[CVE-2026-46207][MODERATE REGULAR] vsock/virtio: fix empty payload in tap skb for non-linear buffers

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

CVE: CVE-2026-46207
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: vsock/virtio: fix empty payload in tap skb for non-linear buffers
Commit: 06747f52ab157591cec7e5623a759473b66ef6f6
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=06747f52ab157591cec7e5623a759473b66ef6f6
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46207
Analysis date: Thu, 28 May 2026 09:28:49 -0400
ActionableScore: 3
ActionableScore lower bound: 1
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
`virtio_transport_build_skb()` could deliver uninitialized kernel heap bytes to the vsockmon tap for non-linear skbs because the manual `iov_iter` setup copied zero payload bytes.

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

The original Linux kernel CVE announcement for CVE-2026-46207 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46207

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-2026-46207	MODERATE	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:N/A:N';CWE-908;*CWE-457;*CWE-200;CWE-665;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:H/I:N/A:N';BEST CVSS score: '4.7';DESCR 'virtio vsock tap packet construction failed to copy payload data for non-linear skbs because the manually initialized iov_iter had a zero count. The new skb delivered to the vsockmon tap could therefore contain uninitialized kernel heap bytes instead of the original payload. For the CVSS the PR:L is used for the paranoid score when a reduced privilege user or container has delegated access to vsockmon or packet capture facilities. The issue is not directly Internet reachable, although a vsock peer may help create non-linear traffic that reaches the monitor path. Impact is kernel information disclosure only, with no supported memory corruption, arbitrary write, or privilege escalation primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 2) 	MAYBE	VIRT SIMPLEFIX SKB HARDWARE PACKET  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=3
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 3
* Final recommended bucket: **Borderline, manual review recommended**::

## 2. Signal breakdown

Conservative signals:

* Confidentiality impact plausible: +1. The tap skb may contain uninitialized kernel heap bytes instead of the intended vsock payload.
* Requires privileged capture or monitor access in typical deployments: -2. Reading vsockmon or packet monitor output normally requires elevated local privileges or delegated monitoring rights.

Paranoid additional signals:

* Local delegated trigger or read access: +1. In some container or observability setups, packet capture or vsock monitoring may be delegated to a reduced privilege user.
* Adjacent vsock peer influence: +1. A vsock peer may help create non-linear skb traffic that reaches the monitor path, although the peer does not automatically receive the leaked bytes.

Not counted:

* No memory corruption.
* No UAF, OOB write, arbitrary write, or privilege escalation primitive.
* No direct Internet reachability.
* No availability impact.

## 3. Reachability analysis

The leak is visible through the vsockmon tap path, not through the normal vsock payload receiver. A peer may influence the traffic pattern by sending non-linear vsock data, but exploiting the information disclosure requires the attacker to read the monitor output.

In typical systems, that means root, CAP_NET_ADMIN, packet capture permissions, or an explicitly delegated monitoring interface. Containers matter if vsockmon or capture access is passed into the container. Without such access, this is mostly a correctness bug in monitoring output rather than a directly exploitable disclosure.

## 4. Severity interpretation

This behaves like a borderline Moderate information disclosure issue. The realistic impact is possible kernel heap byte exposure to a local monitor reader.

It does not behave like an Important memory corruption issue. The bug is an uninitialized data exposure in a tap skb construction path, with no write primitive or LPE evidence.

## 5. One-sentence report phrase

`virtio_transport_build_skb()` could deliver uninitialized kernel heap bytes to the vsockmon tap for non-linear skbs because the manual `iov_iter` setup copied zero payload bytes.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Review is recommended to determine whether untrusted users can access vsockmon or packet capture output in the target environment. If capture access is strictly privileged, this can likely be treated as low-priority information disclosure.

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

Patch: vsock/virtio: fix empty payload in tap skb for non-linear buffers
Commit: 06747f52ab157591cec7e5623a759473b66ef6f6
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=06747f52ab157591cec7e5623a759473b66ef6f6

Commit description:

For non-linear skbs, virtio_transport_build_skb() goes through
virtio_transport_copy_nonlinear_skb() to copy the original payload
in the new skb to be delivered to the vsockmon tap device.
This manually initializes an iov_iter but does not set iov_iter.count.
Since the iov_iter is zero-initialized, the copy length is zero and no
payload is actually copied to the monitor interface, leaving data
un-initialized.

Fix this by removing the linear vs non-linear split and using
skb_copy_datagram_iter() with iov_iter_kvec() for all cases, as
vhost-vsock already does. This handles both linear and non-linear skbs,
properly initializes the iov_iter, and removes the now unused
virtio_transport_copy_nonlinear_skb().

While touching this code, let's also check the return value of
skb_copy_datagram_iter(), even though it's unlikely to fail.

Fixes: 4b0bf10 ("vsock/virtio: non-linear skb handling for tap")
Reported-by: Yiqi Sun <sunyiqixm@gmail.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Bobby Eshleman <bobbyeshleman@meta.com>
Reviewed-by: Arseniy Krasnov <avkrasnov@rulkc.org>
Link: https://patch.msgid.link/20260508164411.261440-3-sgarzare@redhat.com
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Cc: Luigi Leonardi <leonardi@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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=06747f52ab157591cec7e5623a759473b66ef6f6

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

The original Linux kernel CVE announcement for CVE-2026-46207 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46207

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:H/I:N/A:N
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:N/A:N
  The Best / paranoid CVSS score: 4.7

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: 1
  Paranoid ActionableScore: 3

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).