[CVE-2026-80588][MODERATE REGULAR] mptcp: reclaim forward-allocated memory on RX path errors

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

CVE: CVE-2026-80588
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: mptcp: reclaim forward-allocated memory on RX path errors
Commit: 473f1a5ab2abc98dd9e74b95b9c23c66c47535cc
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=473f1a5ab2abc98dd9e74b95b9c23c66c47535cc
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80588
Analysis date: Wed, 26 Aug 2026 12:00:44 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
MPTCP RX error handling can fail to reclaim forward allocated socket memory after mptcp_borrow_fwdmem, allowing a remote peer with access to an exposed MPTCP endpoint to cause sustained memory pressure and denial of service under pathological drop conditions.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-80588 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80588

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-2026-80588	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	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-400;*CWE-772;CWE-404;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 RX path errors can leak forward allocated socket memory because mptcp_drop may free an skb after mptcp_borrow_fwdmem transferred memory accounting to the MPTCP socket but before an skb destructor was installed. Repeated pathological drops can make the forward memory accounting grow without proper reclaim, causing memory pressure and denial of service. For the CVSS the PR:N is used because a remote peer does not need local privileges if it can reach an MPTCP endpoint and drive the RX error path. The issue is network reachable in deployments where MPTCP is enabled and exposed, but practical exploitation likely requires sustained traffic and specific RX error conditions. Impact is resource exhaustion and denial of service only, with no clear confidentiality or integrity impact from the patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 4)  SKIP CVE-2026-80588 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	REMOTE SIMPLEFIX NETWORK SKB  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline, manual review recommended**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2. The affected path is MPTCP RX handling, and a remote peer may influence receive-path drops if it can reach an MPTCP endpoint.
* Reliable kernel crash / strong DoS: +1. The commit explicitly says forward memory can grow unboundedly in pathological drop scenarios, which can cause memory pressure and denial of service.

Paranoid additional signal:

* Availability impact realistic: +1. In the worst still-defensible case, sustained triggering could create system-wide memory pressure beyond a single transient failure.

Signals not awarded:

* Generic memory corruption: +0. The patch shows missing memory-accounting reclaim, not UAF, OOB write, double-free, type confusion, or arbitrary write.
* Privilege escalation plausible: +0. No supported LPE primitive is visible.
* Confidentiality impact: +0.
* Integrity impact: +0.
* Common networking core path: +0. MPTCP is network-facing, but this is not the ordinary TCP/IP fast path for most deployments.

## 3. Reachability analysis

A remote peer can plausibly trigger the vulnerable path only if MPTCP is enabled and an MPTCP-capable endpoint is reachable. No local privileges are required in that network-reachable model, so PR:N is reasonable for CVSS-style discussion. Exploitation appears to require sustained traffic and specific RX error or drop conditions before skb queueing, so it is not a simple one-packet crash. Namespaces do not materially lower the requirement because the relevant attacker model is remote reachability rather than local socket administration.

## 4. Severity interpretation

This behaves like an actionable network-reachable resource-exhaustion issue, not an Important-class memory corruption vulnerability. The practical impact is DoS through unbounded forward-allocated memory growth under pathological RX drop scenarios. Theoretical escalation is not supported by the patch because the fix only restores socket memory reclaim and does not indicate stale object reuse, overwrite, refcount takeover, or attacker-controlled memory corruption.

## 5. One-sentence report phrase

MPTCP RX error handling can fail to reclaim forward allocated socket memory after mptcp_borrow_fwdmem, allowing a remote peer with access to an exposed MPTCP endpoint to cause sustained memory pressure and denial of service under pathological drop conditions.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the bug is network reachable in deployments that expose MPTCP, and the commit explicitly describes unbounded memory growth. However, review should focus on confirming realistic triggerability and deployment exposure, not on privilege escalation, because the patch supports resource exhaustion only.

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

Patch: mptcp: reclaim forward-allocated memory on RX path errors
Commit: 473f1a5ab2abc98dd9e74b95b9c23c66c47535cc
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=473f1a5ab2abc98dd9e74b95b9c23c66c47535cc

Commit description:

After commit 9db5b3c ("mptcp: borrow forward memory from subflow"),
errors in the receive path prior to queueing skbs into the receive
queue do not trigger forward-allocated memory reclaiming.

Prevent forward memory from growing unboundedly in pathological drop
scenarios by explicitly reclaiming memory when skbs are dropped.

Fixes: 9db5b3c ("mptcp: borrow forward memory from subflow")
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260803-net-mptcp-misc-fixes-7-2-rc6-v2-8-b8f496d71664@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/mptcp/protocol.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=473f1a5ab2abc98dd9e74b95b9c23c66c47535cc

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

The original Linux kernel CVE announcement for CVE-2026-80588 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80588

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: 3
  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).