[CVE-2026-52977][MODERATE REGULAR] futex: Prevent lockup in requeue-PI during signal/ timeout wakeup [ Upstream

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

CVE: CVE-2026-52977
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: futex: Prevent lockup in requeue-PI during signal/ timeout wakeup [ Upstream
Commit: 4e0ed44e51727d56244a822ab941efe507c47966
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4e0ed44e51727d56244a822ab941efe507c47966
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52977
Analysis date: Fri, 26 Jun 2026 15:09:37 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
A local unprivileged futex wait-requeue-PI race during signal or timeout wakeup can cause the requeue side to busy loop and livelock the system, resulting in a local denial of service without evidence of memory corruption or privilege escalation.

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

The original Linux kernel CVE announcement for CVE-2026-52977 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52977

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-2026-52977	MODERATE	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:N/I:N/A:H';CWE-362;CWE-667;*CWE-835;BEST CVSS score: '4.7';DESCR 'A race in futex wait-requeue-PI can cause a livelock when one task leaves the wait path early due to a signal or timeout while another task is doing requeue-PI and holds the futex hash bucket locks. The exiting waiter can mark itself with Q_REQUEUE_PI_IGNORE but may be unable to remove itself from the queue before the requeue side retries, causing the requeue side to repeatedly hit EAGAIN and busy loop. For the CVSS the PR:L is used because a local unprivileged process can invoke futex operations, but some local code execution on the target is still required. AC:H is used because reliable triggering depends on a race window and scheduling conditions such as priority differences or a UP system. The issue is not network reachable. Impact is denial of service through CPU livelock or system lockup. No concrete UAF, OOB access, arbitrary write, information leak, or privilege escalation primitive is evident from the patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 4) 	MAYBE	DEADLOCK DANGER RACE LINUS  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	-	-	checked

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

ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline, manual review recommended**

## 2. Signal breakdown

* Local unprivileged trigger: +1
  Futex operations are reachable from local user space without administrative privileges.

* Reliable kernel crash / strong DoS: +1
  The described race can make the requeue side busy loop and livelock the system.

* Availability impact realistic: +1
  The impact can be system-wide lockup, especially on UP systems or under unfavorable scheduling priority conditions.

* Broad/default/common subsystem: +1
  Futex is a core kernel synchronization interface and is broadly exposed to local processes and containers.

* Hard or unreliable race / special timing required: -1 for conservative score
  Triggering depends on a signal or timeout racing with requeue-PI and on scheduling conditions. For the paranoid score, this penalty is not applied because the commit describes a realistic livelock scenario.

No memory corruption, UAF, OOB access, arbitrary write, information leak, or privilege escalation primitive is supported by the patch context.

## 3. Reachability analysis

A local unprivileged process can invoke the relevant futex wait-requeue-PI and requeue-PI operations. Containers and namespaces generally do not remove futex syscall reachability, so containerized workloads may still reach the affected code path. The path is default kernel functionality, but the specific trigger requires a timing race involving early wakeup by signal or timeout while another task is performing requeue-PI. Exploitation is realistic as a local DoS, but there is no evidence that it crosses into memory corruption or LPE.

## 4. Severity interpretation

This behaves like an actionable local DoS rather than an Important-class vulnerability. The bug is in a sensitive core kernel subsystem and is reachable by local unprivileged code, so it should not be dismissed as a trivial low-risk issue. However, the demonstrated impact is livelock or system lockup only, and the patch does not indicate a concrete corruption primitive or privilege escalation path.

## 5. One-sentence report phrase

A local unprivileged futex wait-requeue-PI race during signal or timeout wakeup can cause the requeue side to busy loop and livelock the system, resulting in a local denial of service without evidence of memory corruption or privilege escalation.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: this is a local unprivileged race in core futex code with potential system-wide DoS, but current evidence supports DoS only and does not justify Important classification without additional memory corruption or LPE evidence.

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

Patch: futex: Prevent lockup in requeue-PI during signal/ timeout wakeup [ Upstream
Commit: 4e0ed44e51727d56244a822ab941efe507c47966
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4e0ed44e51727d56244a822ab941efe507c47966

Changed files:
  kernel/futex/requeue.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=4e0ed44e51727d56244a822ab941efe507c47966

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

The original Linux kernel CVE announcement for CVE-2026-52977 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52977

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: Not available
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: 3
  Paranoid ActionableScore: 4

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).