[CVE-2026-64477][MODERATE REGULAR] x86,fs/resctrl: Prevent out-of-bounds access while offlining CPU when SNC enabled

[CVE-2026-64477][MODERATE REGULAR] x86,fs/resctrl: Prevent out-of-bounds access while offlining CPU when SNC enabled

[AL-KERNEL]

CVE: CVE-2026-64477
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: x86,fs/resctrl: Prevent out-of-bounds access while offlining CPU when SNC enabled
Commit: ebc300b7ee0c669fa76a7a8858298ff32e296103
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ebc300b7ee0c669fa76a7a8858298ff32e296103
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64477
Analysis date: Sat, 25 Jul 2026 12:52:24 -0400
ActionableScore: 3
ActionableScore lower bound: 1
Actionable bucket: Borderline manual review recommended / Ordinary Moderate likely
Manual review required: YES

Summary:
An out-of-bounds access in x86 resctrl can occur during CPU offline on SNC-enabled systems when an empty monitoring-domain CPU mask causes `cpu_to_node()` to be called with `nr_cpu_ids`, potentially leading to a local DoS under privileged or delegated management conditions.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64477 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64477

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-64477	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:L/UI:N/S:U/C:L/I:L/A:H';CWE-125;*CWE-20;CWE-754;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.8';DESCR 'An out-of-bounds access can occur in x86 resctrl while offlining a CPU on systems with SNC enabled. When a monitoring domain is being removed, its cpu mask can become empty, but the limbo handler may still try to read RMID occupancy and the architecture code can call cpu_to_node() with nr_cpu_ids instead of a valid CPU number. This can cause an invalid kernel read and may lead to a warning, oops, panic, or other availability impact during CPU hotplug operations. For the CVSS the PR:L is used in the paranoid score because a conservative deployment may delegate CPU hotplug or resctrl control to a less privileged local management process, although typical systems require administrator privileges. The issue is not network reachable and requires local control over CPU offline or related resctrl management paths. Impact is at least local denial of service, with limited confidentiality or integrity impact kept only as a manual review sensitive worst case due to the out-of-bounds kernel access.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review recommended / Ordinary Moderate likely (with actual score 2)	YES	OOB DANGER INIT HARDWARE  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=3
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 3
* Final recommended bucket: **Borderline manual review recommended / Ordinary Moderate likely**::

## 2. Signal breakdown

Conservative signals:

* Memory corruption, weak/indirect corruption candidate: +1. The bug is an out-of-bounds access caused by passing `nr_cpu_ids` to `cpu_to_node()` when the resctrl monitoring domain CPU mask is empty.
* Reliable kernel crash / strong DoS: +1. The invalid CPU index can plausibly lead to warning, oops, panic, or failed CPU hotplug/offline handling.
* Requires admin/root/CAP in typical deployments: -2. CPU offline and resctrl domain management normally require administrative control.
* Availability impact realistic: +1. The issue occurs in CPU offline handling and can affect host availability during that operation.

Paranoid additional interpretation:

* Conservative deployment may reduce the privilege penalty. If CPU hotplug or resctrl control is delegated to a local management service, container root, or reduced-capability operator, the practical PR may be closer to PR:L than full host-root.
* The issue remains a weak OOB access candidate, not a strong corruption primitive. No write primitive, UAF reclaim, type confusion, callback control, or arbitrary memory access is shown.
* SNC-enabled resctrl monitoring is configuration and hardware dependent, so this does not reach Actionable Moderate or Important on the provided evidence.

## 3. Reachability analysis

The bug is local only. It is triggered while offlining CPUs on systems with SNC enabled and resctrl monitoring domains active. Typical triggering requires administrator-level control over CPU hotplug or platform resource management. Namespaces or containers usually do not expose this directly, but delegated infrastructure management could lower the practical privilege requirement in some environments. The path is not network reachable and is not a default general-purpose user API. Realistic exploitation conditions require specific hardware or platform configuration and a CPU offline event that empties the monitoring domain CPU mask.

## 4. Severity interpretation

This behaves more like an ordinary Moderate issue with manual-review relevance than an Important-class vulnerability. The theoretical concern is a kernel out-of-bounds read through an invalid CPU index in a privileged resource-management path. The realistic evidence supports local DoS or unstable CPU offline behavior, not privilege escalation. There is no demonstrated attacker-controlled overwrite, memory reclaim, stale object reuse, or information disclosure path.

## 5. One-sentence report phrase

An out-of-bounds access in x86 resctrl can occur during CPU offline on SNC-enabled systems when an empty monitoring-domain CPU mask causes `cpu_to_node()` to be called with `nr_cpu_ids`, potentially leading to a local DoS under privileged or delegated management conditions.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: this is a concrete kernel out-of-bounds access in CPU/resctrl management code, but the available evidence indicates weak exploitability, local privileged reachability, hardware/configuration dependence, and no clear LPE primitive.

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

Patch: x86,fs/resctrl: Prevent out-of-bounds access while offlining CPU when SNC enabled
Commit: ebc300b7ee0c669fa76a7a8858298ff32e296103
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ebc300b7ee0c669fa76a7a8858298ff32e296103

Commit description:

The architecture updates the cpu_mask in a domain's header to track which
online CPUs are associated with the domain. When this mask becomes empty
the architecture initiates offline of the domain that includes calling
on resctrl fs to offline the domain. If it is a monitoring domain in
which LLC occupancy is tracked resctrl fs forces the limbo handler to
clear all busy RMID state associated with the domain.

The limbo handler always reads the current event value associated with a
busy RMID irrespective of it being checked as part of regular "is it still
busy" check or whether it will be forced released anyway. When reading an
RMID on a system with SNC enabled the "logical RMID" is converted to the
"physical RMID" and this conversion requires the NUMA node ID of the
resctrl monitoring domain that is in turn determined by querying the NUMA
node ID of any CPU belonging to the monitoring domain.

When the monitoring domain is going offline its cpu_mask is empty causing
the NUMA node ID query via cpu_to_node() to be done with "nr_cpu_ids" as
argument resulting in an out-of-bounds access.

Refactor the limbo handler to skip reading the RMID when the RMID will
just be forced to no longer be dirty in the domain anyway. Add a safety
check to the architecture's RMID reader to protect against this scenario.

Fixes: e13db55 ("x86/resctrl: Introduce snc_nodes_per_l3_cache")
Closes: https://sashiko.dev/#/patchset/cover.1780456704.git.reinette.chatre%40intel.com?part=9
Reported-by: Sashiko <sashiko-bot@kernel.org>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Cc: <stable@kernel.org>
Link: https://patch.msgid.link/16137433df42f85013b2f7a53626795cbd6637b9.1781029125.git.reinette.chatre@intel.com
Signed-off-by: Sasha Levin <sashal@kernel.org>

Changed files:
  arch/x86/kernel/cpu/resctrl/monitor.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=ebc300b7ee0c669fa76a7a8858298ff32e296103

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

The original Linux kernel CVE announcement for CVE-2026-64477 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64477

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:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

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: 3

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).