[CVE-2026-74310][IMPORTANT] vhost/net: complete zerocopy ubufs only once [ Upstream

[CVE-2026-74310][IMPORTANT] vhost/net: complete zerocopy ubufs only once [ Upstream

[AL-KERNEL]

CVE: CVE-2026-74310
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: vhost/net: complete zerocopy ubufs only once [ Upstream
Commit: ea71f873423fb73e66ad88936d6759ac0ad4aa53
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ea71f873423fb73e66ad88936d6759ac0ad4aa53
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74310
Analysis date: Sat, 15 Aug 2026 20:35:13 -0400
ActionableScore: 8
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A vhost-net zerocopy refcounting bug can allow cloned skb completions to access freed vhost state after backend reset or removal, causing a host-kernel use-after-free and at least a reliable DoS with possible higher impact under controlled object reuse.

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

The original Linux kernel CVE announcement for CVE-2026-74310 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74310

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
======================================================================

CVE-2026-74310	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:H';*CWE-416;CWE-362;*CWE-672;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7.8';DESCR 'A use-after-free in the vhost/net zerocopy completion path can occur when the same ubuf_info is shared by cloned zerocopy skbs and vhost_zerocopy_complete treats a non-final ubuf callback as a completed vhost descriptor. A backend reset or removal can free vhost_net_ubuf_ref while another cloned skb still carries the same ubuf_info, and a later completion can dereference freed vhost state in the host kernel. For the CVSS the PR:L is used because the attacker normally needs local guest or vhost-net access to generate zerocopy TX traffic and influence the vulnerable path, but does not need full host administrator privileges. The issue is not directly reachable from the public network and usually depends on virtualization or internal network configuration with zerocopy and skb cloning paths such as batman-adv fragmentation. Impact is at least denial of service via host kernel crash and in the paranoid case may include confidentiality or integrity impact because this is a real UAF in host kernel memory.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8)	YES	LOCK DANGER UAF NETWORK SKB DMAorINTERRUPT LINUS KPANIC 	NO	NO	checked

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

ActionableScore=8
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: 6
* Paranoid score: 8
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative scoring:

* Local unprivileged trigger: +1. A guest or local workload can potentially generate vhost-net zerocopy TX traffic once the VM and vhost-net backend are configured.
* Generic memory corruption, strong corruption primitive: +2. The commit explicitly reports KASAN slab-use-after-free in `vhost_zerocopy_complete`.
* Real lifetime corruption: +1. This is a refcount and async completion lifetime mismatch where a cloned skb can keep `ubuf_info` alive after `vhost_net_ubuf_ref` was freed.
* Reliable kernel crash / strong DoS: +1. KASAN reports a concrete stale completion path and host kernel UAF.
* Weak LPE concern: +1. The stale completion dereferences freed vhost state and writes descriptor completion state, but the patch does not demonstrate attacker-controlled reclaim or a reliable overwrite primitive.
* Broad/common subsystem exposure: +1. vhost-net is common in KVM virtualization deployments.
* Hard or special timing/configuration required: -1. Triggering requires zerocopy skb cloning plus backend reset/removal timing, with examples involving paths such as batman-adv fragmentation.

Paranoid delta:

* Upgrade weak LPE concern to stronger LPE plausibility: +1 additional. This is justified only for triage sensitivity because it is a real host-kernel UAF in a guest-influenced data path.
* Confidentiality impact plausible: +1. Only in the paranoid case, if freed object reuse can be influenced.
* Integrity impact plausible: +1. Only in the paranoid case, because stale vhost state is dereferenced and descriptor state is written after free.

## 3. Reachability analysis

The bug is not directly public-network reachable. The realistic attacker model is a guest or local actor able to drive vhost-net TX traffic in a VM where zerocopy is enabled and where the networking path can clone zerocopy skbs. Backend reset or removal is also part of the lifetime window, so exploitation depends on configuration and timing.

Namespaces and containers may matter if vhost-net access or VM networking control is delegated to reduced-privilege service accounts. This should not be treated as full host-root only, because the vulnerable host-kernel completion path can be influenced from the guest data plane after setup.

Call-site confidence: high. The commit provides the callback path, the freeing path through backend removal, and KASAN reports in `vhost_zerocopy_complete`.

## 4. Severity interpretation

This is stronger than an ordinary Moderate because it is a confirmed UAF in a host kernel virtualization/networking path, not a simple NULL dereference or validation-only fix. Realistic exploitation beyond DoS is not demonstrated, because the patch does not show controlled reclaim, type confusion, or an arbitrary write. However, the combination of async completion, refcount misuse, stale callback state, and guest-influenced vhost-net traffic makes it an Actionable Moderate at minimum and a defensible Strong Important candidate for manual review.

## 5. One-sentence report phrase

A vhost-net zerocopy refcounting bug can allow cloned skb completions to access freed vhost state after backend reset or removal, causing a host-kernel use-after-free and at least a reliable DoS with possible higher impact under controlled object reuse.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: confirmed host-kernel UAF, virtualization guest-to-host relevance, stale async callback lifetime mismatch, and possible descriptor-state write after free make this unsuitable for auto-close even though exploitation beyond DoS is not proven.

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

Patch: vhost/net: complete zerocopy ubufs only once [ Upstream
Commit: ea71f873423fb73e66ad88936d6759ac0ad4aa53
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ea71f873423fb73e66ad88936d6759ac0ad4aa53

Changed files:
  drivers/vhost/net.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=ea71f873423fb73e66ad88936d6759ac0ad4aa53

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

The original Linux kernel CVE announcement for CVE-2026-74310 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74310

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:C/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: 6
  Paranoid ActionableScore: 8

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).