not. ImportantCandidate is YES because the bug affects shared audit lifetime state and post-lifetime pathname/rule processing, so automatic downgrade would risk losing a meaningful unresolved kernel lifetime primitive. AutoDowngradeAllowed is NO for the same reason.

Likely operational severity is Actionable Moderate, with manual review needed to decide whether the audit_tree/pathname stale-use path can be developed beyond crash in realistic configurations.

5. One-sentence report phrase

An audit_tree lifetime bug in fsnotify audit rule autoremove can prematurely drop a live shared tree reference, leaving another installed rule with a dangling tree pointer and causing a later autoremove to dereference freed pathname state, resulting in at least local kernel DoS and requiring manual review for unresolved UAF exploitation leverage.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a real UAF/lifetime ownership bug in audit rule state with shared references and post-lifetime structured pathname access, and the below-host-admin triggerability depends on existing audit rule configuration and filesystem autoremove conditions.

7. Call-site confidence

Call-site confidence: high

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

Patch: audit: avoid dropping live tree ref on fsnotify rule autoremove
Commit: cf37b61b5420fd4600fe78cf8c24da6e4d9bfe28
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=cf37b61b5420fd4600fe78cf8c24da6e4d9bfe28

Commit description:

audit_del_rule() is used for both netlink deletion templates and internal
fsnotify autoremove.  The former passes a parsed template which owns a
temporary tree reference; the latter passes the installed entry itself.

The unconditional audit_put_tree() at the end of audit_del_rule() assumes
the template case.  For mixed AUDIT_DIR plus AUDIT_EXE rules, an fsnotify
autoremove event therefore drops the installed rule's live tree reference.
Repeating this across rules sharing the same tree can free the tree while
another rule still references it, and a later autoremove dereferences the
freed pathname while comparing rules.

Move the temporary-tree put to audit_rule_change(), the caller that owns
deletion templates.  Keep it in the AUDIT_DEL_RULE cleanup so both
successful deletion and -ENOENT still release the parser-owned tree.

Cc: stable@kernel.org
Fixes: 34d99af ("audit: implement audit by executable")
Assisted-by: Codex:gpt-5
Signed-off-by: Jérémy Jean <Jeremy.Jean@oss.cyber.gouv.fr>
Reviewed-by: Ricardo Robaina <rrobaina@redhat.com>
Tested-by: Ricardo Robaina <rrobaina@redhat.com>
[PM: dropped unnecessary comment for line length reasons]
Signed-off-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  kernel/auditfilter.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=cf37b61b5420fd4600fe78cf8c24da6e4d9bfe28

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

The original Linux kernel CVE announcement for CVE-2026-89643 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89643

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:H
  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: 4
  Paranoid ActionableScore: 5

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).