[CVE-2026-46259][IMPORTANT] procfs: fix missing RCU protection when reading real_parent in do_task_stat() [ Upstream

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

CVE: CVE-2026-46259
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: procfs: fix missing RCU protection when reading real_parent in do_task_stat() [ Upstream
Commit: fefa0fcd78be465b7ad4c497fa6ec90d64194c04
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fefa0fcd78be465b7ad4c497fa6ec90d64194c04
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46259
Analysis date: Wed, 03 Jun 2026 14:54:26 -0400
ActionableScore: 8
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A local procfs UAF race in `do_task_stat()` can dereference a stale `task->real_parent` while a task exits, allowing an unprivileged local user to potentially crash the kernel and requiring manual review for limited confidentiality or integrity risk.

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

The original Linux kernel CVE announcement for CVE-2026-46259 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46259

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-2026-46259	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:L/I:L/A:H';*CWE-416;CWE-362;CWE-667;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:H';BEST CVSS score: '5.8';DESCR 'A use-after-free race exists in procfs do_task_stat() because task real_parent can be read before the required RCU protection is active. A local user can potentially trigger the bug by repeatedly reading /proc pid stat while related tasks are exiting, causing do_task_stat() to dereference a stale parent task pointer. For the CVSS the PR:L value is used because local code execution is required, but no administrative privilege is normally needed to read permitted procfs stat files. The issue is not network reachable and is triggered through a local procfs read path. Impact is at least local denial of service via kernel crash. In the paranoid interpretation, limited confidentiality or integrity impact is kept possible due to a task_struct UAF dereference, but there is no strong evidence of an arbitrary write primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8) 	MAYBE	DISK UAF IMPROVEONLY KPANIC  KPANIC LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7	-	-	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. Reading permitted `/proc/[pid]/stat` normally does not require administrative privileges.
* Memory corruption, Strong corruption primitive: +2. The commit explicitly describes a UAF on `task->real_parent`.
* Real lifetime corruption: +1. This is an RCU lifetime bug involving `release_task()` and delayed freeing via `call_rcu()`.
* Reliable kernel crash / strong DoS: +1. A stale `task_struct` dereference can plausibly crash the kernel.
* Broad/default/common subsystem: +1. procfs and `/proc/[pid]/stat` are broadly enabled and commonly exposed.
* Hard or unreliable race / special timing required: -1. Triggering requires racing procfs reads with task exit and parent task teardown.
* Confidentiality impact plausible: +1. Stale `task_struct` data may be read through a dereference path, but no direct disclosure primitive is demonstrated.

Conservative total: 6.

Paranoid additional signals:

* Weak LPE concern: +1. The primitive is a task lifetime UAF in a broadly reachable local kernel path, but no controlled reclaim or write primitive is shown.
* Integrity impact plausible: +1. Limited integrity concern is defensible for manual triage because UAF on `task_struct` may interact with object reuse, but this is not a demonstrated arbitrary write.

Paranoid total: 8.

## 3. Reachability analysis

A local user can potentially trigger the bug by repeatedly reading permitted `/proc/[pid]/stat` entries while related processes are exiting. No network reachability is indicated. The path is normally available on standard Linux systems because procfs is widely mounted, although `hidepid`, namespaces, and procfs permission policy may restrict which tasks are visible. Containers do not eliminate concern because procfs is commonly exposed inside containers, but namespace visibility may reduce the set of target tasks.

The race is timing dependent. However, it is in a common procfs read path and involves normal process exit behavior, so it should not be treated as an obscure hardware or admin-only condition.

## 4. Severity interpretation

This behaves above an ordinary Moderate because the patch explicitly fixes a UAF caused by missing RCU protection in a broadly reachable local interface. The realistic demonstrated impact is local DoS through kernel crash or invalid dereference. Theoretical escalation is weaker because the patch does not show attacker-controlled reclaim, arbitrary write, or a stable object-confusion primitive.

Conservative handling is Actionable Moderate. Paranoid handling is Strong Important candidate because broad local reachability plus task lifetime UAF is a class that deserves manual review even when LPE is not proven.

## 5. One-sentence report phrase

A local procfs UAF race in `do_task_stat()` can dereference a stale `task->real_parent` while a task exits, allowing an unprivileged local user to potentially crash the kernel and requiring manual review for limited confidentiality or integrity risk.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED.

Reason: the issue is an explicitly documented RCU lifetime UAF in a broadly reachable local procfs path, with realistic DoS and nonzero but unproven LPE or data exposure concern.

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

Patch: procfs: fix missing RCU protection when reading real_parent in do_task_stat() [ Upstream
Commit: fefa0fcd78be465b7ad4c497fa6ec90d64194c04
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fefa0fcd78be465b7ad4c497fa6ec90d64194c04

Changed files:
  fs/proc/array.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=fefa0fcd78be465b7ad4c497fa6ec90d64194c04

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

The original Linux kernel CVE announcement for CVE-2026-46259 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46259

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:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

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).