[CVE-2026-64180][LOW] mm/memory_hotplug: fix memory block reference leak on remove

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

CVE: CVE-2026-64180
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: mm/memory_hotplug: fix memory block reference leak on remove
Commit: b8ab30c79fc00147125b9c39f928561d9dd13d06
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b8ab30c79fc00147125b9c39f928561d9dd13d06
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64180
Analysis date: Sun, 19 Jul 2026 15:35:35 -0400
ActionableScore: 0
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
A missing `put_device()` in the memory hotplug removal path leaks a memory block device reference after `find_memory_block()`, causing a privileged resource cleanup leak rather than memory corruption.

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

The original Linux kernel CVE announcement for CVE-2026-64180 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64180

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-2026-64180	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-772;CWE-404;*CWE-401;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.4';DESCR 'A memory block device reference leak can occur in remove_memory_blocks_and_altmaps because find_memory_block takes a device reference that was not released on the removal path. This is a resource lifetime bug rather than memory corruption because the object is kept referenced instead of being freed too early. For the CVSS the PR:H is used because reliable triggering normally requires administrative memory hotplug control or a specialized environment with removable memory blocks. The issue is not network reachable. Impact is denial of service through resource leakage or failed cleanup after repeated memory remove operations. For the paranoid score, A:H is used only for sustained repeated hot remove cycles that can realistically accumulate leaked references.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 0)	YES	READ LOCK SIMPLEFIX LEAK HARDWARE MEMORY  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
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ActionableScore=0

## 1. ActionableScore

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

## 2. Signal breakdown

Triggered signals:

* Real lifetime / reference-management bug: +1
  `find_memory_block()` takes a device reference, and the old removal path failed to drop it with `put_device()`.

* Requires admin/root/CAP-level control in typical deployments: -2
  Reliable triggering requires memory hotplug or memory hotremove control, which is normally restricted to administrators or highly privileged infrastructure.

* Rare / difficult-to-reach subsystem or configuration: -1
  The affected path depends on memory hotplug removal with memory blocks and altmaps, not a common unprivileged interface.

Not triggered:

* No remote reachability.
* No local unprivileged trigger.
* No UAF. This is a leaked reference, not premature free.
* No double-free, OOB write, arbitrary write, type confusion, or page-cache corruption.
* No plausible privilege escalation primitive.
* No confidentiality or integrity impact.
* Availability impact is only theoretical through repeated privileged hotremove cycles, not an immediate reliable crash.

Score floor applied at 0.

## 3. Reachability analysis

The bug is reachable through the memory hotplug remove path, specifically `remove_memory_blocks_and_altmaps()`. A normal local user cannot trigger this path directly in typical deployments. The attacker would need administrative control over memory hotremove operations or a specialized virtualization / hotplug environment where such operations are delegated.

Namespaces and containers generally do not reduce the privilege requirement here, because host memory hotplug management is not normally exposed to unprivileged containers. The path is not network reachable and is not part of a default packet-processing or common syscall data plane.

Call-site confidence: high. The commit message and diff directly identify the missing `put_device()` after `find_memory_block()`.

## 4. Severity interpretation

This behaves like a low-priority resource lifetime leak, not an Important-class kernel vulnerability. The object is kept alive longer than intended because a reference is leaked. That is materially different from UAF or memory corruption, where an object is freed too early and later reused or accessed.

The realistic impact is resource leakage or incomplete cleanup after repeated privileged memory hotremove operations. Theoretical DoS from accumulated leaks is possible, but it requires privileged and repeated operations in a specialized configuration. There is no evidence of privilege escalation, sensitive data exposure, or attacker-controlled memory corruption.

## 5. One-sentence report phrase

A missing `put_device()` in the memory hotplug removal path leaks a memory block device reference after `find_memory_block()`, causing a privileged resource cleanup leak rather than memory corruption.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This can be auto-closed or handled as low priority for most environments because it is privileged, non-network-reachable, configuration-dependent, and limited to a reference leak without evidence of UAF, corruption, or privilege escalation.

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

Patch: mm/memory_hotplug: fix memory block reference leak on remove
Commit: b8ab30c79fc00147125b9c39f928561d9dd13d06
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b8ab30c79fc00147125b9c39f928561d9dd13d06

Commit description:

Patch series "mm: Fix memory block leaks and locking", v2.

This series fixes two memory block device reference leaks and one locking
issue around the per-memory_block hwpoison counter.

This patch (of 2):

remove_memory_blocks_and_altmaps() looks up each memory block with
find_memory_block(), which acquires a reference to the memory block
device.

That reference is never dropped on this path, resulting in a leaked device
reference when removing memory blocks and their altmaps.  Drop the
reference after retrieving mem->altmap and clearing mem->altmap, before
removing the memory block device.

Link: https://lore.kernel.org/20260428085219.1316047-1-songmuchun@bytedance.com
Link: https://lore.kernel.org/20260428085219.1316047-2-songmuchun@bytedance.com
Fixes: 6b8f079 ("mm/memory_hotplug: split memmap_on_memory requests across memblocks")
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Acked-by: Oscar Salvador <osalvador@suse.de>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Danilo Krummrich <dakr@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: "Huang, Ying" <huang.ying.caritas@gmail.com>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Naoya Horiguchi <nao.horiguchi@gmail.com>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: Vishal Verma <vishal.l.verma@intel.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  mm/memory_hotplug.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=b8ab30c79fc00147125b9c39f928561d9dd13d06

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

The original Linux kernel CVE announcement for CVE-2026-64180 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64180

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).