[CVE-2026-46159][MODERATE REGULAR] btrfs: fix btrfs_ioctl_space_info() slot_count TOCTOU which can lead to info-leak [ Upstream

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

CVE: CVE-2026-46159
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: btrfs: fix btrfs_ioctl_space_info() slot_count TOCTOU which can lead to info-leak [ Upstream
Commit: f5ee467b56764964027c361641f64953fc0f8f9a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f5ee467b56764964027c361641f64953fc0f8f9a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46159
Analysis date: Thu, 28 May 2026 11:49:05 -0400
ActionableScore: 4
ActionableScore lower bound: unknown
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
`btrfs_ioctl_space_info()` could race with block group removal and copy trailing uninitialized heap bytes to userspace after filling fewer entries than originally counted.

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

The original Linux kernel CVE announcement for CVE-2026-46159 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46159

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

CVE-2026-46159	MODERATE	CHECK	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:N/A:N';CWE-367;*CWE-200;CWE-908;*CWE-457;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N';BEST CVSS score: '4.7';DESCR 'btrfs_ioctl_space_info had a TOCTOU race between counting space info entries and filling the userspace buffer. If block groups were removed between the two passes, the second pass could fill fewer entries but copy the original allocation size, exposing trailing uninitialized kmalloc bytes to userspace. For the CVSS the PR:L is used because a local user with access to a Btrfs filesystem ioctl can request space info, although reliable exploitation requires a concurrent block group removal condition. The issue is not network reachable. Impact is information disclosure only, with no supported memory corruption, write primitive, or privilege escalation path.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 4) 	MAYBE	DISK WARNONLY SIMPLEFIX RACE LEAK SKIP  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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

ActionableScore=4

## 1. ActionableScore

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

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1. A local user with access to a mounted Btrfs filesystem can call the space-info ioctl.
* Confidentiality impact plausible: +1. The race can expose trailing uninitialized `kmalloc` bytes through `copy_to_user()`.
* Important filesystem/storage path: +1. This is a Btrfs ioctl reporting filesystem allocation state.
* Local filesystem ioctl exposure: +1. The ioctl is reachable through ordinary local filesystem interaction.

Limiting factors:

* Hard/racy condition: -1. Reliable leakage requires concurrent block group removal between the counting and filling passes.
* No memory corruption.
* No write primitive.
* No privilege escalation.
* No availability impact shown.
* No network reachability.

Net result is kept at 4 because the primitive is a local kernel heap information leak but depends on a TOCTOU condition.

## 3. Reachability analysis

The issue is local. A process needs access to the Btrfs filesystem and the ability to invoke `BTRFS_IOC_SPACE_INFO` or equivalent ioctl path.

Reliable exploitation requires a concurrent operation that removes block groups between the two passes. That may require privileged or administrative filesystem maintenance operations, or a workload where such changes occur naturally. Containers matter if Btrfs ioctls are exposed inside containerized environments.

## 4. Severity interpretation

This behaves like a borderline Moderate information disclosure issue. The supported impact is kernel heap byte exposure to userspace.

It does not behave like an Important memory-corruption vulnerability because the patch shows no UAF, OOB write, arbitrary write, or LPE primitive.

## 5. One-sentence report phrase

`btrfs_ioctl_space_info()` could race with block group removal and copy trailing uninitialized heap bytes to userspace after filling fewer entries than originally counted.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Review whether untrusted local users can call the Btrfs space-info ioctl and whether block group removal can occur concurrently in the target environment.

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

Patch: btrfs: fix btrfs_ioctl_space_info() slot_count TOCTOU which can lead to info-leak [ Upstream
Commit: f5ee467b56764964027c361641f64953fc0f8f9a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f5ee467b56764964027c361641f64953fc0f8f9a

Changed files:
  fs/btrfs/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=f5ee467b56764964027c361641f64953fc0f8f9a

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

The original Linux kernel CVE announcement for CVE-2026-46159 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46159

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:N/A:N
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:N/A:N
  The Best / paranoid CVSS score: 4.7

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: unknown
  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).