[CVE-2026-53181][MODERATE 7.0] vsock/vmci: fix sk_ack_backlog leak on failed handshake

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

CVE: CVE-2026-53181
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: vsock/vmci: fix sk_ack_backlog leak on failed handshake
Commit: 22c587aa3ab1ab5264daff3ec32136fd30436c13
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=22c587aa3ab1ab5264daff3ec32136fd30436c13
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53181
Analysis date: Mon, 29 Jun 2026 09:52:03 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: NO

Summary:
A VSOCK/VMCI failed-handshake path can leak `sk_ack_backlog` accounting, allowing a reachable peer to repeatedly force handshake errors until the listener permanently refuses new connections with `ECONNREFUSED`.

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

The original Linux kernel CVE announcement for CVE-2026-53181 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53181

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: NO

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

======================================================================
AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-53181	MODERATE	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H';*CWE-772;**CWE-400;CWE-770;Other CVSS 'AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '6.5';DESCR 'A VSOCK/VMCI listener can leak sk_ack_backlog accounting when a connection handshake fails after the socket was placed on the pending queue. Repeated malformed or failing handshakes can drive sk_ack_backlog up to sk_max_ack_backlog and make the listener refuse all new connections with ECONNREFUSED until the process is restarted. For the CVSS the PR:N is used because the attacker only needs VSOCK/VMCI reachability to the listening endpoint and does not need an account on the target system. The issue is adjacent network reachable in a virtualization environment rather than public Internet reachable. Impact is denial of service for the affected listener, with no evidence of memory corruption, information disclosure, or privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5) 	MAYBE	SIMPLEFIX LEAK ERRORPATH IMPROVEONLY PACKET  KPANIC INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

* +2 Remote reachable / network-triggerable: VSOCK/VMCI is not public Internet reachable, but the bug is triggerable by handshake traffic across a virtualization communication boundary.
* +1 Local unprivileged trigger: a peer with access to the VSOCK/VMCI endpoint may not need privileges on the target listener host or service.
* +1 Reliable strong DoS: repeated failed handshakes can permanently exhaust `sk_ack_backlog` accounting for the listener.
* +1 Availability impact realistic: the affected listener refuses new connections with `ECONNREFUSED` until process restart, so the DoS persists beyond a single failed request.
* -1 Conservative deployment constraint: VMCI transport and an exposed listener are required, so this is deployment-dependent rather than universally exposed.

No memory corruption, UAF, OOB write, information leak, or privilege escalation primitive is supported by the patch.

## 3. Reachability analysis

The bug can be triggered by an endpoint capable of sending VSOCK/VMCI connection attempts to a vulnerable listening socket. This is adjacent virtualization reachability rather than ordinary IP remote reachability. Namespaces do not appear to be the main gating factor. The practical requirement is access to the VMCI/VSOCK communication path and a listener using this transport. In affected deployments, repeated malformed or failing handshakes can be enough to poison the listener backlog accounting.

## 4. Severity interpretation

This behaves like an actionable Moderate DoS issue rather than an Important memory-corruption issue. The impact is persistent denial of service for the affected listener, not kernel-wide arbitrary memory corruption. The paranoid score is higher because guest-host or VMCI-adjacent services may be operationally critical and the attack does not require target-side credentials once the endpoint is reachable.

## 5. One-sentence report phrase

A VSOCK/VMCI failed-handshake path can leak `sk_ack_backlog` accounting, allowing a reachable peer to repeatedly force handshake errors until the listener permanently refuses new connections with `ECONNREFUSED`.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: no memory corruption is shown, but the issue is reachable across a virtualization communication boundary and can cause persistent listener-level denial of service in affected deployments.

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

Patch: vsock/vmci: fix sk_ack_backlog leak on failed handshake
Commit: 22c587aa3ab1ab5264daff3ec32136fd30436c13
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=22c587aa3ab1ab5264daff3ec32136fd30436c13

Commit description:

When vmci_transport_recv_connecting_server() returns an error,
vmci_transport_recv_listen() calls vsock_remove_pending() but never
calls sk_acceptq_removed(). This leaves sk_ack_backlog incremented
permanently.

Repeated handshake failures (malformed packets, queue pair alloc
failure, event subscribe failure) cause sk_ack_backlog to climb
toward sk_max_ack_backlog. Once it reaches the limit the listener
permanently refuses all new connections with -ECONNREFUSED, a
silent denial of service requiring a process restart to recover.

The two existing sk_acceptq_removed() calls in af_vsock.c do not
cover this path: line 764 checks vsock_is_pending() which returns
false after vsock_remove_pending(), and line 1889 is only reached
on successful accept().

Fix by balancing sk_acceptq_added() with sk_acceptq_removed() on
the error path.

Fixes: d021c34 ("VSOCK: Introduce VM Sockets")
Cc: stable@vger.kernel.org
Signed-off-by: Raf Dickson <rafdog35@gmail.com>
Acked-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://patch.msgid.link/20260526104356.469928-1-rafdog35@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/vmw_vsock/vmci_transport.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=22c587aa3ab1ab5264daff3ec32136fd30436c13

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

The original Linux kernel CVE announcement for CVE-2026-53181 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53181

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:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 6.5

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: 4
  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: YES
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).