[CVE-2026-72041][MODERATE 7.0] espintcp: use sk_msg_free_partial to fix partial send

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

CVE: CVE-2026-72041
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: espintcp: use sk_msg_free_partial to fix partial send
Commit: 54d73f18f8919735f4d04d6f43374f75756c0180
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=54d73f18f8919735f4d04d6f43374f75756c0180
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72041
Analysis date: Sat, 15 Aug 2026 06:42:09 -0400
ActionableScore: 5
ActionableScore lower bound: 2
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
ESP-in-TCP partial sends could leave sk_msg offsets, page references, and socket memory accounting inconsistent, allowing a reachable configured peer to cause resource-accounting failure or DoS, but the patch does not show a concrete privilege-escalation or information-disclosure primitive.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-72041 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72041

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: 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-2026-72041	MODERATE	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:N/I:N/A:H';CWE-682;*CWE-772;**CWE-400;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.9';DESCR 'ESP in TCP used manual sk_msg offset and socket memory accounting when tcp_sendmsg_locked sent only part of a scatterlist entry. Partial sends could leave the sk_msg state inconsistent with the bytes already accepted by TCP, which may cause wrong uncharge behavior, retained page references, or resource accounting drift. For the CVSS the PR:N is used for the paranoid score because a remote ESP in TCP peer may not need privileges on the victim once the tunnel endpoint is configured and reachable. The issue is network reachable only in deployments using RFC 8229 ESP in TCP and normally requires an existing IPsec or XFRM setup rather than exposure of a generic public service. Impact is primarily denial of service through resource exhaustion or broken socket memory accounting, without a concrete primitive for information disclosure or privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 4)	YES	REMOTE INIT SIMPLEFIX NETWORK LINUS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Paranoid score signals:

* Remote reachable / network-triggerable: +2
  ESP-in-TCP is a network transport path. A remote peer may influence partial-send behavior once an ESP-in-TCP endpoint is configured and reachable.
* Memory layout invariant restoration: +1
  The patch restores consistency between sk_msg scatterlist position, offsets, sent byte count, page release, and socket memory accounting.
* Real lifetime/accounting corruption: +1
  The old code manually handled put_page(), sk_mem_uncharge(), sg.start, and emsg.offset. Incorrect partial-send handling can leave page references or socket accounting inconsistent.
* Availability impact realistic: +1
  The most plausible impact is DoS through broken socket memory accounting, retained resources, or stuck ESP-in-TCP send state.

Conservative score signals:

* Memory layout invariant restoration: +1
* Availability impact realistic: +1
* Requires admin/root/CAP_NET_ADMIN/XFRM setup in typical deployments: -2
* Real lifetime/accounting concern: +1
* Practical lower result rounded to 2 because ordinary users usually cannot configure ESP-in-TCP or XFRM state.

Not awarded:

* No Strong LPE plausibility. No attacker-controlled reclaim, overwrite, stale callback, type confusion, or refcount takeover is shown.
* No generic strong memory corruption. The patch shows accounting and partial-send state inconsistency, not a demonstrated UAF, double-free, OOB write, or arbitrary write.
* No confidentiality or integrity impact is supported by the patch.

## 3. Reachability analysis

The vulnerable path is in XFRM ESP-in-TCP, RFC 8229. In typical deployments, creating the IPsec/XFRM configuration requires administrative privileges such as CAP_NET_ADMIN, so the conservative view is privileged local setup. After such a tunnel is configured, a remote ESP-in-TCP peer or network conditions may plausibly influence partial TCP sends and trigger the buggy accounting path without privileges on the victim.

This is not a generic public TCP service exposure. It requires ESP-in-TCP to be enabled and reachable. However, XFRM/IPsec is a real networking subsystem and should not be dismissed as an obscure or purely local path.

Call-site confidence: medium. The provided patch includes the affected send loop and the replacement with sk_msg_free_partial(), but not the full surrounding XFRM socket lifecycle.

## 4. Severity interpretation

This behaves more like an actionable Moderate than an ordinary low-risk cleanup. The realistic impact is DoS through inconsistent sk_msg and socket memory accounting during partial sends. The patch does not justify Important-class treatment because there is no concrete exploit primitive for privilege escalation, information disclosure, arbitrary write, UAF reclaim, or type confusion.

The paranoid score is higher because the affected code is in a network transport path and the bug involves page/reference/accounting state in sk_msg handling. This supports manual review, but not automatic escalation to Important without additional evidence.

## 5. One-sentence report phrase

ESP-in-TCP partial sends could leave sk_msg offsets, page references, and socket memory accounting inconsistent, allowing a reachable configured peer to cause resource-accounting failure or DoS, but the patch does not show a concrete privilege-escalation or information-disclosure primitive.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the issue is in a network transport path and involves sk_msg/page/accounting lifetime semantics. The current evidence supports DoS only, but manual review is needed to confirm whether repeated remote partial-send conditions can reliably exhaust resources or trigger stronger lifetime side effects.

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

Patch: espintcp: use sk_msg_free_partial to fix partial send
Commit: 54d73f18f8919735f4d04d6f43374f75756c0180
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=54d73f18f8919735f4d04d6f43374f75756c0180

Commit description:

sk_msg_free_partial() ensures consistency of the skmsg at every
iteration, without having to manually handle uncharges and offsets.
This simplifies the code, and fixes some bugs in skmsg accounting when
we don't send the full contents.

Cc: stable@vger.kernel.org
Fixes: e27cca9 ("xfrm: add espintcp (RFC 8229)")
Reported-by: Aaron Esau <aaron1esau@gmail.com>
Reported-by: Yiming Qian <yimingqian591@gmail.com>
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/xfrm/espintcp.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=54d73f18f8919735f4d04d6f43374f75756c0180

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

The original Linux kernel CVE announcement for CVE-2026-72041 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72041

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: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: 2
  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: NO
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).