[CVE-2026-74318][LOW] btrfs: fix deadlock cloning inline extent when using flushoncommit [ Upstream

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

CVE: CVE-2026-74318
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: btrfs: fix deadlock cloning inline extent when using flushoncommit [ Upstream
Commit: 06283034cec0fa357cbd85784ef7d3f5b2ce0790
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=06283034cec0fa357cbd85784ef7d3f5b2ce0790
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74318
Analysis date: Sat, 15 Aug 2026 06:27:39 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
A local user with access to an affected Btrfs mount using flushoncommit may trigger a reflink inline-extent deadlock between writeback and transaction commit, causing a local denial of service without evidence of memory corruption or privilege escalation.

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

The original Linux kernel CVE announcement for CVE-2026-74318 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74318

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: LOW
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-2026-74318	LOW	CHECK WITH IMPACT FROM ORIG NN NONE	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-833;CWE-667;**CWE-400;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'Btrfs reflink of inline extents can deadlock with transaction commit when the filesystem is mounted with flushoncommit and the destination offset is beyond the current file size. A local user who can write files and issue clone range ioctls on such a Btrfs mount may create dirty folios beyond EOF and cause writeback and commit paths to wait on each other. For the CVSS the PR:L is used because reliable triggering requires local filesystem access but does not require full administrator privileges once the affected mount is available. The issue is not network reachable. Impact is denial of service only through hung tasks and blocked writeback or commit paths.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2)	YES	DEADLOCK DANGER DISK INIT SKIP HARDWARE SYZBOT  INCREASED_FROM_LOW_BASED_ON_HIGHCVSSSCORE DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	YES	NO	checked

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

ActionableScore=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: not materially different
* Final recommended bucket: **Ordinary Moderate / Low-like**::

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  A local user with write access to an affected Btrfs mount can use reflink / clone range operations.

* Reliable kernel crash / strong DoS: +1
  The demonstrated effect is a deadlock with hung tasks in writeback, transaction commit, and reflink paths.

* Availability impact realistic: +1
  The deadlock can block Btrfs writeback and transaction commit, so the impact can persist beyond one failed syscall.

* Important filesystem/storage path: +1
  The issue is in Btrfs reflink, inline extent handling, writeback, and transaction commit interaction.

Subtracted signals:

* Hard or timing-dependent condition: -1
  The deadlock requires interaction between reflink copying of an inline extent and transaction commit / delalloc flushing.

* Non-default runtime condition: -1
  The issue depends on the filesystem being mounted with the flushoncommit option.

Not triggered:

* No generic memory corruption.
  The patch and trace show deadlock, not UAF, OOB write, double free, type confusion, or refcount misuse.

* No Dirty-Pipe-like ownership / COW violation.
  The copied data goes into the destination inode folio and the bug is about i_size update ordering causing deadlock, not unauthorized writes across reflink ownership boundaries.

* No confidentiality or integrity impact.
  No supported primitive for data disclosure, metadata corruption, unauthorized file modification, or privilege escalation is shown.

Call-site confidence: high. The patch includes the affected helper logic and the syzbot trace shows the relevant reflink, writeback, and transaction commit call paths.

## 3. Reachability analysis

The bug is locally reachable by a user who can operate on files in an affected Btrfs filesystem and issue clone range / reflink operations. It does not require network reachability. It does not appear to require full root privileges once the vulnerable mount is already available, but creating or configuring the Btrfs mount with flushoncommit is normally an administrative action.

Namespaces or containers may matter only if an untrusted workload is given access to a host Btrfs mount with reflink support and the relevant mount option. The trigger is not default-universal because flushoncommit is a specific mount option.

## 4. Severity interpretation

This behaves like an ordinary Moderate / Low-like kernel DoS issue, not an Important-class vulnerability. The realistic impact is hung tasks and blocked filesystem progress. The patch does not support memory corruption, privilege escalation, cross-file COW bypass, or sensitive data exposure.

Theoretical risk remains limited because the fixed condition is an ordering issue around i_size update before starting a transaction. It prevents a lock dependency deadlock rather than fixing stale ownership, stale extent mapping, UAF, or attacker-controlled corruption.

## 5. One-sentence report phrase

A local user with access to an affected Btrfs mount using flushoncommit may trigger a reflink inline-extent deadlock between writeback and transaction commit, causing a local denial of service without evidence of memory corruption or privilege escalation.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This can be treated as an ordinary local DoS because the patch and trace show a deadlock only, with no UAF, OOB access, shared-page ownership bypass, information leak, or privilege escalation primitive.

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

Patch: btrfs: fix deadlock cloning inline extent when using flushoncommit [ Upstream
Commit: 06283034cec0fa357cbd85784ef7d3f5b2ce0790
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=06283034cec0fa357cbd85784ef7d3f5b2ce0790

Changed files:
  kernel/sched/core.c
  fs/btrfs/extent-io-tree.c
  fs/btrfs/extent-io-tree.h
  fs/btrfs/inode.c
  fs/btrfs/extent_io.c
  mm/page-writeback.c
  fs/fs-writeback.c
  kernel/workqueue.c
  kernel/kthread.c
  arch/x86/kernel/process.c
  arch/x86/entry/entry_64.S
  fs/btrfs/transaction.c
  fs/btrfs/file.c
  include/linux/fs.h
  fs/read_write.c
  arch/x86/entry/syscall_64.c
  fs/btrfs/reflink.c
  fs/remap_range.c
  fs/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=06283034cec0fa357cbd85784ef7d3f5b2ce0790

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

The original Linux kernel CVE announcement for CVE-2026-74318 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74318

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: 2

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).