[CVE-2026-90272][MODERATE REGULAR] perf: arm_pmuv3: Zero initialize hw_id branch stack field [ Upstream

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-90272
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: perf: arm_pmuv3: Zero initialize hw_id branch stack field [ Upstream
Commit: 57b72442c1672963fd51ab441ca1b71f6dc79b66
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=57b72442c1672963fd51ab441ca1b71f6dc79b66
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90272
Analysis date: Fri, 18 Sep 2026 07:19:47 -0400
ActionableScore: 4
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-90272 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90272

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-2026-90272	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-90272 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 LEGACY=MODERATE RESULT=DIFF (independent 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:H/I:N/A:N';*CWE-457;CWE-908;*CWE-200;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N';BEST CVSS score: '5.5';DESCR 'The ARM PMU BRBE branch stack can expose an uninitialized hw_id field because the backing allocation contains stale kernel heap data and the driver does not explicitly initialize that field. A local user able to create a perf event with PERF_SAMPLE_BRANCH_HW_INDEX may receive these bytes through branch records. For the CVSS the PR:L value is used because a local process and permission to access the relevant perf interface are required. Actual access depends on perf_event_paranoid and system policy. The issue is not network reachable. Impact is an information disclosure with no demonstrated write primitive or direct denial of service. The base assessment uses C:L because the leaked content is allocation dependent. The paranoid assessment uses C:H because repeated samples could expose sensitive kernel data or addresses and should be reviewed manually. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 5.5 and the production ActionableScore returned 4 with recommendation Actionable Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were KP and FD specialist checks stage MODERATE_KPANIC to MODERATE because KP specialist did not establish a qualifying panic. The legacy KP specialist produced a review artifact and its decision was processed by the applicable KP rules. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP. The independent weaker Aegis shadow evaluated CVSS 3.3 and ActionableScore 1 with primitive class CLASS_C and manual review RECOMMENDED. 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 Actionable Moderate (with actual score 4)	MAYBE	DANGER SIMPLEFIX HARDWARE LINUS  DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP	-	-	checked

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

ActionableScore=4
PrimitiveClass=CLASS_B
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 4
* Raw paranoid score: 4
* Paranoid score: 4
* No floor or cap was applied.
* Final recommended bucket: **Actionable Moderate**::

2. Signal breakdown

* +2 Concrete user-visible kernel heap disclosure primitive. `branch_stack` was allocated with `kmalloc()`, while the BRBE driver left `hw_id` uninitialized and exported it through `PERF_SAMPLE_BRANCH_HW_INDEX`.
* +1 Confidentiality impact. Uninitialized allocation contents can cross the kernel-to-userspace boundary through perf branch records.
* +1 Practical disclosure chain. The established sequence is allocation without initialization, incomplete field population, and copying of the stale field to userspace.
* No local-unprivileged bonus. Access below host administrator is plausible through `perf_event_open()`, but the supplied evidence does not resolve `perf_event_paranoid`, `CAP_PERFMON`, and event-scope restrictions for the complete trigger.
* No additional paranoid point. Repeated sampling may expose multiple stale values, but predictable sensitive data, allocator shaping, and substantial disclosure are not established.
* No UAF, OOB write, arbitrary write, controlled reclaim, callback influence, crash, or privilege escalation is demonstrated.
* B5-R does not apply because there is no retry-accumulated write.
* B3-PENDING does not apply because there is no shared-backing write.
* Authorization bypass and B4-PENDING do not apply because no DENY-to-ALLOW security decision is changed.

3. Reachability analysis

The trigger requires an ARM system implementing BRBE and permission to create a perf event requesting `PERF_SAMPLE_BRANCH_HW_INDEX`. A local process can receive the uninitialized field if the applicable perf policy permits the event. The supplied context does not establish whether an ordinary user, a process with `CAP_PERFMON`, or a host administrator is required on the evaluated system, so `HostAdminRequired=UNKNOWN` and `RequiresHostUID0=UNKNOWN`.

The operation is not based on an attacker-obtainable namespace capability, so `NamespaceScopedPrivilege=NO`. No network trigger exists. BRBE-capable ARM hardware and kernel support are prerequisites, but the evidence does not justify a rare-hardware penalty. There is no race or timing requirement.

4. Primitive and severity interpretation

The exact primitive is disclosure of uninitialized kernel heap bytes through a userspace-visible perf branch record. It is not a UAF, stale-object operation, repeated release, or write primitive. The vulnerable storage is an allocated branch-record array whose initialized records contain at least one field retaining prior heap contents.

This is `CLASS_B` because a concrete information-disclosure primitive exists, but the evidence does not establish substantial sensitive-data disclosure, a stable kernel-pointer leak, arbitrary kernel reading, or an exploit chain. The demonstrated consequence is confidentiality loss. Potential KASLR assistance or disclosure of other sensitive heap values remains plausible but unproven.

`ImportantCandidate=NO` because Important-grade disclosure or exploitation leverage is not established. `AutoDowngradeAllowed=NO` because the issue should not be automatically dismissed before confirming the exposed width, number of records, allocation behavior, perf permission boundary, and whether sensitive kernel values are reproducibly disclosed.

5. One-sentence report phrase

The ARM PMU BRBE driver can return uninitialized kernel heap contents in the hw_id field of userspace-visible perf branch records when PERF_SAMPLE_BRANCH_HW_INDEX is requested.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Confirm the exact leaked byte width, number of affected records, repeatability across allocation cycles, possible kernel-pointer exposure, and whether ordinary local users can reach the affected event under relevant `perf_event_paranoid` and `CAP_PERFMON` policies.

7. Call-site confidence

Call-site confidence: high

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

Patch: perf: arm_pmuv3: Zero initialize hw_id branch stack field [ Upstream
Commit: 57b72442c1672963fd51ab441ca1b71f6dc79b66
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=57b72442c1672963fd51ab441ca1b71f6dc79b66

Changed files:
  drivers/perf/arm_pmuv3.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=57b72442c1672963fd51ab441ca1b71f6dc79b66

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

The original Linux kernel CVE announcement for CVE-2026-90272 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90272

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:L/UI:N/S:U/C:L/I:N/A:N
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N
  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: 4

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).