[CVE-2026-90136][LOW] platform/x86/amd/hsmp: Reject negative power cap writes in hwmon [ Upstream

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

CVE: CVE-2026-90136
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: platform/x86/amd/hsmp: Reject negative power cap writes in hwmon [ Upstream
Commit: 2c09cadec116eba3fdbcb5d8d8641f6777d4a11f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2c09cadec116eba3fdbcb5d8d8641f6777d4a11f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90136
Analysis date: Fri, 18 Sep 2026 06:41:30 -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-2026-90136 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90136

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-2026-90136	LOW	https://www.kernelcve.org/list/?q=CVE-2026-90136 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:N/I:L/A:H';*CWE-20;CWE-681;CWE-190;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '5.3';DESCR 'A signed to unsigned conversion bug in amd hsmp hwmon handling can accept a negative power1_cap value and convert it into a huge socket power limit before sending it to the SMU. This may bypass the intended validation of power limits and can result in an invalid multi gigawatt power cap being applied by firmware. For the CVSS the PR:L paranoid case covers systems where hwmon power cap writes are delegated to a reduced capability local account or a management service. In normal deployments the access requirement is closer to PR:H because the sysfs power cap attribute is usually writable only by root. The issue is not network reachable. Impact is mainly integrity impact to power management policy and possible availability impact from unsafe or unstable power configuration, not memory corruption or privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)	YES	WRITE OOB HARDWARE  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	YES	NO	checked

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

ActionableScore=2
ActionableScoreLower=1
PrimitiveClass=NONE
RealUAF=NO
HostAdminRequired=YES
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 1
* Conservative score: 1. No floor or cap applied.
* Raw paranoid score: 2
* Paranoid score: 2. No floor or cap applied.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

Conservative positive signals:

* +1. Concrete input validation / integer conversion defect: a negative signed `long` value written to `power1_cap` is converted through unsigned arithmetic and sent to firmware as a huge socket power limit.

Paranoid additional signals:

* +1. Limited availability concern: if firmware accepts the bogus multi-gigawatt cap, platform power management could become unstable or unsafe. This is not a demonstrated kernel crash or memory corruption primitive.

Negative or non-awarded signals:

* Host-admin-equivalent access is positively established for the normal sysfs hwmon write path, but no extra negative was applied because the total score is already primitive-free and no reachability bonus was awarded.
* No UAF, OOB, double free, refcount mutation, list corruption, or arbitrary write is established.
* No controlled reclaim or attacker-controlled kernel corruption is demonstrated.
* No confidentiality impact is established.
* No privilege escalation path is established.
* No remote or network reachability is established.
* No reliable kernel crash is demonstrated.
* No extra availability point is awarded because the availability impact is firmware/platform-behavior dependent.

3. Reachability analysis

The trigger is a local write to the AMD HSMP hwmon `power1_cap` sysfs attribute. In normal deployments, writable hwmon control attributes are root/admin controlled, so the complete trigger requires host-admin-equivalent privileges. A delegated management service or reduced local account could theoretically expose this write, but that is a deployment-specific delegation and is not established by the patch.

No namespace-scoped capability path is indicated. There is no network trigger. The issue requires AMD HSMP support and a system exposing this hwmon attribute, but no rare physical attack path is required beyond having affected platform support. There is no race and no timing dependency.

4. Primitive and severity interpretation

The exact primitive is invalid numeric conversion and missing validation of a negative power cap value before sending a firmware command. This is not memory corruption and not a lifetime or ownership bug. It is an invalid firmware-mediated configuration value that can bypass intended validation of power-management policy.

PrimitiveClass is `NONE` because no meaningful kernel corruption, authorization bypass, stale object, COW violation, or security-boundary primitive is established. Demonstrated consequence is acceptance of a bogus power limit by the SMU. Speculative consequence is platform instability or reduced availability if firmware applies the value in a harmful way. ImportantCandidate is `NO` because there is no evidence of LPE, kernel corruption, protected resource access, or host-wide reliable crash. AutoDowngradeAllowed is `YES` because this is a host-admin local input validation issue with low practical security impact.

5. One-sentence report phrase

A signed to unsigned conversion bug in amd hsmp hwmon allowed negative power1_cap writes to become huge firmware power limit values, but exploitation normally requires local administrative write access and no memory corruption or privilege escalation primitive is shown.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

Reason: the patch establishes a local host-admin input validation bug only, with no UAF, no kernel memory corruption, no authorization boundary bypass, no remote reachability, and no demonstrated LPE.

7. Call-site confidence

Call-site confidence: high

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

Patch: platform/x86/amd/hsmp: Reject negative power cap writes in hwmon [ Upstream
Commit: 2c09cadec116eba3fdbcb5d8d8641f6777d4a11f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2c09cadec116eba3fdbcb5d8d8641f6777d4a11f

Changed files:
  drivers/platform/x86/amd/hsmp/hwmon.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=2c09cadec116eba3fdbcb5d8d8641f6777d4a11f

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

The original Linux kernel CVE announcement for CVE-2026-90136 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90136

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:L/A:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:H
  The Best / paranoid CVSS score: 5.3

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).