[CVE-2026-46107][MODERATE 7.0] dm-thin: fix metadata refcount underflow

[CVE-2026-46107][MODERATE 7.0] dm-thin: fix metadata refcount underflow

[AL-KERNEL]

CVE: CVE-2026-46107
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: dm-thin: fix metadata refcount underflow
Commit: 12161e03d33afce781f68fa11cc6060538862fad
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=12161e03d33afce781f68fa11cc6060538862fad
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46107
Analysis date: Thu, 28 May 2026 06:39:33 -0400
ActionableScore: 5
ActionableScore lower bound: 3
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: NO

Summary:
dm-thin can undercount references to shared btree grandchildren during rebalance, causing persistent metadata refcount underflow and possible thin pool integrity or availability failure.

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

The original Linux kernel CVE announcement for CVE-2026-46107 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46107

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 7.0
Manual review required: NO

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-2026-46107	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:L/I:H/A:H';CWE-911;*CWE-664;CWE-682;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:L/A:H';BEST CVSS score: '6.5';DESCR 'dm-thin rebalance_children can copy entries from a shared btree child into its parent without increasing the reference counts of the grandchildren blocks. This breaks persistent metadata refcount accounting and can later cause refcount underflow, space map errors, thin pool failure, or storage metadata corruption. For the CVSS the PR:L is used for the paranoid score because a less privileged local user may influence an already configured thin volume through filesystem writes, deletes, discard, or workload patterns, although creating the thin pool and snapshots is normally administrator controlled. The issue is not network reachable. Impact is mainly integrity and availability of storage metadata, with limited confidentiality concern only in worst case if broken sharing leads to unintended block reuse or exposure.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 4)	YES	OOB DISK SIMPLEFIX LINUS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	YES	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: **3**
* Paranoid score: **5**
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

* **Important filesystem/storage path: +1**
  The bug is in dm-thin persistent metadata btree handling, affecting thin pool metadata integrity.

* **Filesystem/storage metadata corruption reachable by local workload: +1**
  The refcount mismatch can corrupt persistent metadata accounting for shared btree grandchildren.

* **Integrity impact plausible: +1**
  Incorrect metadata refcounts can cause wrong block accounting, metadata inconsistency, or thin pool corruption.

* **Availability impact realistic: +1**
  The described result includes space map errors such as unable to decrement block, which can degrade or break thin pool operation.

* **Local unprivileged trigger, paranoid: +1**
  If a thin volume and snapshots are already configured by an administrator, a less privileged local user may influence metadata paths through writes, deletes, discard, or workload patterns.

* **Hard or specific trigger conditions: -1**
  The bug requires a specific btree rebalance condition with a shared child node and metadata shape.

* **Requires admin setup, conservative: -1 to -2**
  Creating dm-thin pools and snapshots is normally administrator controlled. The conservative score assumes the attacker cannot directly create the needed storage topology.

No kernel UAF, OOB write, arbitrary write, or direct LPE primitive is shown.

## 3. Reachability analysis

The issue is local and not network reachable. Direct setup of dm-thin pools, snapshots, and device-mapper configuration normally requires administrative privileges. However, once a thin volume is provisioned and mounted for users or services, ordinary filesystem activity may indirectly drive dm-thin metadata removal and rebalance paths. Containers matter only if they are backed by dm-thin storage or can generate workload against thin-provisioned volumes.

## 4. Severity interpretation

This is not ordinary kernel memory corruption, but it is more serious than a transient crash-only bug because it affects persistent storage metadata. Realistic impact is thin pool metadata inconsistency and availability loss. Paranoid impact reaches Actionable Moderate because storage metadata corruption can produce durable integrity and availability effects even without a demonstrated privilege escalation primitive.

## 5. One-sentence report phrase

dm-thin can undercount references to shared btree grandchildren during rebalance, causing persistent metadata refcount underflow and possible thin pool integrity or availability failure.

## 6. Manual review recommendation

**MANUAL CHECK RECOMMENDED**

Persistent storage metadata corruption should not be auto-closed as a simple DoS, even though no kernel code execution or direct privilege escalation primitive is shown.

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

Patch: dm-thin: fix metadata refcount underflow
Commit: 12161e03d33afce781f68fa11cc6060538862fad
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=12161e03d33afce781f68fa11cc6060538862fad

Commit description:

There's a bug in dm-thin in the function rebalance_children. If the
internal btree node has one entry, the code tries to copy all btree
entries from the node's child to the node itself and then decrement the
child's reference count.

If the child node is shared (it has reference count > 1), we won't free
it, so there would be two pointers to each of the grandchildren nodes.
But the reference counts of the grandchildren is not increased, thus the
reference count doesn't match the number of pointers that point to the
grandchildren. This results in "device mapper: space map common: unable
to decrement block" errors.

Fix this bug by incrementing reference counts on the grandchildren if the
btree node is shared.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Fixes: 3241b1d ("dm: add persistent data library")
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/md/persistent-data/dm-btree-remove.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=12161e03d33afce781f68fa11cc6060538862fad

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

The original Linux kernel CVE announcement for CVE-2026-46107 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46107

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:H/UI:N/S:U/C:N/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:H/A:H
  The Best / paranoid CVSS score: 6.5

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: 5

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 7.0

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).