[CVE-2026-64036][MODERATE REGULAR] cgroup/rstat: validate cpu before css_rstat_cpu() access [ Upstream

[CVE-2026-64036][MODERATE REGULAR] cgroup/rstat: validate cpu before css_rstat_cpu() access [ Upstream

[AL-KERNEL]

CVE: CVE-2026-64036
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: cgroup/rstat: validate cpu before css_rstat_cpu() access [ Upstream
Commit: 6a01413a4e8fcb0263d7bef5075c5f8f4eb3a8b6
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6a01413a4e8fcb0263d7bef5075c5f8f4eb3a8b6
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64036
Analysis date: Sun, 19 Jul 2026 15:22:39 -0400
ActionableScore: 3
ActionableScore lower bound: 1
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
A privileged BPF iter cgroup program can pass an invalid CPU value to `css_rstat_updated()`, causing out-of-bounds per-CPU rstat access and at least a local kernel DoS, with limited manual-review concern for possible corruption in rstat list handling.

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

The original Linux kernel CVE announcement for CVE-2026-64036 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64036

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
======================================================================

CVE-2026-64036	MODERATE	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:H/I:H/A:H';CWE-129;*CWE-787;*CWE-20;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'css_rstat_updated is exposed as a BPF kfunc and accepted a caller supplied cpu value without validating that it refers to a possible CPU. A local BPF iter cgroup program can pass an invalid cpu and force out of bounds per CPU rstat access, which was observed as a UBSAN array index out of bounds report and can crash the kernel. For the CVSS the PR:L is used because reliable triggering requires a local process with CAP_BPF and CAP_PERFMON, which is privileged but not necessarily full administrative control. The issue is not network reachable and requires local BPF execution on the target. Impact is at least local denial of service via kernel crash. In the worst case it may allow confidentiality or integrity impact because the invalid cpu is used in per CPU rstat lookups and lockless list update paths, making this a possible memory corruption candidate that should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 2)	YES	WRITE KASAN OOB INIT BPF HARDWARE LINUS TEST  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	YES	NO	checked

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

ActionableScore=3
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 3
* Final recommended bucket: **Borderline, manual review recommended**::

## 2. Signal breakdown

Conservative signals:

* Memory corruption, weak/indirect corruption candidate: +1
  The BPF-facing kfunc accepts an attacker-supplied CPU index and uses it for per-CPU rstat lookup. The demonstrated failure is UBSAN array-index-out-of-bounds, not a proven controlled overwrite.

* Reliable kernel crash / strong DoS: +1
  The commit shows a reproducible UBSAN_BOUNDS report through `css_rstat_updated`, `bpf_iter_run_prog`, `cgroup_iter_seq_show`, and `bpf_seq_read`.

* Requires CAP_BPF and CAP_PERFMON in typical deployments: -2
  This is not an unprivileged BPF path. The described trigger requires a BPF iter/cgroup program with elevated BPF-related capabilities.

Paranoid additional signals:

* Weak LPE concern: +1
  The invalid CPU value can influence per-CPU rstat object selection and lockless list update paths. This is a possible corruption sink, but the patch does not show attacker-controlled reclaim, arbitrary write, callback control, or a demonstrated LPE primitive.

* Availability impact realistic: +1
  Once the required BPF privileges are available, the trigger is simple and can affect the host kernel, not only the calling process.

## 3. Reachability analysis

The bug is locally reachable through a BPF iter/cgroup program calling the BPF kfunc `css_rstat_updated()` with an invalid CPU value. It is not network reachable.

In typical hardened or enterprise configurations, unprivileged BPF is disabled, and loading or attaching the relevant BPF program requires `CAP_BPF` and `CAP_PERFMON`. Namespaces or containers matter only if these capabilities are delegated into the container or service context. Without such delegation, an ordinary local user cannot trigger the issue.

The affected subsystem, cgroup rstat, is common, but the vulnerable externally callable path is specifically the privileged BPF kfunc wrapper. The in-kernel callers are moved to `__css_rstat_updated()` and continue to use trusted CPU values.

Call-site confidence: high. The patch includes both the BPF-facing wrapper and the in-kernel call-site changes.

## 4. Severity interpretation

This behaves more like a privileged local DoS with a weak memory-corruption concern than a confirmed Important-class vulnerability.

The realistic demonstrated impact is kernel crash or warning due to invalid per-CPU indexing. The theoretical concern is that the invalid CPU value may reach per-CPU rstat and lockless list manipulation, which could make the bug more serious than a pure read-only bounds check failure. However, the patch and description do not demonstrate attacker-controlled object replacement, arbitrary write, type confusion, callback dispatch, or reliable privilege escalation.

Because the trigger is gated by `CAP_BPF` and `CAP_PERFMON`, this should not be treated as local unprivileged in the conservative score.

## 5. One-sentence report phrase

A privileged BPF iter cgroup program can pass an invalid CPU value to `css_rstat_updated()`, causing out-of-bounds per-CPU rstat access and at least a local kernel DoS, with limited manual-review concern for possible corruption in rstat list handling.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended because this is a BPF-reachable invalid per-CPU access in kernel cgroup rstat code, and the invalid index may affect lockless list update paths. It is not automatically Important because the trigger requires privileged BPF capabilities and no concrete LPE primitive is shown.

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

Patch: cgroup/rstat: validate cpu before css_rstat_cpu() access [ Upstream
Commit: 6a01413a4e8fcb0263d7bef5075c5f8f4eb3a8b6
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6a01413a4e8fcb0263d7bef5075c5f8f4eb3a8b6

Changed files:
  kernel/cgroup/rstat.c
  block/blk-cgroup.c
  include/linux/cgroup.h
  mm/memcontrol.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=6a01413a4e8fcb0263d7bef5075c5f8f4eb3a8b6

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

The original Linux kernel CVE announcement for CVE-2026-64036 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64036

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:L/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:H/I:H/A:H
  The Best / paranoid CVSS score: 7

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).