[CVE-2026-31669][IMPORTANT] mptcp: fix slab-use-after-free in __inet_lookup_established

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

CVE: CVE-2026-31669
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: mptcp: fix slab-use-after-free in __inet_lookup_established
Commit: f6e1f25fa5e733570f6d6fe37a4dfed2a0deba47
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f6e1f25fa5e733570f6d6fe37a4dfed2a0deba47
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-31669
Analysis date: Fri, 07 Aug 2026 12:00:39 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A network-reachable slab-use-after-free in the MPTCP IPv6 subflow path can occur because child sockets may be allocated without `SLAB_TYPESAFE_BY_RCU`, allowing lockless established-socket lookups to access freed memory and potentially crash the kernel.

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

The original Linux kernel CVE announcement for CVE-2026-31669 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-31669

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: IMPORTANT
Manual review required: YES

A detailed explanation of the methodology and priority rules is included
at the end of this message.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-31669	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-362;*CWE-664;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '8.1';DESCR 'A slab use-after-free in the MPTCP IPv6 subflow path can occur because tcpv6_prot_override copied tcpv6_prot before tcpv6_prot had a registered SLAB_TYPESAFE_BY_RCU slab cache. MPTCP v6 child sockets may then be allocated from kmalloc-4k without RCU type safe lifetime guarantees, while lockless ehash lookups in __inet_lookup_established still assume socket memory remains stable during rcu_read_lock. A remote peer can potentially trigger the race by creating and closing MPTCP IPv6 subflows while concurrent packets cause established socket lookups. For the CVSS the PR:N is used because no local account is required when an attacker can reach an exposed MPTCP IPv6 listener. The issue is network reachable but depends on MPTCP IPv6 being enabled and reachable, and the race timing makes AC:H appropriate. Impact is at least denial of service via kernel crash and in the paranoid score may include confidentiality and integrity impact due to a real UAF primitive in network socket memory.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7) 	MAYBE	REMOTE DANGER INIT UAF NETWORK LINUS IPV6 KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	-	-	checked

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

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 scoring:

* Remote reachable / network-triggerable: **+2**
  MPTCP IPv6 subflows and established socket lookups can be influenced by a remote peer when an MPTCP IPv6 endpoint is reachable.

* Memory corruption, weak/indirect corruption candidate: **+1**
  The bug is a real slab-use-after-free, but the patch does not show attacker-controlled reclaim, overwrite, callback control, or a reliable object replacement primitive.

* Real lifetime corruption: **+1**
  The root cause is incorrect socket allocation lifetime semantics: MPTCP v6 child sockets are allocated from kmalloc-4k without `SLAB_TYPESAFE_BY_RCU`, while lockless ehash lookups assume RCU-safe socket lifetime.

* Reliable kernel crash / strong DoS: **+1**
  The commit explicitly states that concurrent ehash lookup can access freed memory and trigger slab-use-after-free in `__inet_lookup_established`.

* Hard or unreliable race / special timing required: **-1**
  Triggering depends on a race between socket free/reuse and concurrent lockless established-hash lookup.

* Common networking core path or packet-processing path: **+1**
  The affected failure occurs in `__inet_lookup_established`, a central TCP socket lookup path.

Paranoid scoring differences:

* Memory corruption, strong corruption primitive: **+2 instead of +1**
  In the paranoid interpretation, this is treated as a stronger UAF because freed socket memory can be immediately reused while still reachable from lockless networking lookup.

* Weak LPE concern: **+1**
  Not because privilege escalation is demonstrated, but because a network-reachable UAF in socket memory historically deserves manual review for possible object-reuse consequences.

## 3. Reachability analysis

A remote peer can potentially trigger the bug by interacting with an exposed MPTCP IPv6 endpoint and causing MPTCP subflow creation and teardown while packets drive concurrent established-socket lookups. No local account is required if the vulnerable MPTCP IPv6 service is reachable, so the practical PR interpretation is **PR:N** for exposed services.

The issue is configuration-dependent: MPTCP and IPv6 must be enabled and an MPTCP-capable listener must be reachable. It is not necessarily Internet-exposed in typical deployments, but it is a network-facing kernel socket lifetime bug once MPTCP IPv6 is in use. Namespaces do not materially reduce the remote-trigger concern, though containerized services using host networking or exposed MPTCP endpoints could increase practical exposure.

Call-site confidence: **high**, because the commit directly identifies the affected lookup path, allocation source, missing `SLAB_TYPESAFE_BY_RCU`, and the UAF site in `__inet_lookup_established`.

## 4. Severity interpretation

This should not be treated as an ordinary Moderate or auto-closed DoS-only issue. Realistic demonstrated impact is likely kernel crash or host DoS due to slab-use-after-free under race conditions. Theoretical impact is higher because the bug is a real lifetime violation in network socket memory, but there is no demonstrated arbitrary write, controlled reclaim, type confusion, or reliable privilege-escalation primitive in the patch.

Conservative handling: **Actionable Moderate**.
Paranoid/manual-review handling: **Strong Important candidate**.

## 5. One-sentence report phrase

A network-reachable slab-use-after-free in the MPTCP IPv6 subflow path can occur because child sockets may be allocated without `SLAB_TYPESAFE_BY_RCU`, allowing lockless established-socket lookups to access freed memory and potentially crash the kernel.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

Reason: this is a network-facing race-dependent UAF in a central socket lookup path. Even if the most realistic impact is DoS, the lifetime corruption primitive is strong enough to require human review before closure.

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

Patch: mptcp: fix slab-use-after-free in __inet_lookup_established
Commit: f6e1f25fa5e733570f6d6fe37a4dfed2a0deba47
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f6e1f25fa5e733570f6d6fe37a4dfed2a0deba47

Commit description:

The ehash table lookups are lockless and rely on
SLAB_TYPESAFE_BY_RCU to guarantee socket memory stability
during RCU read-side critical sections. Both tcp_prot and
tcpv6_prot have their slab caches created with this flag
via proto_register().

However, MPTCP's mptcp_subflow_init() copies tcpv6_prot into
tcpv6_prot_override during inet_init() (fs_initcall, level 5),
before inet6_init() (module_init/device_initcall, level 6) has
called proto_register(&tcpv6_prot). At that point,
tcpv6_prot.slab is still NULL, so tcpv6_prot_override.slab
remains NULL permanently.

This causes MPTCP v6 subflow child sockets to be allocated via
kmalloc (falling into kmalloc-4k) instead of the TCPv6 slab
cache. The kmalloc-4k cache lacks SLAB_TYPESAFE_BY_RCU, so
when these sockets are freed without SOCK_RCU_FREE (which is
cleared for child sockets by design), the memory can be
immediately reused. Concurrent ehash lookups under
rcu_read_lock can then access freed memory, triggering a
slab-use-after-free in __inet_lookup_established.

Fix this by splitting the IPv6-specific initialization out of
mptcp_subflow_init() into a new mptcp_subflow_v6_init(), called
from mptcp_proto_v6_init() before protocol registration. This
ensures tcpv6_prot_override.slab correctly inherits the
SLAB_TYPESAFE_BY_RCU slab cache.

Fixes: b19bc29 ("mptcp: implement delegated actions")
Cc: stable@vger.kernel.org
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260406031512.189159-1-jiayuan.chen@linux.dev
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/mptcp/protocol.c
  net/mptcp/protocol.h
  net/mptcp/subflow.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=f6e1f25fa5e733570f6d6fe37a4dfed2a0deba47

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

The original Linux kernel CVE announcement for CVE-2026-31669 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-31669

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:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 8.1

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).