[CVE-2026-52973][IMPORTANT] futex: Drop CLONE_THREAD requirement for private default hash alloc [ Upstream

[CVE-2026-52973][IMPORTANT] futex: Drop CLONE_THREAD requirement for private default hash alloc [ Upstream

[AL-KERNEL]

CVE: CVE-2026-52973
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: futex: Drop CLONE_THREAD requirement for private default hash alloc [ Upstream
Commit: 1dcd36420af2da5bd59306dba9caf78e3d248b1d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1dcd36420af2da5bd59306dba9caf78e3d248b1d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52973
Analysis date: Thu, 25 Jun 2026 16:35:25 -0400
ActionableScore: 8
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A local user can trigger a futex mm lifetime bug by using CLONE_VM without CLONE_THREAD, causing stale mm futex_ref accounting and a slab use after free in futex_hash_put with realistic DoS impact and plausible LPE concern.

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

The original Linux kernel CVE announcement for CVE-2026-52973 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52973

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-2026-52973	IMPORTANT	CHECK	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-416;CWE-362;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'An incorrect futex default hash allocation condition can leave an mm shared by CLONE_VM tasks without the expected private futex hash allocation. In that state mm futex_ref accounting can place a reference bias on a percpu counter that is no longer referenced by mm futex_ref, and later futex_hash_put can hit a slab use after free. A local user can potentially trigger this by creating CLONE_VM siblings that are not CLONE_THREAD and exercising futex lifetime paths while tasks exit. For the CVSS the PR:L is used because reliable triggering requires local code execution by an unprivileged user, not administrator level access. The issue is not network reachable. Impact is at least local denial of service via kernel crash. Because the primitive is a kernel use after free in futex reference accounting, the paranoid score treats privilege escalation as plausible and uses high confidentiality and integrity impact for manual review sensitivity.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8) 	MAYBE	DANGER RACE UAF IMPROVEONLY LINUS KPANIC 	-	-	checked

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

ActionableScore=8
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: 6
* Paranoid score: 8
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. A local process can use clone with CLONE_VM without CLONE_THREAD.
* Generic memory corruption, strong corruption primitive: +2. The commit explicitly reports KASAN slab-use-after-free in futex_hash_put.
* Real lifetime corruption: +1. The bug is a stale lifetime/reference accounting issue involving mm->futex_ref and percpu counters.
* Reliable kernel crash / strong DoS: +1. The demonstrated symptom is a KASAN UAF and likely kernel crash.
* Broad/default/common subsystem: +1. Futex and fork/clone/mm lifetime paths are core Linux kernel functionality.
* Hard or unreliable race / special lifetime timing required: -1. Triggering depends on a specific CLONE_VM sharing pattern and task/mm teardown timing.

Paranoid additional signals:

* Privilege escalation plausible: +2. This is a kernel UAF in futex reference accounting, and futex/mm lifetime bugs are sensitive enough to require manual LPE review even though no controlled reclaim primitive is shown.
* Confidentiality impact plausible: +1. Only in the paranoid interpretation, because successful exploitation of a kernel UAF may expose kernel or privileged process data.
* Integrity impact plausible: +1. Only in the paranoid interpretation, because successful exploitation of a kernel UAF may corrupt kernel state.

## 3. Reachability analysis

The bug is locally reachable by an unprivileged process that can create CLONE_VM siblings without CLONE_THREAD and exercise futex/mm lifetime paths during task exit. It is not network reachable. It does not require CAP_SYS_ADMIN or CAP_NET_ADMIN in the normal model, because clone and futex-related behavior are ordinary local process interfaces. Namespaces or containers do not remove concern, because the affected state is kernel mm/futex lifetime management and can still be reachable from contained local workloads unless blocked by sandbox policy. The path is broad and common, but reliable exploitation beyond crash likely requires careful timing and object lifetime control.

## 4. Severity interpretation

This is above ordinary Moderate because the patch fixes a real kernel slab-use-after-free, not just a warning, leak, or NULL dereference. The realistic demonstrated impact is local DoS via kernel crash. The theoretical higher impact is local privilege escalation due to UAF in a core kernel subsystem, but the patch does not show an arbitrary write, controlled reclaim, or deterministic object confusion primitive. Therefore the conservative interpretation is Actionable Moderate, while the paranoid interpretation is a Strong Important candidate requiring manual review.

## 5. One-sentence report phrase

A local user can trigger a futex mm lifetime bug by using CLONE_VM without CLONE_THREAD, causing stale mm futex_ref accounting and a slab use after free in futex_hash_put with realistic DoS impact and plausible LPE concern.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: confirmed kernel UAF in a broad futex/mm lifetime path, locally reachable without administrative privileges, with DoS demonstrated and privilege escalation plausible enough for Important-class manual triage.

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

Patch: futex: Drop CLONE_THREAD requirement for private default hash alloc [ Upstream
Commit: 1dcd36420af2da5bd59306dba9caf78e3d248b1d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1dcd36420af2da5bd59306dba9caf78e3d248b1d

Changed files:
  kernel/fork.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=1dcd36420af2da5bd59306dba9caf78e3d248b1d

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

The original Linux kernel CVE announcement for CVE-2026-52973 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52973

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:L/I:L/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: 6
  Paranoid ActionableScore: 8

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).