[CVE-2026-89502][LOW] ring-buffer: Free cpu_buffer::free_page with subbuf_order

[CVE-2026-89502][LOW] ring-buffer: Free cpu_buffer::free_page with subbuf_order

[AL-KERNEL]

CVE: CVE-2026-89502
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: ring-buffer: Free cpu_buffer::free_page with subbuf_order
Commit: 77275ccea06eaf297e8e6ac3fc830cb72086379a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=77275ccea06eaf297e8e6ac3fc830cb72086379a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89502
Analysis date: Sat, 12 Sep 2026 05:21:52 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89502 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89502

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: LOW
Manual review required: NO

A detailed explanation of the methodology and priority rules is included
at the end of this message.

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

CVE-2026-89502	LOW	https://www.kernelcve.org/list/?q=CVE-2026-89502 CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';*CWE-401;CWE-762;*CWE-772;**CWE-400;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'The ring buffer free path used free_page for cpu_buffer free_page even though that allocation can use subbuf_order greater than 0. This can free only an order 0 page from a larger allocation and leave the remaining pages leaked when the per CPU ring buffer is destroyed. For the CVSS the PR:L is used in the paranoid case because delegated tracing access, reduced capability root, or a service account with tracing control could exercise this local control plane without being full host root. The issue is not network reachable and requires local ability to configure or destroy tracing ring buffers. Impact is denial of service through memory exhaustion after repeated operations, with no concrete evidence of UAF, OOB write, information disclosure, or privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)  SKIP CVE-2026-89502 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	TRACE SIMPLEFIX HARDWARE LINUS  INCREASED_FROM_LOW_BASED_ON_HIGHCVSSSCORE DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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

ActionableScore=2
ActionableScoreLower=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 1
* Conservative score: 1
* Raw paranoid score: 2
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate**::

No floor or cap changed the score.

2. Signal breakdown

Positive signals:

* +1 Concrete persistent kernel resource leak. `cpu_buffer->free_page` can be allocated with `subbuf_order > 0`, but the old code released it with `free_page()`, effectively freeing only order 0 and leaking the remaining pages.
* +1 Paranoid sustained resource exhaustion. If an actor can repeatedly create, resize, or destroy affected ring buffers with larger sub-buffer order, the leak can accumulate and plausibly cause memory pressure or DoS.

Negative or non-awarded signals:

* No UAF awarded. The patch shows wrong-order freeing and leaked pages, not stale post-lifetime use.
* No repeated release/refcount signal. There is no first release followed by a second lifetime mutation on the same dangling reference.
* No strong corruption primitive. No OOB write, controlled reclaim, arbitrary write, type confusion, or callback influence is shown.
* No C/I impact. The evidence supports availability/resource exhaustion only.
* No reliable immediate crash point. The demonstrated issue is a leak, not a direct panic.
* No remote reachability. This is tracing/ring-buffer control-plane behavior, not packet or network input.
* No host-admin penalty applied because the supplied patch does not fully prove the complete required privilege boundary, although ordinary deployments likely restrict tracing control to root or CAP_SYS_ADMIN.

3. Reachability analysis

The likely trigger is local control over tracing ring-buffer configuration, destruction, or resizing in a setup where `subbuf_order` can be greater than 0. In typical systems, tracefs and ftrace/ring-buffer management are administrator-controlled, but the supplied evidence does not fully establish the exact complete user-facing operation and privilege check, so `HostAdminRequired=UNKNOWN` is used rather than assuming host-admin as a scoring penalty.

No namespace-scoped trigger is established. There is no evidence that an ordinary user can create a user namespace and perform the full vulnerable tracing operation there. No network trigger, special device input, firmware mediation, or race is involved.

4. Primitive and severity interpretation

The primitive is a wrong-order page free causing a persistent kernel memory leak. It is not an active structured-object UAF and not fixed stale storage accessed after free. The demonstrated consequence is resource leakage that can become DoS only through repeated operations.

This is `CLASS_C`: primarily DoS/resource exhaustion behavior without an independently useful corruption primitive. `ImportantCandidate=NO` because there is no evidence of memory corruption, security-boundary bypass, privilege escalation, or unresolved lifetime/refcount mutation. `AutoDowngradeAllowed=YES` because the issue is suitable for ordinary Moderate/Low-like handling unless product-specific evidence proves unprivileged repeated triggering at scale.

5. One-sentence report phrase

The ring buffer free path used `free_page()` for an allocation that may require `free_pages(..., subbuf_order)`, causing a local resource leak that can accumulate into memory-exhaustion DoS but does not show UAF, corruption, information disclosure, or privilege escalation.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

Reason: this is a concrete memory/resource leak with no demonstrated corruption primitive, no UAF, no remote reachability, and no security-boundary bypass.

7. Call-site confidence

Call-site confidence: high

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

Patch: ring-buffer: Free cpu_buffer::free_page with subbuf_order
Commit: 77275ccea06eaf297e8e6ac3fc830cb72086379a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=77275ccea06eaf297e8e6ac3fc830cb72086379a

Commit description:

When sub-buffers use an order greater than 0, cpu_buffer->free_page is
allocated with subbuf_order. Use the correct order for
cpu_buffer->free_page.

Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260813131152.3589632-2-vdonnefort@google.com
Fixes: f9b94da ("ring-buffer: Set new size of the ring buffer sub page")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260806211306.3704194-1-vdonnefort%40google.com # patch 3
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  kernel/trace/ring_buffer.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=77275ccea06eaf297e8e6ac3fc830cb72086379a

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

The original Linux kernel CVE announcement for CVE-2026-89502 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89502

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:L/PR:H/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: 1
  Paranoid ActionableScore: 2

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).