[CVE-2026-74307][MODERATE REGULAR] ext4: validate donor file superblock early in EXT4_IOC_MOVE_EXT [ Upstream

[CVE-2026-74307][MODERATE REGULAR] ext4: validate donor file superblock early in EXT4_IOC_MOVE_EXT [ Upstream

[AL-KERNEL]

CVE: CVE-2026-74307
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: ext4: validate donor file superblock early in EXT4_IOC_MOVE_EXT [ Upstream
Commit: 74796e886ca39fcb0d3fd36ea6a39c62784ab6fb
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=74796e886ca39fcb0d3fd36ea6a39c62784ab6fb
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74307
Analysis date: Sun, 16 Aug 2026 02:01:51 -0400
ActionableScore: 3
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate / Low priority DoS
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-74307 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74307

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: NO

A detailed explanation of the methodology and priority rules is included
at the end of this message.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-74307	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	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-667;*CWE-833;CWE-703;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'EXT4_IOC_MOVE_EXT can enter a circular lock dependency because the donor file superblock check was done after write accounting and inode locks were already taken. A local user can trigger the risky path by passing a donor fd from another filesystem, for example an overlayfs file, while a concurrent freeze operation interacts with ext4 sb_writers and overlay inode locking. For the CVSS the PR:L is used in the paranoid score because the ioctl path can be reached by a local process with writable files, although reliable deadlock conditions may depend on mount layout and freeze activity. The issue is not network reachable and is triggered through a local filesystem ioctl rather than remote traffic. Impact is denial of service through a filesystem or kernel lockup, with no supported evidence of memory corruption, information disclosure, or privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low priority DoS (with actual score 3)	YES	DEADLOCK DANGER DISK HARDWARE LINUS SYZBOT  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	NO	NO	checked

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

ActionableScore=3

1. ActionableScore
* Conservative score: 3
* Paranoid score: 3
* Final recommended bucket: **Ordinary Moderate / Low priority DoS**

2. Signal breakdown
* Local unprivileged trigger: +1. A local process can reach `EXT4_IOC_MOVE_EXT` with writable files and can pass a donor fd from another filesystem.
* Availability impact realistic: +1. The described circular dependency can deadlock filesystem operations and cause a local DoS.
* Important filesystem/storage path: +1. The bug is in ext4 ioctl locking around extent movement and superblock write accounting.
* Hard or unreliable race / special timing required: -1. Reliable impact requires a specific cross-filesystem donor setup plus a concurrent freeze operation and lock ordering.
* Broad/default/common subsystem: +1. ext4 and overlayfs-style deployments are common enough to avoid treating this as rare.
* No memory corruption: +0. The patch only moves validation before locks and does not show UAF, OOB, arbitrary write, stale callback, or refcount misuse.
* No privilege escalation plausible: +0. The supported primitive is deadlock only.
* No confidentiality or integrity impact: +0. There is no evidence of data disclosure or unauthorized file modification.

3. Reachability analysis
A local user with access to writable files can invoke the ioctl path, but the full deadlock scenario also depends on a donor fd from another filesystem and a concurrent freeze operation. Filesystem freeze is typically privileged or performed by storage management tooling, so a fully self-contained reliable trigger is not available to an ordinary unprivileged user in most deployments. Containers and namespaces do not automatically make this host-impacting path unprivileged unless the environment delegates relevant filesystem and mount control. The path is not network reachable.

4. Severity interpretation
This behaves like an ordinary Moderate local DoS, not an Important-class kernel vulnerability. Theoretical impact is a lock dependency deadlock involving ext4, overlayfs-style locking, and freeze interaction. Realistic exploitation evidence supports availability impact only, with no demonstrated memory corruption, privilege escalation, information leak, or persistent filesystem metadata corruption.

5. One-sentence report phrase
`EXT4_IOC_MOVE_EXT performed the donor superblock validation too late, allowing a local cross-filesystem donor fd plus concurrent freeze activity to create a circular lock dependency and cause a local filesystem DoS.`

6. Manual review recommendation
NO MANUAL CHECK NEEDED
Reason: this is a local locking DoS with specific timing and freeze prerequisites, no network exposure, and no supported memory corruption or privilege escalation primitive.

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

Patch: ext4: validate donor file superblock early in EXT4_IOC_MOVE_EXT [ Upstream
Commit: 74796e886ca39fcb0d3fd36ea6a39c62784ab6fb
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=74796e886ca39fcb0d3fd36ea6a39c62784ab6fb

Changed files:
  fs/ext4/ioctl.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=74796e886ca39fcb0d3fd36ea6a39c62784ab6fb

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

The original Linux kernel CVE announcement for CVE-2026-74307 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74307

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: unknown
  Paranoid ActionableScore: 3

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).