[CVE-2026-72307][LOW] mlxsw: fix refcount leak in mlxsw_sp_vrs_lpm_tree_replace()

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

CVE: CVE-2026-72307
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: mlxsw: fix refcount leak in mlxsw_sp_vrs_lpm_tree_replace()
Commit: c2c75c45b54f3b12eafb28a4eb47f8821512c1aa
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c2c75c45b54f3b12eafb28a4eb47f8821512c1aa
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72307
Analysis date: Sun, 16 Aug 2026 00:54:43 -0400
ActionableScore: 2
ActionableScore lower bound: 0
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
A rollback bug in mlxsw_sp_vrs_lpm_tree_replace can leak LPM tree references after partial VR replacement failure, potentially causing resource exhaustion DoS in privileged or delegated routing-control scenarios.

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

The original Linux kernel CVE announcement for CVE-2026-72307 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72307

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: LOW
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
======================================================================

CVE-2026-72307	LOW	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:N/I:N/A:H';*CWE-772;*CWE-401;**CWE-400;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'mlxsw spectrum router error rollback can leak references to a new LPM tree when mlxsw_sp_vrs_lpm_tree_replace fails after replacing some virtual routers. The rollback loop decrements the index but can keep using the stale vr pointer, so earlier VR replacements may not be reverted and their references to new_tree remain held. For the CVSS the PR:L is used in the paranoid interpretation because delegated network administration or a routing service account may be able to trigger routing state changes without full host root. The issue is not directly network reachable through packet traffic and requires local or control plane ability to change the mlxsw routing configuration. Impact is denial of service through resource exhaustion only. No concrete UAF, OOB access, information leak, or privilege escalation primitive is indicated by the patch context.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 1)	YES	REMOTE SIMPLEFIX LEAK NETWORK  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	NO	NO	checked

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

ActionableScore=2
ActionableScoreLower=0

## 1. ActionableScore

* Conservative score: 0
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like**::

## 2. Signal breakdown

Conservative signals:

* Real lifetime or refcount issue: +1. The bug leaks references to `new_tree` because earlier VR replacements are not rolled back correctly.
* Requires admin or CAP_NET_ADMIN in typical deployments: -2. Triggering this path normally requires privileged routing or mlxsw device configuration changes.
* Rare hardware or difficult-to-reach subsystem: -1. `mlxsw` is specific to Mellanox Spectrum switch hardware and not a broad default kernel path.
* Availability impact realistic: +1. Repeated failures could leak references and eventually cause resource exhaustion, but this is not shown as immediate or reliable.

Conservative total is below zero, clamped to 0.

Paranoid signals:

* Real lifetime or refcount issue: +1. The refcount leak is real and affects LPM tree lifetime management.
* Availability impact realistic: +1. In a delegated routing-control environment, repeated partial failures could make this a practical resource exhaustion DoS.
* Requires admin or CAP_NET_ADMIN in typical deployments: not fully applied in paranoid mode. Delegated network administration or routing daemons may lower the practical privilege boundary.
* No memory corruption bonus. The patch shows a reference leak, not UAF, double free, OOB access, arbitrary write, or type confusion.
* No LPE bonus. There is no supported privilege escalation primitive.

Paranoid total: 2.

## 3. Reachability analysis

The bug is reachable through mlxsw Spectrum router configuration paths involving VR and LPM tree replacement. In typical deployments, this requires privileged local control such as root or CAP_NET_ADMIN, or a trusted routing daemon managing switchdev state. It is not directly reachable by ordinary network packets.

Namespaces or containers only matter if network administration is delegated to a less trusted actor. In that case, the practical privilege level may be lower than full host root, but the trigger still requires control-plane routing configuration rather than unprivileged local code.

The affected subsystem is hardware-specific and depends on Mellanox Spectrum mlxsw usage. Realistic exploitation also requires an error after some VRs were already replaced, so conditions are narrower than a common packet-processing bug.

Call-site confidence: medium. The provided patch and commit message explain the rollback path and refcount consequence, but not all callers or failure-injection conditions are shown.

## 4. Severity interpretation

This behaves like an ordinary Moderate or Low-like resource leak, not an Important-class vulnerability. The realistic impact is availability degradation through leaked LPM tree references after repeated failed replacement attempts.

There is no concrete evidence of memory corruption, unauthorized access, information disclosure, integrity impact, or privilege escalation. The paranoid score keeps it visible only because it is a refcount lifetime bug in a privileged networking driver control path, but it remains below Actionable Moderate.

## 5. One-sentence report phrase

A rollback bug in mlxsw_sp_vrs_lpm_tree_replace can leak LPM tree references after partial VR replacement failure, potentially causing resource exhaustion DoS in privileged or delegated routing-control scenarios.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This is a hardware-specific privileged control-plane refcount leak with no demonstrated memory corruption or privilege escalation primitive. It can be handled as non-urgent unless the affected product heavily relies on mlxsw Spectrum routing with delegated administration.

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

Patch: mlxsw: fix refcount leak in mlxsw_sp_vrs_lpm_tree_replace()
Commit: c2c75c45b54f3b12eafb28a4eb47f8821512c1aa
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c2c75c45b54f3b12eafb28a4eb47f8821512c1aa

Commit description:

When mlxsw_sp_vrs_lpm_tree_replace() fails after replacing some VRs,
the error rollback loop does not correctly revert the preceding
replacements. The loop decrements the index but fails to update the
vr pointer, which still points to the VR that caused the failure. As
a result, the condition and the rollback call always operate on the
same VR, potentially calling mlxsw_sp_vr_lpm_tree_replace() multiple
times on it while never rolling back the earlier VRs. Those VRs
continue to hold a reference to new_tree acquired via
mlxsw_sp_lpm_tree_hold(), leaking the reference count of new_tree.

Fix by reinitializing vr inside the error loop with the updated index:

	vr = &mlxsw_sp->router->vrs[i];

so that the loop correctly iterates over all VRs that were actually
replaced.

Cc: stable@vger.kernel.org
Fixes: fc922bb ("mlxsw: spectrum_router: Use one LPM tree for all virtual routers")
Signed-off-by: Wentao Liang <vulab@iscas.ac.cn>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260609084730.215732-1-vulab@iscas.ac.cn
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/net/ethernet/mellanox/mlxsw/spectrum_router.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=c2c75c45b54f3b12eafb28a4eb47f8821512c1aa

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

The original Linux kernel CVE announcement for CVE-2026-72307 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72307

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:N/I:N/A:H
  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: 0
  Paranoid ActionableScore: 2

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).