[CVE-2026-63970][IMPORTANT] vsock/virtio: bind uarg before filling zerocopy skb [ Upstream

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

CVE: CVE-2026-63970
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: vsock/virtio: bind uarg before filling zerocopy skb [ Upstream
Commit: b62e2b2b4a50953ca952f3cd3f77dd62dc50fd5d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b62e2b2b4a50953ca952f3cd3f77dd62dc50fd5d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63970
Analysis date: Sun, 19 Jul 2026 16:45:09 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A virtio-vsock MSG_ZEROCOPY lifetime bug can free managed skb fragments before the zerocopy uarg is bound, allowing a local vsock user to trigger wrong page lifetime handling on the EMSGSIZE rollback path and potentially cause kernel memory corruption or DoS.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-63970 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63970

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-63970	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN MODERATE	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:H/A:H';*CWE-416;CWE-911;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A lifetime bug in the virtio vsock zerocopy send path can free an skb with SKBFL_MANAGED_FRAG_REFS before the zerocopy uarg is bound. When fixed buffer vectored zerocopy reaches MAX_SKB_FRAGS, io_sg_from_iter may partially attach managed frags and return EMSGSIZE, so rollback through kfree_skb can use the ordinary frag unref path instead of the uarg aware path. For the CVSS the PR:L is used because a local process may be able to open a vsock socket and request zerocopy send operations when the feature is available. The issue is not a normal IP network reachable bug, but in virtualized deployments the practical boundary may involve guest to host or host to guest vsock communication depending on where the vulnerable transport runs. Impact is at least local denial of service and in the paranoid case may include confidentiality or integrity impact due to managed page lifetime corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	VIRT SIMPLEFIX SKB  KPANIC INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1
  A local process may be able to use AF_VSOCK and MSG_ZEROCOPY when virtio-vsock is available.
* Generic memory corruption: +2
  The bug affects skb fragment lifetime handling and can release managed fixed-buffer frags through the wrong path.
* Real lifetime corruption: +1
  The skb may carry SKBFL_MANAGED_FRAG_REFS before the zerocopy uarg is bound, creating a real ownership and lifetime mismatch.
* Reliable kernel crash / strong DoS: +1
  Wrong frag release during rollback is a credible kernel crash or memory-management failure path.
* Local high-control kernel data-plane style API: +1
  The trigger involves vectored zerocopy, fixed buffers, skb fragments, and controlled payload sizing up to MAX_SKB_FRAGS.
* Hard or special triggering condition: -1
  Triggering depends on fixed-buffer vectored zerocopy, MAX_SKB_FRAGS, partial io_sg_from_iter attachment, and the EMSGSIZE rollback path.

Conservative total: 5

Paranoid additions:

* Weak-to-moderate LPE concern: +1
  This is not a mere NULL dereference. It is page and skb fragment lifetime corruption, so limited confidentiality or integrity impact is technically plausible even without a demonstrated exploit primitive.
* Cross-boundary virtualization relevance: +1
  virtio-vsock is commonly used across guest/host or VM/service boundaries, so the affected path deserves more scrutiny than an ordinary local-only socket bug.

Paranoid total: 7

## 3. Reachability analysis

The bug is not a normal IP network reachable issue. It is reachable through the virtio-vsock send path with zerocopy enabled. A realistic trigger likely requires a local process in a VM or host environment that can open a vsock socket and issue a fixed-buffer vectored zerocopy send with enough fragments to hit MAX_SKB_FRAGS and force the partial rollback path.

Namespaces do not automatically make this root-only. If AF_VSOCK and the relevant zerocopy path are available to an unprivileged local process, PR is effectively low. In virtualized deployments, the security boundary may be guest-to-host, host-to-guest, or guest-to-service depending on where the vulnerable transport instance runs.

The path is configuration dependent because it requires virtio-vsock and MSG_ZEROCOPY support, but virtio-vsock is common enough in VM environments that this should not be treated as a rare hardware-only issue.

Call-site confidence: high. The provided patch includes the allocator, the send-loop caller, the rollback-relevant ordering, and the reset no-payload caller.

## 4. Severity interpretation

This behaves like an actionable Moderate at minimum and a Strong Important candidate under paranoid triage. The realistic demonstrated impact is kernel memory lifetime corruption that can plausibly lead to local or cross-VM denial of service. The patch context supports more than a simple crash-only bug because the failure involves managed skb fragments and uarg lifetime rules.

Theoretical privilege escalation is not proven. There is no demonstrated arbitrary write, callback control, or attacker-controlled reclaim in the patch text. However, because the primitive involves page reference ownership and skb fragment lifetime corruption, manual review is warranted and the paranoid score should preserve sensitivity for possible confidentiality or integrity impact.

## 5. One-sentence report phrase

A virtio-vsock MSG_ZEROCOPY lifetime bug can free managed skb fragments before the zerocopy uarg is bound, allowing a local vsock user to trigger wrong page lifetime handling on the EMSGSIZE rollback path and potentially cause kernel memory corruption or DoS.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This should not be auto-closed because the bug is a real skb/page lifetime corruption issue in a zerocopy path, with plausible VM boundary relevance and possible impact beyond a simple crash even though a full LPE primitive is not demonstrated.

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

Patch: vsock/virtio: bind uarg before filling zerocopy skb [ Upstream
Commit: b62e2b2b4a50953ca952f3cd3f77dd62dc50fd5d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b62e2b2b4a50953ca952f3cd3f77dd62dc50fd5d

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=b62e2b2b4a50953ca952f3cd3f77dd62dc50fd5d

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

The original Linux kernel CVE announcement for CVE-2026-63970 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63970

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:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 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: 5
  Paranoid ActionableScore: 7

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).