[CVE-2026-64556][IMPORTANT] perf/core: Detach event groups during remove_on_exec [ Upstream

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

CVE: CVE-2026-64556
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: perf/core: Detach event groups during remove_on_exec [ Upstream
Commit: 4cdb1b3ab96eb1b7eb70bc5c82fede334bd60df2
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4cdb1b3ab96eb1b7eb70bc5c82fede334bd60df2
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64556
Analysis date: Wed, 29 Jul 2026 04:55:34 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A local perf user may create an inconsistent remove_on_exec event group state that later double-adds an active_list entry in the PMU context, causing kernel list corruption and at least a local DoS, with privilege escalation potential requiring manual review.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64556 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64556

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: IMPORTANT
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-2026-64556	IMPORTANT	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:H/I:H/A:H';*CWE-664;*CWE-672;CWE-362;*CWE-416;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'perf remove_on_exec can leave an active event group in an inconsistent state when the removed event is a group leader and surviving siblings do not have remove_on_exec. A later close of the removed leader can promote siblings from this stale group state, and the next schedule in can add an already linked active_list entry again, corrupting the PMU context active list. For the CVSS the PR:L is used for the paranoid score because a local user or process with access to perf_event_open can create perf event groups and trigger exec and close paths, although practical exposure depends on perf_event_paranoid and system policy. The issue is not network reachable and requires local execution. Impact is at least local denial of service via kernel crash, and because the patch context shows kernel list corruption rather than only a warning, worst case confidentiality and integrity impact should be manually reviewed.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	DANGER INIT HARDWARE LINUS KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1
  A local process with access to `perf_event_open()` can create event groups, use `remove_on_exec`, execute `exec`, and later close events. Practical reachability depends on `perf_event_paranoid` and perf access policy.

* Memory corruption, weak or indirect corruption candidate: +1
  The bug corrupts the PMU context `active_list` by adding an already linked list entry. This is concrete kernel list corruption, but the patch does not show attacker controlled overwrite payload or target selection.

* Real lifetime or ownership state corruption: +1
  A removed group leader can leave surviving siblings attached to stale group state, causing later promotion and invalid active list linkage.

* Reliable kernel crash / strong DoS: +1
  With `DEBUG_LIST`, this is caught as a `list_add` double add in `merge_sched_in()`. Without list debugging, kernel list corruption can plausibly crash the system.

* Broad/default/common subsystem exposure: +1
  `perf/core` is a common kernel subsystem, although access is often controlled by `perf_event_paranoid`, capabilities, and distribution policy.

Paranoid additional interpretation:

* Stronger memory corruption concern: +1 over conservative
  The affected object is a core kernel linked list used during scheduling of perf events. Even though no arbitrary write is shown, list corruption in this context is more serious than a pure warning or bookkeeping bug.

* Weak LPE concern: +1
  Privilege escalation is not demonstrated, but the primitive is kernel list corruption reachable from a local perf workflow. This is enough for manual review sensitivity, but not enough to claim reliable LPE.

## 3. Reachability analysis

The likely trigger is local. An attacker needs the ability to create perf event groups and set up a group leader with `remove_on_exec` while siblings survive the exec path. Then a later close and schedule in can hit the stale group state.

This is not network reachable. Containers and namespaces matter because some environments delegate limited perf access or run reduced capability service accounts. In locked down deployments with restrictive `perf_event_paranoid` or missing `CAP_PERFMON`, reachability may be reduced. In permissive developer, CI, performance monitoring, or containerized environments, a non root local user may still have enough access to trigger at least user scoped perf events.

The path is common kernel code, but the exact vulnerable sequence requires a specific perf event grouping and lifecycle pattern. It is not a random timing race, so the trigger may be more reproducible than classic race only bugs.

## 4. Severity interpretation

This is more than an ordinary Moderate because the commit describes concrete kernel list corruption, not only a warning or resource leak. The realistic demonstrated impact is local DoS through kernel crash or list debugging failure.

The theoretical worst case is higher because corrupted kernel linked lists can sometimes become exploitable depending on list hardening, object lifetime, and attacker control over event objects. However, the patch does not demonstrate arbitrary write, controlled reclaim, callback control, or a reliable privilege escalation primitive. Therefore the conservative interpretation is Actionable Moderate, while the paranoid interpretation is Strong Important candidate for manual review.

## 5. One-sentence report phrase

A local perf user may create an inconsistent remove_on_exec event group state that later double-adds an active_list entry in the PMU context, causing kernel list corruption and at least a local DoS, with privilege escalation potential requiring manual review.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This should not be auto-closed because the patch shows concrete kernel list corruption in `perf/core`, reachable through local perf event lifecycle operations. The confirmed impact is DoS, but the corrupted active list state is a plausible memory corruption sink that warrants manual analysis for LPE feasibility.

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UPSTREAM PATCH SUMMARY
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Patch: perf/core: Detach event groups during remove_on_exec [ Upstream
Commit: 4cdb1b3ab96eb1b7eb70bc5c82fede334bd60df2
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4cdb1b3ab96eb1b7eb70bc5c82fede334bd60df2

Changed files:
  kernel/events/core.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=4cdb1b3ab96eb1b7eb70bc5c82fede334bd60df2

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

The original Linux kernel CVE announcement for CVE-2026-64556 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64556

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: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: 5
  Paranoid ActionableScore: 7

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).