[CVE-2026-52980][MODERATE 7.0] sched/fair: Clear rel_deadline when initializing forked entities [ Upstream

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

CVE: CVE-2026-52980
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: sched/fair: Clear rel_deadline when initializing forked entities [ Upstream
Commit: c71bf35caba12bfd9bc23e32b0bcd9e02d1cf1ac
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c71bf35caba12bfd9bc23e32b0bcd9e02d1cf1ac
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52980
Analysis date: Fri, 26 Jun 2026 14:42:23 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A stale rel_deadline flag inherited by a forked sched_entity can corrupt CFS/EEVDF scheduling accounting after sched_yield, making pick_next_entity return NULL and causing a local unprivileged kernel crash.

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

The original Linux kernel CVE announcement for CVE-2026-52980 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52980

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 7.0
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-2026-52980	MODERATE	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-665;CWE-682;CWE-754;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'A stale rel_deadline flag can be inherited by a newly forked sched_entity and can make the first fair class enqueue treat an invalid deadline as a relative deadline. A later sched_yield can copy the inflated deadline into vruntime, which can overflow vruntime based accounting and break EEVDF eligibility calculations. This can make pick_next_entity return NULL unexpectedly and lead to a kernel NULL pointer dereference and system crash. For the CVSS the PR:L is used because a local unprivileged process can normally fork and call sched_yield, while no administrator privileges are implied by the triggering path. The issue is not network reachable and the impact is denial of service only based on the visible primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5) 	MAYBE	DANGER NULLPTR  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH KPANIC INCREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN5	-	-	checked

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**

## 2. Signal breakdown

* Local unprivileged trigger: +1
  Fork and sched_yield are normally available to an unprivileged local process.

* Reliable kernel crash / strong DoS: +1
  The described path can make pick_next_entity return NULL and later cause a NULL pointer dereference in core scheduler code.

* Availability impact realistic: +1
  The crash is system-wide and occurs in the scheduler, not in an isolated optional helper path.

* Broad/default/common subsystem: +1
  CFS/EEVDF scheduler code is a core default kernel subsystem.

* Practical triggering sequence strongly implied: +1 paranoid only
  The commit message gives a concrete sequence: fork with stale rel_deadline, first enqueue, sched_yield, vruntime inflation, accounting overflow, eligibility failure, NULL dereference.

No generic memory corruption score is applied. The visible primitive is scheduler accounting corruption leading to NULL dereference, not UAF, OOB write, arbitrary write, type confusion, or reclaimable lifetime corruption.

## 3. Reachability analysis

A local unprivileged user can plausibly reach the relevant scheduler paths by creating tasks and calling sched_yield. No network reachability is indicated. Containers and user namespaces do not eliminate concern because normal task scheduling still occurs in the host kernel, although exact reproducibility may depend on scheduler state and whether rel_deadline can remain set across the fork scenario. The affected code is default scheduler functionality, so exposure is broad.

## 4. Severity interpretation

This behaves like an actionable Moderate rather than ordinary Low or full Important. The realistic impact is local denial of service through a kernel crash. Theoretical privilege escalation is not supported by the patch because there is no demonstrated memory safety primitive such as UAF, OOB write, controlled overwrite, or object confusion. The paranoid score is raised because the bug is local-user reachable, affects core scheduler accounting, and has a concrete crash chain.

## 5. One-sentence report phrase

A stale rel_deadline flag inherited by a forked sched_entity can corrupt CFS/EEVDF scheduling accounting after sched_yield, making pick_next_entity return NULL and causing a local unprivileged kernel crash.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Manual review is required because this is a local-user reachable crash in core scheduler code with broad default exposure, even though the currently visible primitive supports DoS rather than privilege escalation.

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

Patch: sched/fair: Clear rel_deadline when initializing forked entities [ Upstream
Commit: c71bf35caba12bfd9bc23e32b0bcd9e02d1cf1ac
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c71bf35caba12bfd9bc23e32b0bcd9e02d1cf1ac

Changed files:
  kernel/sched/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=c71bf35caba12bfd9bc23e32b0bcd9e02d1cf1ac

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

The original Linux kernel CVE announcement for CVE-2026-52980 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52980

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:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: 4
  Paranoid ActionableScore: 5

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: YES
Published priority for this report (same as in Subject): MODERATE 7.0

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).