[CVE-2026-99159][MODERATE REGULAR] ring-buffer: Fix the sub-buffer array base in rb_meta_subbuf_idx()

[CVE-2026-99159][MODERATE REGULAR] ring-buffer: Fix the sub-buffer array base in rb_meta_subbuf_idx()

[AL-KERNEL]

CVE: CVE-2026-99159
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: ring-buffer: Fix the sub-buffer array base in rb_meta_subbuf_idx()
Commit: 2f1248b728b56d583ea454fedf239550c49f7b18
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2f1248b728b56d583ea454fedf239550c49f7b18
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99159
Analysis date: Wed, 28 Oct 2026 22:14:27 -0400
ActionableScore: 3
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
An incorrect persistent ring-buffer base calculation shifts sub-buffer indices, enabling a permitted local mapper to trigger a NULL dereference and causing prior-boot trace loss.

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

The original Linux kernel CVE announcement for CVE-2026-99159 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99159

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-2026-99159	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-99159 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 LEGACY=MODERATE RESULT=DIFF (final Legacy and Aegis differ; FINAL: MODERATE, Aegis: MODERATE_KPANIC)	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-682;**CWE-476;CWE-129;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'An incorrect base calculation in rb_meta_subbuf_idx can return an index one sub buffer too high for a persistent ring buffer. This shifts entries written to subbuf_ids and leaves entry zero NULL, which __rb_map_vma can dereference when userspace maps trace_pipe_raw and may cause a kernel oops or panic. The invalid indices can also make the persistent metadata fail validation after reboot and discard the previous boot trace. For the CVSS the PR:L value reflects that a local user or process must have permission to access and map the trace interface. Exploitation also requires persistent ring buffer metadata with the affected alignment layout. There is no evidence of UAF, OOB write, information disclosure, or a privilege escalation primitive. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 5.5 and the production ActionableScore returned 3 with recommendation Borderline / manual review recommended. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage LOW to MODERATE_KPANIC because the manual review requirement was applied. ActionableScore thresholds stage MODERATE_KPANIC to MODERATE because the ActionableScore threshold rule was applied. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK and DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 and V41_INHERITED_KPANIC_PROMOTION_BLOCKED and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 2.7 and ActionableScore 1 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. The models disagreed but the aggregation rules retained the production result. The final reported impact is MODERATE.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3)	MAYBE	OOB TRACE SIMPLEFIX LEAK HARDWARE LINUS  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 V41_INHERITED_KPANIC_PROMOTION_BLOCKED MANUAL_REVIEW_REQUIRED	-	-	checked

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

ActionableScore=3
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 3.
* Conservative score: 3; no floor or cap applied.
* Raw paranoid score: 3.
* Paranoid score: 3; unsupported memory-corruption effects were not added.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

* +1 Local below-host-admin trigger: supplied evidence grounds access by a local user or process with permission to map `trace_pipe_raw`; this may depend on delegated tracefs permissions.
* +1 Reliable kernel crash/strong DoS: the shifted `subbuf_ids[]` leaves entry zero NULL, which `__rb_map_vma()` dereferences.
* +1 Distinct persistent data loss: invalid indices written into `meta->buffers[]` can fail validation after reboot and discard the previous boot’s trace.
* No UAF, heap OOB write, arbitrary write, disclosure, controlled reclaim, or privilege escalation is shown.
* NEW52/B5-R does not apply: there is no confirmed retry-accumulated heap OOB write or later consumption of OOB-corrupted structured state.
* No extra availability point was awarded for repeatability of the same NULL dereference.
* No confidentiality or protected-state integrity point was awarded; discarded diagnostic trace data is the only separate persistent effect shown.

3. Reachability analysis

The immediate trigger is mapping `trace_pipe_raw` after an affected persistent ring buffer has been configured with the alignment-sensitive layout. The supplied summary supports a permission-bearing local actor below host-admin, so `HostAdminRequired=NO` and `RequiresHostUID0=NO`. Persistent tracing configuration may require prior privileged setup, but the later mapping trigger need not. No evidence establishes that the complete path is namespace-scoped. No race, retry loop, or special hardware is required.

4. Primitive and severity reading

The primitive is an arithmetic base mismatch producing an index one sub-buffer too high. It causes a shifted pointer array and a later NULL dereference, plus invalid persistent metadata. This is passive index/state corruption with crash and trace-loss consequences, not a confirmed write outside an allocation; therefore it is CLASS_C. No authorization decision changes from DENY to ALLOW, so B4 pending review is inapplicable. There is no material unknown exploit primitive requiring Important preservation, and automatic Moderate downgrade remains permissible.

5. One-sentence report phrase

An incorrect persistent ring-buffer base calculation shifts sub-buffer indices, enabling a permitted local mapper to trigger a NULL dereference and causing prior-boot trace loss.

6. Manual review advice

MANUAL CHECK RECOMMENDED

Confirm product-specific tracefs permissions and whether persistent ring-buffer setup is exposed to delegated users.

7. Call-site confidence

Call-site confidence: high

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

Patch: ring-buffer: Fix the sub-buffer array base in rb_meta_subbuf_idx()
Commit: 2f1248b728b56d583ea454fedf239550c49f7b18
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2f1248b728b56d583ea454fedf239550c49f7b18

Commit description:

On a persistent ring buffer, rb_meta_subbuf_idx() can return an index one
sub-buffer too high.  rb_setup_ids_meta_page() then fills subbuf_ids[]
shifted by one and leaves subbuf_ids[0] NULL, which __rb_map_vma()
dereferences when userspace maps trace_pipe_raw.  rb_update_meta_reader()
writes two such indices into meta->buffers[], which rb_cpu_meta_valid()
would reject on the next boot, discarding the previous boot's trace.

A CPU's sub-buffer array starts after its meta header and the nr_subbufs
integers that follow, aligned up to a sub-buffer.  rb_meta_subbuf_idx()
inverts that calculation but skips only the integers, not the header, so
its base is one sub-buffer low whenever the omitted header size crosses
the alignment boundary.

Invert the calculation with rb_subbufs_from_meta() rather than repeating
it.  rb_range_buffer() maps an index back to an address through the same
helper, so the two directions can no longer disagree.

Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260918071507.1019251-1-donggeunyoo.kernel@gmail.com
Fixes: b14d032 ("ring-buffer: Add ring_buffer_meta data")
Signed-off-by: Donggeun Yoo <donggeunyoo.kernel@gmail.com>
Reviewed-by: Vincent Donnefort <vdonnefort@google.com>
Acked-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=2f1248b728b56d583ea454fedf239550c49f7b18

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

The original Linux kernel CVE announcement for CVE-2026-99159 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99159

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:N/A:H
  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: unknown
  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).