[CVE-2026-46322][MODERATE REGULAR] tun: free page on build_skb failure in tun_xdp_one() [ Upstream

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

CVE: CVE-2026-46322
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: tun: free page on build_skb failure in tun_xdp_one() [ Upstream
Commit: d16e38fac09a47bfcf98c1ad65a1bb53f94540f5
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d16e38fac09a47bfcf98c1ad65a1bb53f94540f5
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46322
Analysis date: Tue, 09 Jun 2026 09:24:04 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Ordinary Moderate / Low-like resource exhaustion issue
Manual review required: NO

Summary:
A missing `put_page()` in `tun_xdp_one()` can leak a page-frag chunk when `build_skb()` fails on the vhost/tun XDP path, allowing a local or guest-controlled workload to cause memory resource exhaustion under allocation failure conditions.

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

The original Linux kernel CVE announcement for CVE-2026-46322 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46322

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-46322	MODERATE	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';*CWE-401;*CWE-772;**CWE-400;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.8';DESCR 'A resource leak in tun_xdp_one() can occur when build_skb() fails after vhost_net_build_xdp() allocated a page for an XDP frame. The error is converted into an ENOMEM path, but without the missing put_page() the page is not released and vhost_tx_batch may still take the success path because tun_sendmsg can return total_len for the batch. For the CVSS the PR:L is used in the paranoid score because reliable triggering normally requires a local process or guest workload able to send through an already configured vhost/tun or TUN/TAP path, not direct unauthenticated network access. The issue is not directly network reachable from outside the host and it requires a configured virtualization or tun data path. Impact is denial of service through memory resource exhaustion. No UAF, OOB access, information leak, or privilege escalation primitive is indicated by the patch context.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like resource exhaustion issue (with actual score 3)	YES	SIMPLEFIX LEAK ERRORPATH NETWORK SKB  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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

ActionableScore=3
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 3
* Final recommended bucket: **Ordinary Moderate / Low-like resource exhaustion issue**

## 2. Signal breakdown

Conservative signals:

* Local or guest-controlled trigger: +1. A local process or guest workload may be able to send traffic through an already configured vhost/tun or TUN/TAP path.
* Availability impact realistic: +1. Repeated failures can leak page-frag chunks and may contribute to memory exhaustion.
* Hard or unreliable condition: -1. The leak requires `build_skb()` failure, which usually depends on memory pressure or allocation failure conditions.
* Common virtualization/networking path: +1. TUN/TAP and vhost are common in virtualization and container networking setups.

Paranoid additional signal:

* Cross-boundary resource impact: +1. In a VM setup, a guest workload may cause host-side page leakage through the vhost/tun data path, but only when the device path is already configured.

Not counted:

* No generic memory corruption. The patch shows a missing `put_page()`, not UAF, double-free, OOB write, type confusion, or arbitrary write.
* No privilege escalation plausible. No reclaim, stale callback, attacker-controlled overwrite, or object confusion is shown.
* No confidentiality or integrity impact. The primitive is resource leakage only.
* No direct remote reachability. This is not triggerable by arbitrary network packets from outside without a configured vhost/tun path.

## 3. Reachability analysis

The realistic trigger is a local process or guest workload that can send through an existing vhost/tun or TUN/TAP data path. In typical host deployments, creating and configuring TUN/TAP or vhost devices requires privileges, but once configured, guest traffic may exercise the affected path. Namespaces or containers can reduce the effective privilege barrier if TUN/TAP access is delegated, but the vulnerable condition still depends on `build_skb()` failure. The path is common in virtualization, but the precise failure condition is not an easy default remote trigger.

## 4. Severity interpretation

This behaves like an ordinary Moderate or Low-like resource exhaustion issue, not an Important-class kernel vulnerability. The theoretical worst case is host memory pressure or DoS from repeated page leaks. Realistic exploitation is constrained by the need for a configured vhost/tun path and allocation failure conditions. There is no evidence of memory corruption, privilege escalation, information disclosure, or attacker-controlled kernel object reuse.

## 5. One-sentence report phrase

A missing `put_page()` in `tun_xdp_one()` can leak a page-frag chunk when `build_skb()` fails on the vhost/tun XDP path, allowing a local or guest-controlled workload to cause memory resource exhaustion under allocation failure conditions.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This is a resource leak without a memory corruption primitive, without direct remote reachability, and without plausible privilege escalation evidence.

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

Patch: tun: free page on build_skb failure in tun_xdp_one() [ Upstream
Commit: d16e38fac09a47bfcf98c1ad65a1bb53f94540f5
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d16e38fac09a47bfcf98c1ad65a1bb53f94540f5

Changed files:
  drivers/net/tun.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=d16e38fac09a47bfcf98c1ad65a1bb53f94540f5

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

The original Linux kernel CVE announcement for CVE-2026-46322 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46322

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:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:A/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.8

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: 2
  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).