[CVE-2026-46130][MODERATE REGULAR] dm-verity-fec: fix reading parity bytes split across blocks (take 3)

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

CVE: CVE-2026-46130
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: dm-verity-fec: fix reading parity bytes split across blocks (take 3)
Commit: 3d1b4e2d8ac0a1a1390a117f61ce0ca1c47e3bcb
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3d1b4e2d8ac0a1a1390a117f61ce0ca1c47e3bcb
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46130
Analysis date: Thu, 28 May 2026 13:39:26 -0400
ActionableScore: 3
ActionableScore lower bound: 1
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
dm-verity FEC can read past a parity block buffer when Reed-Solomon parity bytes are split across blocks under specific non-default FEC and low-memory conditions, causing possible FEC recovery failure or DoS.

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

The original Linux kernel CVE announcement for CVE-2026-46130 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46130

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-46130	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:N/A:H';CWE-125;CWE-190;*CWE-20;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.3';DESCR 'dm verity FEC parity decoding can read past a parity block buffer when Reed Solomon parity bytes for one codeword are split across block boundaries. The issue requires specific non default fec_roots values and reduced nbufs, typically due to low memory, so reliable triggering is constrained. For the CVSS the PR:L is used in the paranoid model because an unprivileged local user may trigger reads from an already configured affected dm verity volume, but setting up such a volume normally requires administrator privileges. The issue is not network reachable. Impact is primarily availability through read failure or kernel fault. Confidentiality is at most low because this is an internal OOB read with no demonstrated disclosure path, and no write primitive or privilege escalation is shown.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 2) 	MAYBE	WRITE OOB DISK LEAK LINUS  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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

ActionableScore=3
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: **1**
* Paranoid score: **3**
* Final recommended bucket: **Borderline, manual review recommended**::

## 2. Signal breakdown

* Local unprivileged trigger, conditional: **+1**. A local user may trigger reads from an already configured affected dm-verity FEC volume.
* Important filesystem/storage path: **+1**. The bug is in dm-verity FEC recovery, which is part of storage integrity and recovery handling.
* Reliable crash / strong DoS, paranoid only: **+1**. The OOB read can plausibly cause read failure, KASAN report, or kernel fault depending on memory layout.
* Availability impact realistic: **+1 paranoid**. Practical impact is failure of FEC recovery or affected-volume I/O.
* Weak read-only OOB: **+0**. The primitive is an OOB read from a parity buffer, not an OOB write, UAF, double-free, or controlled overwrite.
* Hard / special conditions: **-1**. Triggering requires specific non-default `fec_roots` values and reduced `nbufs`, often due to low memory.
* Configuration-dependent setup: **-1 conservative**. Creating the affected dm-verity FEC configuration normally requires administrator control.

## 3. Reachability analysis

The issue is local, not network reachable. It is reachable during dm-verity FEC recovery when parity bytes for an RS codeword cross a parity block boundary. A normal unprivileged user could potentially trigger reads from an already configured affected volume, but creating such a volume and choosing the relevant FEC parameters is typically administrative. Namespaces and containers do not materially lower setup requirements unless the affected block device is already exposed to them.

## 4. Severity interpretation

This behaves like a **borderline Moderate** issue. The theoretical primitive is a real OOB read in a storage integrity path, but the realistic evidence points to read failure or DoS only. There is no demonstrated information disclosure path, no write primitive, no lifetime corruption, and no credible privilege escalation primitive from the patch context.

## 5. One-sentence report phrase

dm-verity FEC can read past a parity block buffer when Reed-Solomon parity bytes are split across blocks under specific non-default FEC and low-memory conditions, causing possible FEC recovery failure or DoS.

## 6. Manual review recommendation

**MANUAL CHECK RECOMMENDED**

Manual review is recommended because this is an OOB read in an important dm-verity storage path, but exploitation appears constrained and current evidence supports DoS only.

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

Patch: dm-verity-fec: fix reading parity bytes split across blocks (take 3)
Commit: 3d1b4e2d8ac0a1a1390a117f61ce0ca1c47e3bcb
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3d1b4e2d8ac0a1a1390a117f61ce0ca1c47e3bcb

Commit description:

fec_decode_bufs() assumes that the parity bytes of the first RS codeword
it decodes are never split across parity blocks.

This assumption is false.  Consider v->fec->block_size == 4096 &&
v->fec->roots == 17 && fio->nbufs == 1, for example.  In that case, each
call to fec_decode_bufs() consumes v->fec->roots * (fio->nbufs <<
DM_VERITY_FEC_BUF_RS_BITS) = 272 parity bytes.

Considering that the parity data for each message block starts on a
block boundary, the byte alignment in the parity data will iterate
through 272*i mod 4096 until the 3 parity blocks have been consumed.  On
the 16th call (i=15), the alignment will be 4080 bytes into the first
block.  Only 16 bytes remain in that block, but 17 parity bytes will be
needed.  The code reads out-of-bounds from the parity block buffer.

Fortunately this doesn't normally happen, since it can occur only for
certain non-default values of fec_roots *and* when the maximum number of
buffers couldn't be allocated due to low memory.  For example with
block_size=4096 only the following cases are affected:

    fec_roots=17: nbufs in [1, 3, 5, 15]
    fec_roots=19: nbufs in [1, 229]
    fec_roots=21: nbufs in [1, 3, 5, 13, 15, 39, 65, 195]
    fec_roots=23: nbufs in [1, 89]

Regardless, fix it by refactoring how the parity blocks are read.

Fixes: 6df90c0 ("dm-verity FEC: Fix RS FEC repair for roots unaligned to block size (take 2)")
Cc: stable@vger.kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/md/dm-verity-fec.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=3d1b4e2d8ac0a1a1390a117f61ce0ca1c47e3bcb

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

The original Linux kernel CVE announcement for CVE-2026-46130 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46130

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:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:H
  The Best / paranoid CVSS score: 5.3

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: 1
  Paranoid ActionableScore: 3

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).