[CVE-2026-64512][LOW] Revert "ACPI: CPPC: Use access_width over bit_width for system memory accesses" This reverts commit 4949aff which is

[CVE-2026-64512][LOW] Revert "ACPI: CPPC: Use access_width over bit_width for system memory accesses" This reverts commit 4949aff which is

[AL-KERNEL]

CVE: CVE-2026-64512
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: Revert "ACPI: CPPC: Use access_width over bit_width for system memory accesses" This reverts commit 4949aff which is
Commit: b54c4632946ae42f2b39ed38abd909bbf78cbcc2
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b54c4632946ae42f2b39ed38abd909bbf78cbcc2
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64512
Analysis date: Sat, 25 Jul 2026 11:23:40 -0400
ActionableScore: 0
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
ACPI CPPC may misinterpret the PCC `access_width` field as an access-size value, causing a shift-out-of-bounds UBSAN warning during CPPC read or write paths, but practical triggering requires firmware or administrator-level control and no memory corruption or privilege escalation primitive is shown.

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

The original Linux kernel CVE announcement for CVE-2026-64512 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64512

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-2026-64512	LOW	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';CWE-682;CWE-758;*CWE-20;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.4';DESCR 'ACPI CPPC can decode reg access_width as an access size even when the register uses ACPI_ADR_SPACE_PLATFORM_COMM, where the same field carries a PCC subspace id. If that value is greater than the normal access width range, GET_BIT_WIDTH can evaluate a shift with an invalid exponent and trigger a UBSAN shift out of bounds warning in cpc_read or cpc_write. For the CVSS the PR:H is used for the paranoid score because reliable triggering normally requires administrator level control over firmware supplied ACPI tables or equivalent platform configuration. The issue is not network reachable and does not provide a demonstrated memory corruption primitive. Impact is mainly denial of service only when warning handling or UBSAN policy turns the warning into a system failure.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 0)	YES	WRITE KASAN OOB INIT HARDWARE  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	YES	NO	checked

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

ActionableScore=0

## 1. ActionableScore

* Conservative score: 0
* Paranoid score: 1
* Final recommended bucket: **Ordinary Moderate / Low-like**::

## 2. Signal breakdown

Conservative scoring:

* Firmware-mediated external influence: +1
  The value comes from firmware supplied ACPI CPPC tables. This is not remotely reachable, but platform firmware can influence the problematic field.

* Requires admin/root/platform control in typical deployments: -2
  Reliable triggering normally requires control over ACPI tables, firmware, boot configuration, or equivalent platform-level configuration. This is effectively administrator or firmware-provider control.

* WARN/UBSAN-only condition: +0
  The patch addresses a shift-out-of-bounds UBSAN warning caused by decoding `access_width` incorrectly for `ACPI_ADR_SPACE_PLATFORM_COMM`. The commit does not show UAF, OOB write, double-free, arbitrary write, info leak, or privilege escalation.

Paranoid adjustment:

* Availability concern under strict UBSAN or panic-on-warning policy: +1
  If the system treats UBSAN warnings as fatal, this can become a local/platform-triggered DoS. This is configuration-dependent and not enough to make the issue actionable Important.

## 3. Reachability analysis

The bug is reached through ACPI CPPC handling, especially `cpc_read()`, `cpc_write()`, `cppc_set_perf()`, and CPU frequency initialization paths. A normal unprivileged local user cannot directly control the ACPI register descriptors needed to trigger this. Practical triggering requires malformed or unusual firmware tables, platform configuration, or administrator-level ability to override ACPI data.

Namespaces and containers do not materially reduce the privilege requirement because ACPI table control is outside ordinary container privilege boundaries. The path may be exercised during normal boot or CPU frequency setup on affected hardware, but attacker-controlled reachability is narrow and firmware-mediated.

## 4. Severity interpretation

This behaves like a Low-like or ordinary Moderate robustness issue, not an Important-class vulnerability. The demonstrated failure mode is an UBSAN shift-out-of-bounds warning due to field misuse. There is no concrete memory corruption primitive, no user-controlled overwrite, no stale lifetime issue, no security-boundary bypass, and no network exposure.

The theoretical worst case is denial of service if warning handling is fatal. Realistic exploitation evidence is weak because the attacker must influence firmware-provided ACPI data or equivalent privileged platform state.

## 5. One-sentence report phrase

ACPI CPPC may misinterpret the PCC `access_width` field as an access-size value, causing a shift-out-of-bounds UBSAN warning during CPPC read or write paths, but practical triggering requires firmware or administrator-level control and no memory corruption or privilege escalation primitive is shown.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This can be auto-closed for practical triage unless the affected product specifically allows untrusted ACPI table injection or treats UBSAN warnings as fatal in production.

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

Patch: Revert "ACPI: CPPC: Use access_width over bit_width for system memory accesses" This reverts commit 4949aff which is
Commit: b54c4632946ae42f2b39ed38abd909bbf78cbcc2
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b54c4632946ae42f2b39ed38abd909bbf78cbcc2

Commit description:

It breaks AmpereOne systems and should not have been added to the stable
tree just yet.

Link: https://lore.kernel.org/r/97d25ef7-dee9-4cc5-842a-273f565869b3@linux.microsoft.com
Reported-by: Easwar Hariharan <eahariha@linux.microsoft.com>
Cc: Jarred White <jarredwhite@linux.microsoft.com>
Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/acpi/cppc_acpi.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=b54c4632946ae42f2b39ed38abd909bbf78cbcc2

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

The original Linux kernel CVE announcement for CVE-2026-64512 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64512

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:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.4

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: 0

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).