[CVE-2026-46158][MODERATE 7.0] mptcp: pm: ADD_ADDR rtx: always decrease sk refcount

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

CVE: CVE-2026-46158
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: mptcp: pm: ADD_ADDR rtx: always decrease sk refcount
Commit: acd3d3562315c99f3c0db16f0fcc5f0306638982
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=acd3d3562315c99f3c0db16f0fcc5f0306638982
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46158
Analysis date: Thu, 28 May 2026 11:51:37 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: NO

Summary:
MPTCP ADD_ADDR retransmission timer error paths could return without dropping the socket reference held by the timer, allowing configured MPTCP connections to leak resources and potentially cause DoS.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-46158 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46158

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-2026-46158	MODERATE	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';*CWE-401;*CWE-772;**CWE-400;*CWE-664;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.9';DESCR 'MPTCP ADD_ADDR retransmission timer handling could return early without dropping the socket reference held by sk_reset_timer. A remote MPTCP peer may contribute to the connection state that reaches this timer path when ADD_ADDR signaling and retransmission are enabled, but reliable triggering depends on specific timing and socket state. For the CVSS the PR:N is used because the peer does not need local privileges on the victim in the network scenario. The issue is network reachable only in configured MPTCP deployments. Impact is availability only through socket reference leaks and resource exhaustion, with no supported memory corruption, information disclosure, or privilege escalation primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5) 	MAYBE	REMOTE INIT SIMPLEFIX LEAK NETWORK KPANIC 	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2. A remote MPTCP peer can participate in ADD_ADDR signaling and retransmission behavior.
* Availability impact realistic: +1. Missing socket refcount drops can leak socket references and related resources.
* Common networking path: +1. MPTCP is part of the network stack, though not universally enabled.

Paranoid additional signal:

* Reliable resource-exhaustion DoS concern: +1. If the early-return path can be repeatedly reached, leaked references may accumulate into meaningful resource exhaustion.

Limiting factors:

* Hard or timing-specific state: -1. The leak depends on an unlikely `TCP_CLOSE` state during the timer callback.
* No memory corruption.
* No information disclosure.
* No privilege escalation.
* Not a generic TCP path. It requires MPTCP ADD_ADDR retransmission.

## 3. Reachability analysis

The issue is relevant when MPTCP is enabled and ADD_ADDR retransmission is active. A remote peer may influence the connection state enough to reach the timer path, but reliable exploitation requires specific socket timing and state.

No local privileges are needed in the peer-triggered scenario, but the target must expose MPTCP behavior and path-management features. Network namespaces matter if MPTCP is enabled inside them.

## 4. Severity interpretation

This behaves like an actionable Moderate availability issue. The realistic impact is socket reference leaks and resource exhaustion over time.

It does not behave like an Important memory-corruption vulnerability because the patch supports a refcount/resource leak only, without UAF, write primitive, information disclosure, or LPE.

## 5. One-sentence report phrase

MPTCP ADD_ADDR retransmission timer error paths could return without dropping the socket reference held by the timer, allowing configured MPTCP connections to leak resources and potentially cause DoS.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Review is recommended for products enabling MPTCP ADD_ADDR path management. The supported impact is availability-only resource exhaustion, not memory corruption or privilege escalation.

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

Patch: mptcp: pm: ADD_ADDR rtx: always decrease sk refcount
Commit: acd3d3562315c99f3c0db16f0fcc5f0306638982
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=acd3d3562315c99f3c0db16f0fcc5f0306638982

Commit description:

When an ADD_ADDR is retransmitted, the sk is held in sk_reset_timer().
It should then be released in all cases at the end.

Some (unlikely) checks were returning directly instead of calling
sock_put() to decrease the refcount. Jump to a new 'exit' label to call
__sock_put() (which will become sock_put() in the next commit) to fix
this potential leak.

While at it, drop the '!msk' check which cannot happen because it is
never reset, and explicitly mark the remaining one as "unlikely".

Fixes: 00cfd77 ("mptcp: retransmit ADD_ADDR when timeout")
Cc: stable@vger.kernel.org
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260505-net-mptcp-pm-fixes-7-1-rc3-v1-4-fca8091060a4@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/mptcp/pm.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=acd3d3562315c99f3c0db16f0fcc5f0306638982

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

The original Linux kernel CVE announcement for CVE-2026-46158 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46158

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:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.9

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: 4
  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: 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).