[CVE-2026-52943][IMPORTANT] net: skbuff: fix missing zerocopy reference in pskb_carve helpers [ Upstream

[CVE-2026-52943][IMPORTANT] net: skbuff: fix missing zerocopy reference in pskb_carve helpers [ Upstream

[AL-KERNEL]

CVE: CVE-2026-52943
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: net: skbuff: fix missing zerocopy reference in pskb_carve helpers [ Upstream
Commit: 8dbed691e43a50903658130bde0fcb5abc425b37
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8dbed691e43a50903658130bde0fcb5abc425b37
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52943
Analysis date: Wed, 24 Jun 2026 05:52:20 -0400
ActionableScore: 10
ActionableScore lower bound: unknown
Actionable bucket: High-end Important / possible Critical candidate
Manual review required: YES

Summary:
A missing zerocopy reference in skbuff carving helpers can free `ubuf_info_msgzc` while TX skbs still reference it, causing a use-after-free that is reported as reliably exploitable for local root privilege escalation from an unprivileged user.

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

The original Linux kernel CVE announcement for CVE-2026-52943 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52943

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-2026-52943	IMPORTANT	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-911;*CWE-664;BEST CVSS score: '7.8';DESCR 'A use-after-free in net/core/skbuff.c occurs because pskb_carve_inside_header() and pskb_carve_inside_nonlinear() copy skb_shared_info including destructor_arg for MSG_ZEROCOPY skbs without taking a matching zerocopy reference. This can prematurely drop the ubuf_info_msgzc reference count to zero while other TX skbs still hold live destructor_arg pointers, causing later skb cleanup to access freed memory. For the CVSS the PR:L is used because an unprivileged local user can trigger the bug through local socket operations, while remote packets alone are not sufficient. Impact is full local privilege escalation on affected default configurations according to the supplied analysis, with kernel crash also possible. The issue is not directly network reachable as AV:N, although it is in networking code and may involve TCP MSG_ZEROCOPY traffic generated locally.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is High-end Important / possible Critical candidate (with actual score 10)	YES	REMOTE READ DANGER ERRORPATH UAF SKB  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	NO	NO	guess

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

ActionableScore=10

## 1. ActionableScore

* Conservative score: **10**
* Paranoid score: **10**
* Final recommended bucket: **High-end Important / possible Critical candidate**::

## 2. Signal breakdown

Triggered signals:

* **Local unprivileged trigger: +1**
  The supplied analysis states that an unprivileged local user can trigger the bug through local socket operations using MSG_ZEROCOPY.

* **Strong LPE plausibility: +2**
  The input explicitly states that a working proof-of-concept achieves full root privilege escalation from an unprivileged local user on a default kernel configuration.

* **Memory corruption, strong corruption primitive: +2**
  This is a concrete use-after-free involving `ubuf_info_msgzc` after incorrect zerocopy reference accounting.

* **Real lifetime corruption: +1**
  The root cause is a refcount/lifetime mismatch: copied `skb_shared_info->destructor_arg` lacks a matching `net_zcopy_get()`.

* **Reliable kernel crash / strong DoS: +1**
  KASAN reports slab-use-after-free in `skb_release_data()`, and kernel crash is a realistic outcome.

* **Common networking core path: +1**
  The bug is in `net/core/skbuff.c`, a central networking buffer management path.

* **Confidentiality impact plausible: +1**
  Full local root privilege escalation implies high confidentiality impact.

* **Integrity impact plausible: +1**
  Full local root privilege escalation implies high integrity impact.

* **Availability impact not separately added**
  Crash impact is already counted under reliable kernel crash / strong DoS, so it is not double-counted.

Total: **10**

## 3. Reachability analysis

The bug is locally reachable by an unprivileged user through socket operations that create MSG_ZEROCOPY skbs and drive the affected skb carving helpers. It is not scored as remotely network-triggerable because remote packets alone are not sufficient to control the local MSG_ZEROCOPY skb lifetime pattern. Network namespaces may make the local trigger more accessible in containerized environments, but the key point is that host-local unprivileged code can reach the vulnerable path according to the supplied analysis. The affected code is in core skbuff handling and the input states exploitation works on a default kernel configuration, so this is not a rare or legacy-only path.

## 4. Severity interpretation

This behaves as an **Important-class kernel vulnerability**, not an ordinary Moderate. The issue has a concrete UAF/refcount lifetime primitive, not merely a warning or constrained crash. Realistic exploitation evidence is strong because the supplied description states that a working PoC achieves full root privilege escalation from an unprivileged local user. The theoretical and practical assessments therefore align: this should be treated as a high-priority local privilege escalation issue.

## 5. One-sentence report phrase

A missing zerocopy reference in skbuff carving helpers can free `ubuf_info_msgzc` while TX skbs still reference it, causing a use-after-free that is reported as reliably exploitable for local root privilege escalation from an unprivileged user.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

Reason: confirmed UAF, refcount/lifetime corruption, local unprivileged reachability, and stated working root privilege escalation PoC on a default kernel configuration.

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

Patch: net: skbuff: fix missing zerocopy reference in pskb_carve helpers [ Upstream
Commit: 8dbed691e43a50903658130bde0fcb5abc425b37
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8dbed691e43a50903658130bde0fcb5abc425b37

Changed files:
  net/core/skbuff.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=8dbed691e43a50903658130bde0fcb5abc425b37

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

The original Linux kernel CVE announcement for CVE-2026-52943 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52943

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: Not available
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.8

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: unknown
  Paranoid ActionableScore: 10

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).