* [CVE-2026-89501][MODERATE 7.0] ring-buffer: Hold cpu_buffer::lock when resizing a subbuf
@ 2026-09-12 5:28 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-12 5:28 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-89501
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: ring-buffer: Hold cpu_buffer::lock when resizing a subbuf
Commit: 8a496aaa3da6e8d545896c2370c7e617cc931ae2
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8a496aaa3da6e8d545896c2370c7e617cc931ae2
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89501
Analysis date: Sat, 12 Sep 2026 01:28:13 -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.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89501 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89501
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-2026-89501 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-89501 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-362;CWE-667;*CWE-416;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A race in the tracing ring buffer resize path can occur because ring_buffer_subbuf_order_set clears cpu_buffer free_page without holding cpu_buffer lock. The same free_page pointer is also used by ring_buffer_alloc_read_page and ring_buffer_free_read_page, so concurrent resize and read page operations may corrupt the lifetime state of the ring buffer page. For the CVSS the PR:L is used for the paranoid score because delegated tracing access, container root, or reduced capability service accounts may be able to trigger trace buffer operations without being full system administrators. The issue is local and is not network reachable. Impact is at least denial of service via kernel crash. In the worst case it should be treated as a use after free candidate with possible confidentiality and integrity impact due to racing page pointer ownership.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5) SKIP CVE-2026-89501 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES NO NO unknown MAYBE LOCK TRACE SIMPLEFIX RACE HARDWARE LINUS KPANIC INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4 - - checked
======================================================================
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
* Applied conservative floor/cap: Real UAF floor 4 applied because a concrete lifetime race around `cpu_buffer->free_page` is directly implied and host-admin-equivalent reachability is not fully proven from the supplied context.
* Raw paranoid score: 4
* Paranoid score: 5
* Applied paranoid floor/cap: Real UAF floor 5 applied because the race is in a runtime page ownership path, not only final teardown, and concurrent resize plus read-page allocation/free activity may be repeatably driven by an actor with tracing control.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
2. Signal breakdown
Positive signals:
* +2 Real UAF / lifetime violation: `ring_buffer_subbuf_order_set()` clears `cpu_buffer->free_page` without the same lock used by `ring_buffer_alloc_read_page()` and `ring_buffer_free_read_page()`. This directly implies a stale page ownership/lifetime race.
* +1 Reliable kernel crash / strong DoS: racing page ownership in the tracing ring buffer can plausibly lead to stale page use, invalid free, or corrupted ring-buffer state with kernel fault/DoS.
* +1 Paranoid attacker-influenced repeatable lifetime manipulation: for the paranoid path, an actor able to resize trace subbuffers and exercise read-page operations can likely repeat the vulnerable lifetime transition. This is not counted conservatively because the complete privilege model is not visible.
Negative or non-awarded signals:
* No controlled reclaim demonstrated.
* No arbitrary or directed kernel write demonstrated.
* No callback, function-pointer, credential, or security-policy object consumption demonstrated.
* C/I impact is not awarded directly. The paranoid concern is unresolved lifetime corruption, not proven confidentiality or integrity impact.
* Extra availability is not awarded because the same crash/DoS is already counted once.
* Hard race penalty is not applied. The issue is a race, but the supplied evidence does not prove a narrow one-shot or externally uncontrollable timing window.
* Host-admin penalty is not applied because the supplied patch does not fully establish the complete user-facing privilege boundary.
* Concrete repeated release/refcount signal is not awarded. The patch shows a page pointer ownership race, not a visible first put followed by a second put on the exact same dangling reference.
* Structured stale-object leverage is not separately awarded because the concrete stale consumption details of `ring_buffer_alloc_read_page()` and `ring_buffer_free_read_page()` are not included.
Repeated release/refcount checklist:
* First release can end lifetime: not established from the supplied diff.
* Exact same reference survives: possible for `free_page`, but not shown as a repeated refcount/release sequence.
* Second operation is a real lifetime/refcount mutation: not established.
* +1 Concrete repeated release/refcount awarded: no.
* Paranoid repeated-refcount leverage awarded: no.
* FLOOR 5 applied: yes, but due runtime page ownership/lifetime race and repeatable active processing, not due repeated refcount.
3. Reachability analysis
The vulnerable operation is local and belongs to the tracing ring-buffer control path. A realistic trigger likely requires the ability to change trace ring-buffer subbuffer order while another path allocates or frees a read page. In normal systems, tracing configuration is commonly restricted to root or privileged tracing users, but the supplied patch does not show the exact tracefs/sysfs entry, file permissions, or capability checks. Therefore `HostAdminRequired` is `UNKNOWN`, not `YES`.
No network trigger is established. No namespace-scoped privilege path is established. The mere possibility of delegated tracing access or container/service control is not enough to set namespace reachability to yes. Prior privileged setup may exist in real deployments if tracefs permissions are delegated, but it is not proven in the input.
The race appears repeatable if an actor can concurrently drive resize and read-page operations. The input does not prove a one-shot teardown-only race, so no timing penalty is applied.
4. Primitive and severity interpretation
The exact primitive is a race on `cpu_buffer->free_page` during ring-buffer subbuffer resizing. The patched operation stores the old `free_page` pointer and clears the shared pointer. Without holding `cpu_buffer->lock`, concurrent allocation or free of the read page can observe or modify the same pointer concurrently, creating a stale page ownership/lifetime violation.
This is best classified as `CLASS_B`: a real lifetime primitive requiring exploitation-leverage analysis. It is stronger than a pure warning or passive bookkeeping bug, but it is not `CLASS_A` because controlled reclaim, arbitrary write, callback control, or demonstrated privilege escalation is not established.
The demonstrated or directly implied consequence is local DoS through kernel crash or corrupted ring-buffer state. The speculative consequence is broader memory corruption if the stale page is reused or later processed in a useful way, but that requires manual source review of the surrounding read-page lifetime rules. Because the bug affects runtime page ownership rather than only a final destructor path, automatic downgrade would risk losing an unresolved kernel lifetime primitive.
`ImportantCandidate=YES` because this is a concrete runtime lifetime race around a kernel page pointer and post-lifetime leverage is unresolved. `AutoDowngradeAllowed=NO` because the issue should not be treated as an ordinary crash-only cleanup bug without reviewing the surrounding `ring_buffer_alloc_read_page()` and `ring_buffer_free_read_page()` paths.
Likely operational severity is Actionable Moderate. It may become Important only if manual review confirms attacker-controlled reuse, meaningful stale page processing, second lifetime mutation, or a below-host-admin trigger path.
5. One-sentence report phrase
A race in the tracing ring buffer resize path can clear `cpu_buffer->free_page` without synchronization against read-page allocation or free operations, creating a local page lifetime UAF candidate that can cause kernel DoS and requires manual review for possible stronger memory-corruption impact.
6. Manual review recommendation
MANUAL CHECK REQUIRED
Reason: the patch directly fixes a synchronization gap around a shared page pointer used by allocation/free paths, which is a concrete lifetime-corruption candidate even though controlled reclaim or privilege escalation is not demonstrated.
7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: ring-buffer: Hold cpu_buffer::lock when resizing a subbuf
Commit: 8a496aaa3da6e8d545896c2370c7e617cc931ae2
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8a496aaa3da6e8d545896c2370c7e617cc931ae2
Commit description:
Because, ring_buffer_subbuf_order_set() can clear cpu_buffer->free_page,
hold cpu_buffer->lock to prevent races with
ring_buffer_alloc_read_page() and ring_buffer_free_read_page().
Cc: [email protected]
Link: https://patch.msgid.link/[email protected]
Fixes: 8e7b58c ("ring-buffer: Just update the subbuffers when changing their allocation order")
Reported-by: Sashiko <[email protected]>
Closes: https://sashiko.dev/#/patchset/20260810125633.3344684-1-vdonnefort%40google.com # patch 3
Signed-off-by: Vincent Donnefort <[email protected]>
Signed-off-by: Steven Rostedt <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
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=8a496aaa3da6e8d545896c2370c7e617cc931ae2
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89501 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89501
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: 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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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