[CVE-2026-72345][MODERATE REGULAR] net/mlx5: LAG, Fix off-by-one in single-FDB error rollback [ Upstream

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

CVE: CVE-2026-72345
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: net/mlx5: LAG, Fix off-by-one in single-FDB error rollback [ Upstream
Commit: 40cc06bf71476932e5d139fa326dcd0372766e16
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=40cc06bf71476932e5d139fa326dcd0372766e16
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72345
Analysis date: Sun, 16 Aug 2026 04:05:33 -0400
ActionableScore: 4
ActionableScore lower bound: 2
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
An off-by-one error in mlx5 LAG single-FDB rollback can make cleanup operate on an uninitialized or already rolled-back rule resource after setup failure, causing a possible driver or kernel DoS and requiring manual review for lifecycle corruption impact.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-72345 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72345

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
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CVE-2026-72345	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	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:L/A:H';CWE-193;*CWE-672;CWE-415;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.8';DESCR 'An off-by-one error in the mlx5 LAG single FDB rollback path can make error cleanup start from the failed index instead of the last successfully installed entry. The failed entry may have uninitialized state or may already have been rolled back by the add path, so cleanup can operate on an invalid or already torn down rule resource. For the CVSS the PR:L is used for the paranoid score because practical triggering may be possible from a reduced capability network administrator context with control over mlx5 LAG or related netdev configuration, while typical production triggering still requires administrative control and specific mlx5 hardware. The issue is not directly network reachable and depends on a local control plane operation plus a failure during FDB rule creation. Impact is at least denial of service via driver or kernel crash, with limited confidentiality or integrity impact kept only for paranoid manual review sensitivity due to the uninitialized state or double teardown bug class.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 3)  SKIP CVE-2026-72345 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	REMOTE SIMPLEFIX NETWORK  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=4
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 4
* Final recommended bucket: **Borderline, manual review recommended**::

## 2. Signal breakdown

Triggered signals:

* Weak/indirect corruption candidate: +1
  Off-by-one rollback may clean the failed index, which the commit says can mean uninitialized state or already rolled-back rule state.

* Real lifetime corruption concern: +1
  The phrase double-tear-down indicates a cleanup/lifetime mismatch for driver rule resources, although no concrete UAF reclaim or arbitrary write primitive is shown.

* Privileged kernel/device-management corruption path: +1
  This is in mlx5 LAG/FDB rule setup and rollback, a trusted NIC driver/device-management path.

* Availability impact plausible: +1
  Invalid or duplicate teardown of rule resources can plausibly crash the driver path or leave device state inconsistent.

Paranoid-only signals:

* Weak LPE concern: +1
  Only for manual-review sensitivity. The bug involves invalid cleanup of kernel driver state, but there is no demonstrated reclaim, overwrite, callback control, or type confusion.

* Firmware-mediated external influence: +1
  The vulnerable path is entered after FDB rule creation failure, which may depend on firmware/device/resource responses rather than purely userspace logic.

Subtractors:

* Requires admin/root/CAP_NET_ADMIN in typical deployments: -2 conservative
  Creating or changing mlx5 LAG/FDB configuration normally requires privileged network administration.

* Reduced capability interpretation: -1 paranoid
  In some deployments, container root, delegated network administration, or service accounts may have enough control over netdev configuration without full host-root equivalence.

* Special hardware/configuration dependency: -1 paranoid
  Requires mlx5 hardware and LAG/single-FDB setup plus an error during rule creation.

## 3. Reachability analysis

The realistic trigger is local control-plane configuration of mlx5 LAG/FDB state, normally requiring root or CAP_NET_ADMIN. It is not directly network-reachable from packet traffic. Namespaces or delegated net administration can reduce the practical privilege barrier, but ordinary unprivileged users should not be able to trigger it in typical deployments. Exploitation also depends on mlx5 hardware, LAG configuration, and a failure at a specific index during FDB rule installation.

Call-site confidence: medium. The patch shows the rollback loop and commit message describes the failure consequence, but full add_one and teardown call paths are not present.

## 4. Severity interpretation

This is not an Important-class candidate based on the provided patch alone. It is more than a pure bookkeeping fix because the rollback can touch uninitialized or already torn-down kernel driver rule state. However, there is no concrete evidence of attacker-controlled reclaim, arbitrary write, type confusion, or privilege escalation. Conservative handling is ordinary Moderate or Low-like. Paranoid handling is borderline manual review due to lifecycle cleanup risk in a privileged driver path.

## 5. One-sentence report phrase

An off-by-one error in mlx5 LAG single-FDB rollback can make cleanup operate on an uninitialized or already rolled-back rule resource after setup failure, causing a possible driver or kernel DoS and requiring manual review for lifecycle corruption impact.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED.
YES REQUIRES MANUAL CHECK for automated triage because this is an off-by-one rollback bug involving uninitialized or double-torn-down kernel driver state, but current evidence does not justify Important severity without additional proof of exploitable memory corruption.

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UPSTREAM PATCH SUMMARY
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Patch: net/mlx5: LAG, Fix off-by-one in single-FDB error rollback [ Upstream
Commit: 40cc06bf71476932e5d139fa326dcd0372766e16
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=40cc06bf71476932e5d139fa326dcd0372766e16

Changed files:
  drivers/net/ethernet/mellanox/mlx5/core/lag/shared_fdb.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=40cc06bf71476932e5d139fa326dcd0372766e16

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

The original Linux kernel CVE announcement for CVE-2026-72345 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72345

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:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

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: 2
  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).