[CVE-2026-53365][MODERATE REGULAR] vsock/virtio: fix zerocopy completion for multi-skb sends [ Upstream

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

CVE: CVE-2026-53365
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: vsock/virtio: fix zerocopy completion for multi-skb sends [ Upstream
Commit: 76b995bc57bd90cb6e954e1966fbd8786da47f0d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=76b995bc57bd90cb6e954e1966fbd8786da47f0d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53365
Analysis date: Mon, 13 Jul 2026 13:52:00 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Ordinary Moderate / Low-like, borderline only under sustained abuse
Manual review required: NO

Summary:
A local virtio-vsock MSG_ZEROCOPY sender can cause incorrect completion tracking for multi-skb sends, potentially leaking pinned pages and causing denial of service through resource exhaustion.

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

The original Linux kernel CVE announcement for CVE-2026-53365 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53365

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: 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-2026-53365	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';*CWE-401;*CWE-772;**CWE-400;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'A resource accounting flaw in virtio vsock MSG_ZEROCOPY handling can occur when a large message is split into multiple skbs and the zerocopy uarg is attached only to the final skb. Non final skbs may carry pinned user pages without proper completion tracking, and an early send loop break can leave pinned pages without a completion notification. This can cause local denial of service through pinned page leakage or stalled zerocopy completion semantics. For the CVSS the PR:L is used because reliable triggering requires a local process that can open an AF_VSOCK socket and send with MSG_ZEROCOPY on the affected system. The issue is not directly network reachable in the CVSS AV:N sense because a remote peer does not by itself force the local kernel to issue zerocopy sends. Impact is primarily availability loss. Confidentiality and integrity impact are not raised because the patch context indicates a resource leak and completion tracking bug rather than UAF, OOB access, or an arbitrary write primitive.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like, borderline only under sustained abuse (with actual score 4)	YES	VIRT INIT NOMEMCHK LEAK SKB IMPROVEONLY LINUS MEMORY  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	YES	NO	guess

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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: **Ordinary Moderate / Low-like, borderline only under sustained abuse**::

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  A local process that can use AF_VSOCK and MSG_ZEROCOPY can trigger the affected send path.

* Real lifetime corruption / async completion lifetime mismatch: +1
  The bug is in zerocopy completion ownership. Non-final skbs may carry pinned user pages without the correct ubuf_info completion tracking.

* Availability impact realistic: +1
  Repeated large multi-skb zerocopy sends can leak pinned pages or leave userspace without completion notification, creating a practical local DoS or resource exhaustion condition.

Paranoid-only signal:

* Reliable strong DoS under sustained abuse: +1
  If the workload can repeatedly hit the early-break or multi-skb path, pinned pages may accumulate enough to cause broader memory pressure.

Not triggered:

* Remote reachable: +0
  A remote peer does not directly force the local kernel to send MSG_ZEROCOPY data.

* Generic memory corruption: +0
  The patch does not show UAF, OOB write, double free, type confusion, arbitrary write, or attacker-controlled object reuse.

* Privilege escalation plausible: +0
  No concrete LPE primitive is visible.

* Confidentiality / integrity impact: +0
  The issue is resource accounting and completion tracking, not data disclosure or unauthorized modification.

## 3. Reachability analysis

The bug is reachable by a local process using virtio-vsock with MSG_ZEROCOPY and sending a large enough message to be fragmented into multiple skbs. It is most relevant inside VM or virtualization environments where virtio-vsock is enabled and available to local workloads.

Namespaces or containers may matter if AF_VSOCK access is exposed to containerized workloads. In typical deployments this is still a local user-triggered path, not a network-triggered one. The affected feature is not a universal default networking path, but it is not exotic in virtualized environments.

Call-site confidence: high. The provided patch includes the affected send loop, skb attachment point, ubuf_info allocation, reference balancing, and abort path.

## 4. Severity interpretation

This behaves like an ordinary Moderate or Low-like resource exhaustion bug, not an Important-class vulnerability. The theoretical worst case is local DoS through pinned-page leakage or stalled zerocopy completion handling. Realistic exploitation evidence does not support privilege escalation, confidentiality impact, integrity impact, or kernel memory corruption beyond resource lifetime/accounting mismatch.

The paranoid score is slightly higher because pinned pages are a meaningful kernel resource and repeated abuse can plausibly create system-wide pressure. It should still not be treated as Strong Important without evidence of UAF, object replacement, write primitive, or cross-boundary security bypass.

## 5. One-sentence report phrase

A local virtio-vsock MSG_ZEROCOPY sender can cause incorrect completion tracking for multi-skb sends, potentially leaking pinned pages and causing denial of service through resource exhaustion.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

Reason: the issue is a resource leak and zerocopy completion accounting bug with no demonstrated memory corruption, information disclosure, privilege escalation, or direct remote reachability.

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

Patch: vsock/virtio: fix zerocopy completion for multi-skb sends [ Upstream
Commit: 76b995bc57bd90cb6e954e1966fbd8786da47f0d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=76b995bc57bd90cb6e954e1966fbd8786da47f0d

Changed files:
  net/vmw_vsock/virtio_transport_common.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=76b995bc57bd90cb6e954e1966fbd8786da47f0d

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

The original Linux kernel CVE announcement for CVE-2026-53365 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53365

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:L/PR:L/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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).