[CVE-2026-74722][LOW] btrfs: fix memory leak in btrfs_do_encoded_write() [ Upstream

[CVE-2026-74722][LOW] btrfs: fix memory leak in btrfs_do_encoded_write() [ Upstream

[AL-KERNEL]

CVE: CVE-2026-74722
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: btrfs: fix memory leak in btrfs_do_encoded_write() [ Upstream
Commit: 0f0da96ccb1b9f35f4d3d4540dcaab0969d9d6b0
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0f0da96ccb1b9f35f4d3d4540dcaab0969d9d6b0
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74722
Analysis date: Sat, 22 Aug 2026 16:10:05 -0400
ActionableScore: 2
ActionableScore lower bound: 0
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
A missing cleanup in `btrfs_do_encoded_write()` leaks the `data_reserved` extent changeset on the Btrfs encoded write path, allowing a local privileged or delegated storage-management actor to cause gradual kernel memory exhaustion through repeated operations.

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

The original Linux kernel CVE announcement for CVE-2026-74722 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74722

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-2026-74722	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-401;*CWE-772;**CWE-400;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'btrfs_do_encoded_write can leak the data_reserved extent_changeset object when returning from the encoded write path. The leak is reached through BTRFS_IOC_ENCODED_WRITE and the qgroup reservation path, and repeated calls can accumulate kernel memory usage over time. For the CVSS the PR:L is used for the paranoid score because a local process with delegated access to the Btrfs encoded write ioctl or storage management path could repeatedly trigger the leak. The issue is not network reachable and requires local execution on a system with an affected Btrfs filesystem. Impact is denial of service through resource exhaustion only. No UAF, OOB access, information leak, or privilege escalation primitive is indicated by the code change.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 1)  SKIP CVE-2026-74722 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	WRITE OOB DISK INIT LEAK SKIP HARDWARE MEMORY  INCREASED_FROM_LOW_BASED_ON_HIGHCVSSSCORE DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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

## 1. ActionableScore

ActionableScore=2
ActionableScoreLower=0

* Conservative score: 0
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like**

## 2. Signal breakdown

Conservative signals:

* Important filesystem/storage path: +1. The bug is in Btrfs encoded write and qgroup reservation cleanup.
* Availability impact realistic: +1. Repeated triggering can leak kernel memory and may eventually contribute to resource exhaustion.
* Requires admin/root/CAP_* in typical deployments: -2. `BTRFS_IOC_ENCODED_WRITE` is a specialized filesystem ioctl and is usually available only to privileged or trusted storage-management paths.

Conservative total: 0

Paranoid signals:

* Local delegated trigger: +1. If a local service account, container root, backup/restore tool, or delegated storage-management process can issue encoded writes, repeated triggering may be possible without full host-root equivalence.
* Availability impact realistic: +1. The leak is small per operation but repeatable, so sustained abuse can plausibly cause memory pressure.
* Important filesystem/storage path: +1. Btrfs write/qgroup reservation paths are storage-integrity relevant, although this specific bug is only a cleanup leak.
* Rare/specialized ioctl path: -1. Encoded write is not the common buffered write path and is typically used by specialized Btrfs tooling.

Paranoid total: 2

## 3. Reachability analysis

The issue is locally reachable through the Btrfs encoded write ioctl path, specifically `BTRFS_IOC_ENCODED_WRITE` reaching `btrfs_do_encoded_write()` and qgroup reservation. It is not network reachable. In typical deployments, the ioctl is expected to be used by privileged filesystem or backup/restore tooling, so conservative reachability is privileged local. Namespaces or containers may matter only if a reduced-privilege service or container root is deliberately delegated access to the affected Btrfs file and ioctl path. The trigger requires repeated local operations to accumulate the leak, not a single reliable crash primitive.

Call-site confidence: high. The provided trace includes the ioctl path and the patch directly frees `data_reserved` before return.

## 4. Severity interpretation

This behaves like an ordinary Moderate or Low-like resource leak, not an Important-class vulnerability. The patch fixes a missing `extent_changeset_free(data_reserved)` and the evidence points to a memory leak only. There is no UAF, double free, OOB access, arbitrary write, page-cache corruption, ownership/COW bypass, information leak, or privilege-escalation primitive supported by the patch. The realistic impact is sustained local resource exhaustion under repeated triggering.

## 5. One-sentence report phrase

A missing cleanup in `btrfs_do_encoded_write()` leaks the `data_reserved` extent changeset on the Btrfs encoded write path, allowing a local privileged or delegated storage-management actor to cause gradual kernel memory exhaustion through repeated operations.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This is a cleanup-only resource leak with no demonstrated memory corruption or security-boundary bypass. It can be handled as an auto-close or deferred Moderate unless the product exposes `BTRFS_IOC_ENCODED_WRITE` to untrusted local users.

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

Patch: btrfs: fix memory leak in btrfs_do_encoded_write() [ Upstream
Commit: 0f0da96ccb1b9f35f4d3d4540dcaab0969d9d6b0
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0f0da96ccb1b9f35f4d3d4540dcaab0969d9d6b0

Changed files:
  include/linux/kmemleak.h
  mm/slub.c
  include/linux/slab.h
  fs/btrfs/extent_io.h
  fs/btrfs/qgroup.c
  fs/btrfs/inode.c
  fs/btrfs/file.c
  fs/btrfs/ioctl.c
  fs/ioctl.c
  arch/x86/entry/common.c
  fs/btrfs/ulist.c
  fs/btrfs/extent-io-tree.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=0f0da96ccb1b9f35f4d3d4540dcaab0969d9d6b0

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

The original Linux kernel CVE announcement for CVE-2026-74722 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74722

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:L/PR:H/UI:N/S:U/C:N/I:N/A:L
  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: 0
  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).