[CVE-2026-64373][MODERATE REGULAR] cpufreq: Fix hotplug-suspend race during reboot

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

CVE: CVE-2026-64373
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: cpufreq: Fix hotplug-suspend race during reboot
Commit: 6d5dd354c37abaf4d60400c55c71f23ba2b33639
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6d5dd354c37abaf4d60400c55c71f23ba2b33639
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64373
Analysis date: Sat, 25 Jul 2026 07:03:51 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate / Low-like with manual review recommended due to UAF candidate
Manual review required: YES

Summary:
A reboot-time race between CPU hotplug and `cpufreq_suspend()` can leave `governor_data` freed while still being accessed, causing a local privileged kernel crash or failed reboot, with no demonstrated practical privilege-escalation primitive.

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

The original Linux kernel CVE announcement for CVE-2026-64373 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64373

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-2026-64373	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:H/UI:N/S:U/C:H/I:H/A:H';CWE-362;*CWE-416;**CWE-476;CWE-667;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '6.4';DESCR 'A CPU hotplug versus reboot suspend race in cpufreq can let CPU hotplug free governor data while cpufreq_suspend() is still using it during kernel_restart() and device_shutdown(). The observed failure is a NULL pointer dereference, but the patch context describes a freed object being accessed, so it should be treated as a UAF candidate for review. For the CVSS the PR:H is used because reliable triggering normally requires privileges to reboot the system and to race CPU hotplug operations. The issue is local only and not network reachable. Base impact is kernel crash or failed reboot. For the paranoid score, the same race and UAF candidate justify C:H/I:H/A:H for manual review, although practical privilege escalation is unlikely because the trigger occurs during reboot and needs high local privileges.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like with manual review recommended due to UAF candidate (with actual score 2)	YES	LOCK DEADLOCK NULLPTR INIT RACE UAF HARDWARE BOOT  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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

ActionableScore=2
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like with manual review recommended due to UAF candidate**

## 2. Signal breakdown

Triggered signals:

* Memory corruption, weak primitive: +1
  The commit says `governor_data` can be freed by CPU hotplug while `cpufreq_suspend()` is still accessing it. This is a UAF candidate, but the observed failure is a NULL pointer dereference and no reclaim, overwrite, callback control, type confusion, or refcount takeover is shown.

* Real lifetime corruption: +1
  The bug is a real lifetime race between CPU hotplug teardown and reboot-time `cpufreq_suspend()` access.

* Reliable kernel crash / strong DoS: +1
  The provided trace shows a kernel NULL pointer dereference in `cpufreq_suspend()` during reboot.

* Broad/default subsystem: +1 in paranoid only
  `cpufreq` is common on many systems, but this specific path is limited to reboot plus concurrent CPU hotplug.

Subtracted signals:

* Hard or unreliable race / special timing required: -1
  The race requires CPU hotplug to overlap with reboot-time device shutdown.

* Requires admin/root/CAP_* in typical deployments: -2
  Reliable triggering normally requires the ability to reboot the host and manipulate CPU hotplug, which is host-admin level in typical deployments.

Not awarded:

* No local unprivileged trigger.
* No remote reachability.
* No practical LPE signal.
* No confidentiality or integrity impact in the conservative score.
* No strong memory corruption primitive because there is no demonstrated attacker-controlled reclaim, write-after-free, object replacement, or callback dispatch.

## 3. Reachability analysis

The bug is locally reachable only during the reboot path through `kernel_restart()` -> `device_shutdown()` -> `cpufreq_suspend()`. A realistic trigger requires concurrent CPU hotplug activity while reboot shutdown is in progress. In normal deployments, both reboot and CPU hotplug control require administrative privileges on the host. Containers or namespaces usually do not provide access to host CPU hotplug or host reboot, so they do not meaningfully lower PR in the typical case. The affected subsystem is common, but the vulnerable timing window is narrow and tied to shutdown/reboot behavior.

## 4. Severity interpretation

This behaves more like an ordinary Moderate or Low-like kernel reliability issue than an Important-class vulnerability. Theoretical memory-safety concern exists because the commit describes access after `governor_data` may have been freed, but the demonstrated outcome is a NULL pointer dereference during reboot. There is no evidence of attacker-controlled reuse, arbitrary write, information disclosure, or a practical privilege-escalation path. The paranoid score keeps a small uplift for UAF-candidate lifetime corruption, but the high privilege requirement and reboot-only race keep it below Actionable Moderate.

## 5. One-sentence report phrase

A reboot-time race between CPU hotplug and `cpufreq_suspend()` can leave `governor_data` freed while still being accessed, causing a local privileged kernel crash or failed reboot, with no demonstrated practical privilege-escalation primitive.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: the issue is low-actionability in practice because it requires host-admin level reboot and CPU-hotplug control, but the patch context describes a freed object being accessed, so a quick manual review is reasonable to confirm that no reclaim or write-after-free primitive exists.

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

Patch: cpufreq: Fix hotplug-suspend race during reboot
Commit: 6d5dd354c37abaf4d60400c55c71f23ba2b33639
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6d5dd354c37abaf4d60400c55c71f23ba2b33639

Commit description:

During system reboot, cpufreq_suspend() is called via the
kernel_restart() -> device_shutdown() path. Unlike the normal system
suspend path, the reboot path does not call freeze_processes(), so
userspace processes and kernel threads remain active.

This allows CPU hotplug operations to run concurrently with
cpufreq_suspend(). The original code has no synchronization with CPU
hotplug, leading to a race condition where governor_data can be freed
by the hotplug path while cpufreq_suspend() is still accessing it,
resulting in a null pointer dereference:

  Unable to handle kernel NULL pointer dereference
  Call Trace:
   do_kernel_fault+0x28/0x3c
   cpufreq_suspend+0xdc/0x160
   device_shutdown+0x18/0x200
   kernel_restart+0x40/0x80
   arm64_sys_reboot+0x1b0/0x200

Fix this by adding cpus_read_lock()/cpus_read_unlock() to
cpufreq_suspend() to block CPU hotplug operations while suspend is in
progress.

Fixes: 65650b3 ("cpufreq: Avoid cpufreq_suspend() deadlock on system shutdown")
Signed-off-by: Tianxiang Chen <nanmu@xiaomi.com>
Reviewed-by: Zhongqiu Han <zhongqiu.han@oss.qualcomm.com>
Cc: All applicable <stable@vger.kernel.org>
[ rjw: Changelog edits ]
Link: https://patch.msgid.link/20260408141914.35281-1-nanmu@xiaomi.com
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/cpufreq/cpufreq.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=6d5dd354c37abaf4d60400c55c71f23ba2b33639

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

The original Linux kernel CVE announcement for CVE-2026-64373 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64373

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:H/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 6.4

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: 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: 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).