[CVE-2026-64552][IMPORTANT] virtio-net: fix len check in receive_big() [ Upstream

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

CVE: CVE-2026-64552
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: virtio-net: fix len check in receive_big() [ Upstream
Commit: f9451d0fd5ba635dcabb49bfe456a6db734a8986
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f9451d0fd5ba635dcabb49bfe456a6db734a8986
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64552
Analysis date: Mon, 27 Jul 2026 18:42:25 -0400
ActionableScore: 8
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A malicious virtio-net backend can announce an oversized RX length that makes `page_to_skb()` walk past the advertised page chain and write past `skb_shinfo()->frags`, causing guest kernel memory corruption and likely DoS, with possible higher impact requiring manual review.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64552 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64552

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-2026-64552	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H';*CWE-787;*CWE-20;**CWE-476;Other CVSS 'AV:A/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '8.8';DESCR 'A malicious virtio net backend can announce an RX length that passes the old receive_big check but is larger than the buffer size actually advertised by add_recvbuf_big. This makes page_to_skb walk one fragment past the page chain and write a NULL derived fragment entry past skb_shinfo frags(MAX_SKB_FRAGS), creating an OOB write in guest kernel memory and a NULL frag in the RX path. For the CVSS the PR:N is used because the attacker does not need privileges inside the victim guest when they control the virtio backend or device emulation side. The issue is not Internet reachable packet input, but it is reachable across the virtio device boundary from a malicious or compromised backend. Impact is at least guest kernel denial of service via crash and in the paranoid case may allow confidentiality and integrity impact because the patch context explicitly describes an out of bounds write primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8)  SKIP CVE-2026-64552 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 0c716703965ffc5ef4311b65cb5d84a703784717 YES  NO NO unknown 	MAYBE	WRITE OOB VIRT SIMPLEFIX NETWORK SKB HARDWARE LINUS KPANIC MEMORY PACKET  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	-	-	checked

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ACTIONABLESCORE ANALYSIS
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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:

* +1 Firmware-mediated or device-mediated external influence: the attacker is a malicious or compromised virtio backend, not ordinary Internet traffic.
* +1 Constrained kernel buffer overwrite: the bug causes an OOB write past `skb_shinfo()->frags[MAX_SKB_FRAGS]`, but the written value is NULL or kernel-derived and the target is relatively constrained.
* +1 Memory layout invariant restoration: the patch fixes inconsistency between advertised receive buffer size, header offset, `hdr_len`, and fragment accounting.
* +1 Reliable kernel crash / strong DoS: the NULL frag can be handed up the RX path and plausibly crash the guest kernel.
* +1 Broad/default/common exposure: virtio-net is widely deployed in virtualized Linux guests.
* +1 Privileged kernel/device-management memory corruption path: the corruption occurs in a trusted network driver path fed by a virtual device/backend.
* -1 No attacker-controlled payload or overwrite target: the overwrite appears narrow and not directly attacker-chosen.

Paranoid scoring differences:

* Upgrade constrained overwrite to +2 memory corruption strong primitive because the commit explicitly states an out-of-bounds write past a static skb frag array.
* Add +1 confidentiality plausible and +1 integrity plausible because kernel memory corruption in skb metadata may have impact beyond DoS after manual analysis.
* Keep -1 constraint penalty because the immediate write is still narrow and NULL-derived.

## 3. Reachability analysis

The bug is triggerable by a malicious virtio-net backend that can announce a receive length in the gap between the old bound and the actual buffer size advertised by `add_recvbuf_big()`. It is not reachable by ordinary network packets from the public Internet. It is reachable across the virtual device boundary, for example from a compromised backend, malicious device model, vhost-user backend, or hostile virtualization environment.

No privileges are needed inside the victim guest. From the guest perspective this is external device input. Namespaces inside the guest do not materially affect reachability because the trigger is below guest userspace and enters through the virtio device path.

## 4. Severity interpretation

This is not an ordinary Moderate bounds-check cleanup. The patch context explicitly describes both an OOB write past the skb frag array and a NULL frag being passed into the RX path. Realistic demonstrated impact is at least guest kernel DoS. Theoretical impact may extend to confidentiality or integrity because skb metadata corruption in kernel memory can sometimes become exploitable, but the immediate primitive is constrained and does not show attacker-controlled arbitrary write.

Therefore the conservative result is Actionable Moderate, and the paranoid result is a Strong Important candidate requiring manual review.

## 5. One-sentence report phrase

A malicious virtio-net backend can announce an oversized RX length that makes `page_to_skb()` walk past the advertised page chain and write past `skb_shinfo()->frags`, causing guest kernel memory corruption and likely DoS, with possible higher impact requiring manual review.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the patch explicitly identifies an OOB write in guest kernel skb metadata across a virtio backend-to-guest boundary, which is a memory-corruption primitive even if the immediate overwrite is constrained.

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UPSTREAM PATCH SUMMARY
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Patch: virtio-net: fix len check in receive_big() [ Upstream
Commit: f9451d0fd5ba635dcabb49bfe456a6db734a8986
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f9451d0fd5ba635dcabb49bfe456a6db734a8986

Changed files:
  drivers/net/virtio_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=f9451d0fd5ba635dcabb49bfe456a6db734a8986

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

The original Linux kernel CVE announcement for CVE-2026-64552 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64552

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:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 8.8

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).