[CVE-2026-53122][MODERATE REGULAR] btrfs: fix deadlock between reflink and transaction commit when using flushoncommit [ Upstream

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

CVE: CVE-2026-53122
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: btrfs: fix deadlock between reflink and transaction commit when using flushoncommit [ Upstream
Commit: 73be4a08306bb84f4d5d16f62cb80e1543109ffa
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=73be4a08306bb84f4d5d16f62cb80e1543109ffa
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53122
Analysis date: Wed, 24 Jun 2026 16:00:11 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
A local user with write access to a Btrfs filesystem mounted with flushoncommit may trigger a reflink versus transaction commit deadlock by copying inline extent data beyond i_size, causing filesystem writeback and commit operations to hang.

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

The original Linux kernel CVE announcement for CVE-2026-53122 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53122

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: 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-2026-53122	MODERATE	CHECK	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-362;**CWE-400;CWE-667;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'A Btrfs deadlock can occur when a reflink operation copies inline extent data to an offset beyond the destination inode i_size while the filesystem is mounted with flushoncommit. The copied data creates a dirty folio beyond EOF, and a concurrent transaction commit can trigger delalloc writeback that tries to invalidate that folio and waits on an extent lock held by the reflink path. The reflink path then waits for the current transaction to unblock before it can update the inode item, creating a circular wait and leaving tasks stuck in D state. For the CVSS the PR:L is used because a local user with write access to files on the affected Btrfs mount can trigger reflink style operations such as copy_file_range(), while no administrative privileges are normally needed for the file operation itself. The issue is not network reachable and does not provide a direct confidentiality or integrity primitive. Impact is denial of service through a hung transaction commit and degraded or blocked filesystem operations.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 4)	YES	DEADLOCK DANGER DISK INIT SKIP HARDWARE SYZBOT  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline, manual review recommended**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1
  A local user with write access to the affected Btrfs mount can trigger reflink style operations via copy_file_range().
* Reliable kernel hang / strong DoS: +1
  The bug creates a circular wait between reflink, writeback, and transaction commit, leaving tasks stuck in D state.
* Important filesystem/storage path: +1
  The affected path is Btrfs reflink, writeback, delalloc flushing, and transaction commit.

Paranoid additional signal:

* Availability impact realistic: +1
  The deadlock can block transaction commit and filesystem writeback, causing persistent filesystem-level availability degradation until recovery or reboot.

Conservative subtraction:

* Hard/specific trigger conditions: -1
  The trigger requires flushoncommit, inline extents, reflink beyond current i_size, and a concurrent transaction commit.

## 3. Reachability analysis

The bug is locally reachable by a user able to operate on files in a Btrfs filesystem and invoke reflink/copy_file_range style operations. It does not require network access and is not remotely triggerable. It does not normally require CAP_SYS_ADMIN for the reflink operation itself, but the vulnerable configuration depends on the filesystem being mounted with flushoncommit, which is an administrator-controlled mount option. Containers may matter only if they expose writable Btrfs-backed paths where reflink operations reach the host filesystem.

## 4. Severity interpretation

This behaves like a borderline actionable Moderate rather than an Important vulnerability. The patch and commit describe a deadlock and hung tasks, not UAF, OOB write, page-cache corruption, credential bypass, or another concrete memory corruption primitive. The realistic impact is denial of service against Btrfs transaction/writeback progress. Theoretical privilege escalation is not supported by the patch context.

## 5. One-sentence report phrase

A local user with write access to a Btrfs filesystem mounted with flushoncommit may trigger a reflink versus transaction commit deadlock by copying inline extent data beyond i_size, causing filesystem writeback and commit operations to hang.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: this is not memory corruption and does not justify Important-class handling by itself, but it affects Btrfs transaction/writeback availability and has syzbot-reported hung tasks, so it should not be blindly auto-closed without confirming deployment exposure to flushoncommit and untrusted local reflink operations.

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UPSTREAM PATCH SUMMARY
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Patch: btrfs: fix deadlock between reflink and transaction commit when using flushoncommit [ Upstream
Commit: 73be4a08306bb84f4d5d16f62cb80e1543109ffa
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=73be4a08306bb84f4d5d16f62cb80e1543109ffa

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/reflink.c
  fs/read_write.c
  arch/x86/entry/syscall_64.c
  fs/btrfs/ioctl.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=73be4a08306bb84f4d5d16f62cb80e1543109ffa

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

The original Linux kernel CVE announcement for CVE-2026-53122 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53122

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: 3
  Paranoid ActionableScore: 4

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).