[CVE-2026-46170][MODERATE 7.0] mptcp: pm: ADD_ADDR rtx: free sk if last

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

CVE: CVE-2026-46170
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: mptcp: pm: ADD_ADDR rtx: free sk if last
Commit: b74ad20198652b6b39a761c277ba65ae82b1e107
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b74ad20198652b6b39a761c277ba65ae82b1e107
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46170
Analysis date: Thu, 28 May 2026 11:09:07 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: NO

Summary:
MPTCP ADD_ADDR retransmission timer handling could fail to free the socket on the last reference or deadlock during timer cleanup, allowing remote MPTCP-triggered resource exhaustion in configured deployments.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-46170 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46170

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-2026-46170	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;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 drop the last socket reference with __sock_put, which decrements the reference count but does not free the socket when it becomes the last reference. Repeated affected timer completions can leak sockets and related resources, causing availability degradation over time. For the CVSS the PR:N is used because a remote MPTCP peer can participate in the connection behavior that reaches ADD_ADDR retransmission once MPTCP path management is configured. The issue is network reachable only in MPTCP deployments using ADD_ADDR signaling and retransmission. Impact is availability only through resource exhaustion or timer cleanup deadlock risk, 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 DANGER INIT NETWORK TIMER 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 connection behavior that reaches ADD_ADDR retransmission.
* Availability impact realistic: +1. Lost socket references can accumulate and degrade resources over time.
* Common networking path: +1. This is in MPTCP path-management timer handling.

Paranoid additional signal:

* Reliable resource-exhaustion / cleanup deadlock concern: +1. If the last-reference timer path can be repeatedly reached, the issue may lead to socket/resource leaks or a timer self-wait deadlock during cleanup.

Not counted:

* No memory corruption.
* No UAF, OOB write, double free, arbitrary write, or information leak.
* No privilege escalation primitive.
* Not a generic TCP path. It requires MPTCP ADD_ADDR retransmission behavior.

## 3. Reachability analysis

The issue is reachable only when MPTCP is enabled and ADD_ADDR signaling/retransmission is in use. A remote MPTCP peer may influence the connection state enough to exercise this timer path, but reliable exploitation depends on path-manager behavior and last-reference timing.

Namespaces can matter if MPTCP is enabled inside network namespaces, but no local privilege on the victim is required in the network-peer scenario. This is not a generic Internet-wide TCP bug unless the target actually exposes MPTCP with the relevant path-management features.

## 4. Severity interpretation

This behaves like an actionable Moderate availability issue. The realistic impact is socket or resource leakage and possible cleanup hang, not kernel memory corruption.

The paranoid score reaches 5 because the issue is network-triggerable in configured MPTCP deployments and can cause sustained resource exhaustion. It should not be upgraded to Important without evidence of memory corruption or privilege escalation.

## 5. One-sentence report phrase

MPTCP ADD_ADDR retransmission timer handling could fail to free the socket on the last reference or deadlock during timer cleanup, allowing remote MPTCP-triggered resource exhaustion in configured deployments.

## 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 or deadlock, with no memory corruption or LPE evidence.

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

Patch: mptcp: pm: ADD_ADDR rtx: free sk if last
Commit: b74ad20198652b6b39a761c277ba65ae82b1e107
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b74ad20198652b6b39a761c277ba65ae82b1e107

Commit description:

When an ADD_ADDR is retransmitted, the sk is held in sk_reset_timer(),
and released at the end.

If at that moment, it was the last reference being held, the sk would
not be freed. sock_put() should then be called instead of __sock_put().

But that's not enough: if it is the last reference, sock_put() will call
sk_free(), which will end up calling sk_stop_timer_sync() on the same
timer, and waiting indefinitely to finish. So it is needed to mark that
the timer is done at the end of the timer handler when it has not been
rescheduled, not to call sk_stop_timer_sync() on "itself".

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-v1-5-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=b74ad20198652b6b39a761c277ba65ae82b1e107

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

The original Linux kernel CVE announcement for CVE-2026-46170 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46170

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).