[CVE-2026-64378][IMPORTANT] writeback: fix race between cgroup_writeback_umount() and inode_switch_wbs() [ Upstream

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

CVE: CVE-2026-64378
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: writeback: fix race between cgroup_writeback_umount() and inode_switch_wbs() [ Upstream
Commit: 087d5b8b501c570f84bf655164e6698c3ce146e0
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=087d5b8b501c570f84bf655164e6698c3ce146e0
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64378
Analysis date: Sat, 25 Jul 2026 08:22:53 -0400
ActionableScore: 7
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A race between cgroup writeback unmount and inode writeback switching can leave queued switch work operating after superblock teardown, causing a UAF in percpu counter handling and at least a local or container-triggerable kernel crash.

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

The original Linux kernel CVE announcement for CVE-2026-64378 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64378

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
======================================================================

CVE-2026-64378	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN MODERATE	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-362;*CWE-416;CWE-667;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A race in committed writeback can occur between cgroup_writeback_umount and inode_switch_wbs or cleanup_offline_cgwb. A switcher can pass the SB_ACTIVE check and take an inode reference before unmount begins, while the unmount path observes no queued switch work and proceeds to evict inodes and destroy superblock resources. The delayed switch work can later drop the inode reference after put_super, reaching eviction paths that update freed percpu counter state and creating a use after free condition. For the CVSS the PR:L is used because reliable triggering needs local code execution or a container workload that can drive cgroup writeback and unmount activity, but not necessarily full host root in delegated container environments. The issue is not network reachable. Impact is at least local denial of service via kernel BUG or crash and in the paranoid case may include confidentiality and integrity impact due to UAF based memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	LOCK DISK INIT RACE UAF HARDWARE KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7	NO	NO	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

Triggered signals:

* **Local unprivileged or reduced-privilege container trigger: +1**
  The reported trigger is container exit with cgroup writeback activity. A workload inside a container may be able to create writeback pressure and cause container teardown, even if it is not full host root.

* **Memory corruption, weak to medium UAF primitive: +1 conservative / +2 paranoid**
  The commit explicitly describes a delayed `iput()` reaching `percpu_counter_dec()` after `put_super()` destroys percpu counters. This is a real UAF sink, but the patch does not show controlled reclaim, controlled replacement, type confusion, callback control, or arbitrary write.

* **Real lifetime corruption: +1**
  This is a lifetime ordering bug between `cgroup_writeback_umount()` and `inode_switch_wbs()` / `cleanup_offline_cgwb()`, where a reference survives past superblock teardown.

* **Reliable kernel crash / strong DoS: +1**
  The observed failure is a host kernel `BUG_ON()` / busy inode failure after unmount, and the described later path can also reach UAF during eviction.

* **Weak LPE concern: +1 paranoid only**
  The UAF includes a write-like update via `percpu_counter_dec()` on freed state. There is no demonstrated exploit primitive, but this is enough to require manual review for possible privilege escalation.

* **Important filesystem/storage path: +1**
  The affected code is core writeback and superblock unmount lifetime handling, relevant to ext4 and containerized storage workloads.

* **Cross-boundary container-to-host availability impact: +1**
  If reachable from a container workload, the bug can affect the host kernel during container teardown.

Negative signals:

* **Hard or unreliable race / special timing required: -1**
  The commit says the issue is occasional and depends on a narrow window between `inode_prepare_wbs_switch()` and `wb_queue_isw()`.

Call-site confidence: **high**. The commit gives the relevant interleaving and the downstream call path to `iput()` and `percpu_counter_dec()`.

## 3. Reachability analysis

The bug is local, not network reachable. The practical trigger requires cgroup writeback activity racing with filesystem unmount during container or mount teardown. In typical host-only deployments, direct unmount control may require elevated privileges. In containerized environments, however, a reduced-privilege workload may influence writeback and exit timing while the container runtime performs teardown on the host. This makes PR:L or reduced-privilege container reachability a defensible paranoid interpretation, rather than treating it as full host-root only.

The path is not an obscure driver path. It is in filesystem writeback and superblock teardown, which are common in container and storage-heavy systems. Exploitation beyond DoS is not demonstrated, but the UAF sink is explicit enough that the issue should not be auto-closed.

## 4. Severity interpretation

This behaves above an ordinary Moderate because it is not merely a warning or resource leak. The patch fixes a real race-driven lifetime violation where delayed writeback switching can operate after superblock resources have been destroyed.

Realistically demonstrated impact is **local/container-triggered host DoS** via BUG or crash. Theoretical higher impact is **possible memory corruption from UAF**, but there is no proven controlled reclaim or arbitrary write primitive in the supplied patch. Therefore the conservative score is Actionable Moderate, while the paranoid score reaches Strong Important candidate due to explicit UAF in a core filesystem/writeback path.

## 5. One-sentence report phrase

A race between cgroup writeback unmount and inode writeback switching can leave queued switch work operating after superblock teardown, causing a UAF in percpu counter handling and at least a local or container-triggerable kernel crash.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

Reason: the issue is a race with an explicit UAF candidate in core writeback/unmount lifetime handling, and container-driven triggering may reduce the practical privilege requirement below full host root.

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

Patch: writeback: fix race between cgroup_writeback_umount() and inode_switch_wbs() [ Upstream
Commit: 087d5b8b501c570f84bf655164e6698c3ce146e0
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=087d5b8b501c570f84bf655164e6698c3ce146e0

Changed files:
  fs/super.c
  fs/fs-writeback.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=087d5b8b501c570f84bf655164e6698c3ce146e0

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

The original Linux kernel CVE announcement for CVE-2026-64378 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64378

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: 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).