[CVE-2026-72098][MODERATE 7.0] dm-verity: fix buffer overflow in FEC calculation

[CVE-2026-72098][MODERATE 7.0] dm-verity: fix buffer overflow in FEC calculation

[AL-KERNEL]

CVE: CVE-2026-72098
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: dm-verity: fix buffer overflow in FEC calculation
Commit: 5488d3a69d205e28f74f18857b853aa12e778e66
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5488d3a69d205e28f74f18857b853aa12e778e66
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72098
Analysis date: Sat, 15 Aug 2026 12:23:23 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A dm-verity FEC off-by-one error can pass too many erasures to the Reed-Solomon decoder, causing a constrained kernel out-of-bounds write into decoder state with realistic DoS impact and possible integrity or privilege impact requiring manual review.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-72098 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72098

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: 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-2026-72098	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	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:H/A:H';*CWE-787;CWE-193;*CWE-20;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A buffer overflow in the dm-verity/FEC path can occur because the erasure counter may reach roots plus one before decode_rs8 is called. The Reed-Solomon decoder can then write one element past the lambda array and corrupt the adjacent syndrome buffer. This is an explicit kernel out-of-bounds write, not only a warning or resource leak. For the CVSS the PR:L is used because a local user or process can trigger reads from an already configured dm-verity volume, while preparing the corrupted backing storage may require stronger control in many real deployments. The issue is not directly network reachable. Impact is at least local denial of service and possible integrity or privilege impact due to kernel memory corruption, so the paranoid score uses the highest still defensible interpretation for manual review.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	WRITE OOB DISK SIMPLEFIX LINUS KPANIC  DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1
  A local process can trigger reads from an already configured dm-verity volume and reach the FEC recovery path, although preparing corrupted backing storage may require stronger control in many deployments.

* Generic memory corruption: +2
  The patch describes an explicit out-of-bounds write in the Reed-Solomon decoder path, where `lambda[j]` can write one element past the valid `lambda[]` range.

* Corruption primitive highly constrained or metadata-only: -1
  The overwrite is described as landing on `syn[0]`, an adjacent syndrome buffer, not an attacker-selected kernel object or arbitrary address.

* Availability impact realistic: +1
  Corruption in dm-verity FEC recovery can cause failed recovery, bad I/O behavior, or kernel instability in a storage integrity path.

* Important filesystem/storage path: +1
  dm-verity is a security-sensitive storage integrity subsystem, and FEC is part of the recovery path for protected block data.

Paranoid additional signals:

* Weak LPE concern: +1
  The primitive is a real kernel OOB write. No controlled overwrite or reclaim primitive is shown, but the class is serious enough to require manual review for possible privilege impact.

* Integrity impact plausible: +1
  The bug corrupts kernel memory in an integrity subsystem. Even though the known overwrite target is constrained, integrity impact cannot be ruled out without deeper source-level review.

## 3. Reachability analysis

The bug is not network reachable. The affected path requires dm-verity with FEC enabled and enough corrupted or unreadable blocks to drive the erasure count to the boundary condition. In a typical system, creating or configuring a dm-verity target requires administrative privilege, but reading from an already configured protected volume can be done by an ordinary local process. Namespaces do not significantly reduce the requirement to configure dm-verity devices, but they also do not prevent a local process from triggering reads if the vulnerable volume is already exposed. Realistic exploitation depends heavily on whether an attacker can influence the corrupted backing storage, supply a malicious image, or otherwise cause reads from crafted damaged blocks.

## 4. Severity interpretation

This is more than an ordinary Moderate issue because the patch fixes an explicit kernel out-of-bounds write, not only a validation warning or resource leak. Realistically, the overwrite appears constrained to Reed-Solomon decoder state and there is no demonstrated arbitrary write, object replacement, UAF reclaim, or direct LPE primitive. Conservatively this is an Actionable Moderate issue. Paranoid triage should treat it as a Strong Important candidate because kernel memory corruption occurs in a storage integrity path and the consequences need manual source review.

## 5. One-sentence report phrase

A dm-verity FEC off-by-one error can pass too many erasures to the Reed-Solomon decoder, causing a constrained kernel out-of-bounds write into decoder state with realistic DoS impact and possible integrity or privilege impact requiring manual review.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the bug is an explicit kernel OOB write in a security-sensitive storage subsystem. The known overwrite is constrained, but memory corruption plus dm-verity/FEC context is sufficient to avoid auto-closure.

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

Patch: dm-verity: fix buffer overflow in FEC calculation
Commit: 5488d3a69d205e28f74f18857b853aa12e778e66
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5488d3a69d205e28f74f18857b853aa12e778e66

Commit description:

There's a buffer overflow in dm-verity-fec:

if (neras && *neras <= v->fec->roots)
	fio->erasures[(*neras)++] = i;

This allows *neras to reach roots + 1 (the post-increment pushes it past
roots). This value is then passed as no_eras to decode_rs8(). Inside the
RS decoder (lib/reed_solomon/decode_rs.c:113-121), the erasure locator
polynomial loop writes lambda[j] where j can reach nroots + 1 — one
element past the end of lambda[] (which is sized nroots + 1, valid
indices 0..nroots). The out-of-bounds write lands on syn[0], corrupting
the syndrome buffer.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Assisted-by: Claude:claude-opus-4-6
Cc: stable@vger.kernel.org
Fixes: a739ff3 ("dm verity: add support for forward error correction")
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Changed files:
  lib/reed_solomon/decode_rs.c
  drivers/md/dm-verity-fec.c
  drivers/md/dm-verity-fec.h

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=5488d3a69d205e28f74f18857b853aa12e778e66

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

The original Linux kernel CVE announcement for CVE-2026-72098 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72098

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:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 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: 5
  Paranoid ActionableScore: 7

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: YES
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).