[CVE-2026-74634][MODERATE 7.0] ring-buffer: Prevent subbuf order change when resizing is disabled

[CVE-2026-74634][MODERATE 7.0] ring-buffer: Prevent subbuf order change when resizing is disabled

[AL-KERNEL]

CVE: CVE-2026-74634
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: ring-buffer: Prevent subbuf order change when resizing is disabled
Commit: b45b91db41379ee5fb36c187d6d7c37b725cbe8e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b45b91db41379ee5fb36c187d6d7c37b725cbe8e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74634
Analysis date: Sat, 22 Aug 2026 11:54:23 -0400
ActionableScore: 5
ActionableScore lower bound: 2
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in the tracing ring buffer can let `ring_buffer_subbuf_order_set()` free sub-buffer pages while a non-consuming reader may still advance over them in `rb_advance_iter()`, creating a local UAF that is realistically a kernel crash DoS and warrants manual review for privilege-escalation potential.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-74634 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74634

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-74634	MODERATE	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:H/I:H/A:H';*CWE-416;CWE-362;CWE-667;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A use-after-free can occur in the tracing ring buffer because ring_buffer_subbuf_order_set() may free buffer pages while resizing is disabled and a non-consuming reader can still advance over the old pages in rb_advance_iter(). For the CVSS the PR:L is used in the paranoid score because some systems delegate tracing access to service accounts, tracing groups, containers, or reduced capability root contexts rather than only full host administrators. The issue is local and is not network reachable. The realistic impact is denial of service via kernel crash. In the worst still defensible interpretation, the UAF may allow confidentiality or integrity impact and should be reviewed manually for privilege escalation risk.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 4)  SKIP CVE-2026-74634 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	TRACE UAF LINUS SYZBOT  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: **2**
* Paranoid score: **5**
* Final recommended bucket: **Actionable Moderate at minimum**

## 2. Signal breakdown

Conservative signals:

* **Memory corruption, weak primitive: +1**. The commit explicitly identifies a UAF risk, but this is a race-dependent reader versus resize case with no demonstrated reclaim, overwrite, callback control, or type confusion.
* **Real lifetime corruption: +1**. `ring_buffer_subbuf_order_set()` can free buffer pages while a non-consuming reader may still reference them through `rb_advance_iter()`.
* **Reliable kernel crash / strong DoS: +1**. A stale ring-buffer page iterator can plausibly lead to an Oops or crash.
* **Availability impact realistic: +1**. The impact is host-kernel availability when the stale page access is hit.
* **Hard or unreliable race / special timing required: -1**. Triggering requires overlap between non-consuming reader state and subbuf order change.
* **Requires admin/root/CAP_SYS_ADMIN in typical deployments: -2**. Tracing ring-buffer configuration is normally restricted to root or privileged tracing control.

Paranoid additional interpretation:

* **Local lower-privileged trigger: +1**. Some environments delegate tracing access to service accounts, tracing groups, reduced-capability root, or controlled debugging roles.
* **No full admin penalty in paranoid case: +2 relative to conservative**. In delegated tracing setups, the attacker may not have full host-root equivalence.
* **Cap applied for race-only UAF: 5**. The patch shows UAF risk, but not attacker-controlled reclaim, write-after-free, stale callback dispatch, or refcount takeover.

Not awarded:

* **No remote/network score**. This is not packet-triggered.
* **No strong LPE score**. UAF exists, but the patch does not show reclaim control, object replacement, arbitrary write, or callback hijack.
* **No confidentiality/integrity score in conservative scoring**. The realistic evidence supports DoS first.

## 3. Reachability analysis

The bug is reachable through local tracing ring-buffer control paths, specifically changing subbuf order while resizing is disabled and a non-consuming reader is active. In normal production deployments, this usually requires root, CAP_SYS_ADMIN, or privileged access to tracefs/debug tracing controls. Containers generally do not get host tracing control by default, but some observability or debugging deployments may delegate tracing access to non-full-root service accounts or reduced-capability contexts.

The path is not network reachable. The race requires a specific local control-plane sequence and timing, likely making exploitation harder than a direct syscall crash. Call-site confidence: **medium**, because the patch and commit directly name the vulnerable operation and the UAF consumer `rb_advance_iter()`, but the full reader and tracefs call paths are not included in the supplied diff.

## 4. Severity interpretation

Realistically, this behaves like a **local privileged or semi-privileged DoS** with explicit UAF risk. The conservative score remains low because typical triggering requires privileged tracing control and a race window.

However, because the bug is an actual kernel lifetime error involving freed ring-buffer pages and a reader iterator, it should not be treated as an ordinary low-priority correctness fix. The paranoid score reaches **Actionable Moderate** because delegated tracing access is plausible in real systems and UAF-class bugs deserve manual review even when no practical LPE primitive is demonstrated.

## 5. One-sentence report phrase

A race in the tracing ring buffer can let `ring_buffer_subbuf_order_set()` free sub-buffer pages while a non-consuming reader may still advance over them in `rb_advance_iter()`, creating a local UAF that is realistically a kernel crash DoS and warrants manual review for privilege-escalation potential.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

Reason: the patch explicitly describes a use-after-free in kernel tracing ring-buffer lifetime handling. Even though the realistic impact appears to be DoS and the race is likely privileged in default deployments, UAF-class bugs should receive manual review before closure.

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

Patch: ring-buffer: Prevent subbuf order change when resizing is disabled
Commit: b45b91db41379ee5fb36c187d6d7c37b725cbe8e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b45b91db41379ee5fb36c187d6d7c37b725cbe8e

Commit description:

Because ring_buffer_subbuf_order_set() frees buffer pages, we can't
allow it when resizing is disabled. A non-consuming reader is at risk of
use-after-free (rb_advance_iter()).

Return -EBUSY on resize_disabled, matching ring_buffer_resize()
behaviour.

Cc: stable@vger.kernel.org
Fixes: f9b94da ("ring-buffer: Set new size of the ring buffer sub page")
Link: https://patch.msgid.link/20260806211306.3704194-3-vdonnefort@google.com
Reported-by: syzbot+e0cc44465d6bae735679@syzkaller.appspotmail.com
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
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=b45b91db41379ee5fb36c187d6d7c37b725cbe8e

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

The original Linux kernel CVE announcement for CVE-2026-74634 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74634

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:H/I:H/A:H
  The Best / paranoid CVSS score: 7

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: 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: NO
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).