[CVE-2026-52991][IMPORTANT] sched/psi: fix race between file release and pressure write [ Upstream

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

CVE: CVE-2026-52991
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: sched/psi: fix race between file release and pressure write [ Upstream
Commit: 03dc070fa0fc3cb4068693f468ccd5f8a7e58282
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=03dc070fa0fc3cb4068693f468ccd5f8a7e58282
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52991
Analysis date: Fri, 26 Jun 2026 08:48:41 -0400
ActionableScore: 7
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A race between PSI cgroup pressure writes and cgroup file release can cause `pressure_write()` to use a freed `cgroup_file_ctx`, resulting in a local kernel UAF with reliable DoS impact and plausible privilege escalation concern in delegated cgroup environments.

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

The original Linux kernel CVE announcement for CVE-2026-52991 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52991

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-2026-52991	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-667;**CWE-476;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A race in the PSI cgroup pressure write path can let pressure_write use cgroup_file_ctx after cgroup_file_release has freed it. The vulnerable state is tied to kernfs_open_file priv and can be reached when a pressure write overlaps with cgroup removal or kernfs open file draining. The observed primitive is a kernel use after free, with a secondary NULL pointer dereference case when the priv pointer is cleared before pressure_write uses it. For the CVSS the PR:L is used because a local user or delegated service needs write access to PSI cgroup pressure files and the ability to race file release or cgroup removal. The issue is not network reachable. Impact is at least a local denial of service via kernel crash and in the paranoid case may allow confidentiality or integrity impact because the bug class is UAF in kernel memory. Manual review is recommended because UAF and race conditions can be exploitability sensitive even when the direct reproducer is a crash.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7) 	MAYBE	WRITE OOB DANGER NULLPTR RACE UAF HARDWARE LINUS SYZBOT KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7	-	-	checked

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

ActionableScore=7
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: 6
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative score 6:

* Local unprivileged or delegated local trigger: +1
  The path is local via cgroup PSI pressure files. In typical locked down systems this may require a cgroup manager or privileged service, but delegated cgroup setups can expose this to non full root users.

* Generic memory corruption, strong corruption primitive: +2
  The report shows a real KASAN slab use after free in `pressure_write()` on `struct cgroup_file_ctx`.

* Real lifetime corruption: +1
  This is a lifetime race between `pressure_write()` and `cgroup_file_release()` freeing `of->priv`.

* Reliable kernel crash / strong DoS: +1
  The demonstrated UAF and secondary NULL dereference can crash the kernel.

* Broad/common subsystem exposure: +1
  cgroups, kernfs, and PSI are common Linux kernel infrastructure, especially on containerized and systemd based systems.

* Weak LPE concern: +1
  The UAF includes later access to a freed object and a store into `ctx->psi.trigger`, but the patch does not demonstrate a reliable reclaim or attacker controlled overwrite primitive.

* Hard race / special timing required: -1
  Triggering requires a race between pressure write and cgroup file release or removal.

Paranoid delta:

* Upgrade weak LPE concern to strong LPE plausibility: +1
  In the paranoid interpretation, the confirmed kernel heap UAF plus a write into the freed `ctx` object is enough to require Important class manual review, even though practical exploitation is not demonstrated.

## 3. Reachability analysis

The bug is locally reachable through writes to PSI cgroup pressure files while the corresponding cgroup file is being released or the cgroup is being removed. It is not network reachable.

Privileges depend heavily on deployment. On a traditional host, creating/removing cgroups and writing the relevant files may be restricted to root or a cgroup manager. In delegated cgroup v2 environments, containers, service managers, or reduced privilege service accounts may have enough control over their own cgroup subtree to trigger the race without full host root privileges. Therefore a full admin penalty is not appropriate for triage, but the bug should be considered less exposed on systems without cgroup delegation or PSI monitor access.

The race is timing dependent, but syzbot/KASAN evidence shows the UAF is real and reachable.

## 4. Severity interpretation

This is not an ordinary resource leak or NULL dereference only issue. The primary bug class is a confirmed kernel heap use after free caused by incorrect synchronization around `kernfs_open_file.priv`.

Realistically, the demonstrated impact is local denial of service via kernel crash. Theoretical exploitation beyond DoS is plausible because the freed `cgroup_file_ctx` can be accessed after release and the code can later store a pointer into the freed object. However, the patch does not show a reliable attacker controlled reclaim, arbitrary write, or full LPE chain. This places it as Actionable Moderate at minimum and a Strong Important candidate under paranoid triage.

## 5. One-sentence report phrase

A race between PSI cgroup pressure writes and cgroup file release can cause `pressure_write()` to use a freed `cgroup_file_ctx`, resulting in a local kernel UAF with reliable DoS impact and plausible privilege escalation concern in delegated cgroup environments.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: confirmed kernel UAF plus race plus possible delegated local reachability. Even if the observed reproducer is a crash, the lifetime bug and write back into the freed context make this unsafe to auto-close.

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

Patch: sched/psi: fix race between file release and pressure write [ Upstream
Commit: 03dc070fa0fc3cb4068693f468ccd5f8a7e58282
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=03dc070fa0fc3cb4068693f468ccd5f8a7e58282

Changed files:
  kernel/cgroup/cgroup.c
  fs/kernfs/file.c
  fs/open.c
  fs/kernfs/dir.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=03dc070fa0fc3cb4068693f468ccd5f8a7e58282

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

The original Linux kernel CVE announcement for CVE-2026-52991 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52991

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: 6
  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).