[CVE-2026-63990][LOW] bonding: refuse to enslave CAN devices [ Upstream

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

CVE: CVE-2026-63990
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: bonding: refuse to enslave CAN devices [ Upstream
Commit: 69b78b5f3033272e53a2dc2dad675962654a5b38
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=69b78b5f3033272e53a2dc2dad675962654a5b38
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63990
Analysis date: Sun, 19 Jul 2026 18:56:09 -0400
ActionableScore: 3
ActionableScore lower bound: 0
Actionable bucket: Ordinary Moderate / Low-like, not Actionable Moderate
Manual review required: NO

Summary:
Bonding CAN or vxcan devices can place the interface into an unsupported half-enslaved state and later trigger a NULL pointer dereference in CAN socket cleanup, causing a local denial of service when an attacker can perform network device administration.

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

The original Linux kernel CVE announcement for CVE-2026-63990 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63990

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: LOW
Manual review required: NO

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-2026-63990	LOW	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';**CWE-476;*CWE-20;CWE-754;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'Bonding a CAN or vxcan network device is architecturally invalid because bonding expects Ethernet like link layer behavior while CAN uses the CAN mid layer private state. If such a device is enslaved, bonding can mutate the net device state without creating or maintaining the CAN receiver list state expected later by CAN socket cleanup paths. A local actor can then hit a NULL pointer dereference in can_rx_unregister during operations such as isotp_release, causing a kernel crash and denial of service. For the CVSS the PR:L is used because reliable triggering requires a local process with network administration capability, but this capability may be delegated to containers or user namespaces in some deployments and is not equivalent to full host root in practical risk terms. The issue is not directly network reachable and is triggered through local network device control plane operations rather than packet traffic. Impact is denial of service only, with no supported confidentiality or integrity impact because the visible primitive is a NULL pointer dereference rather than UAF, OOB access, or arbitrary write.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like, not Actionable Moderate (with actual score 2)	YES	KASAN LOCK LEAK NETWORK KERNEL_PANIC_PLUS_UAF HARDWARE LINUS SYZBOT  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=3
ActionableScoreLower=0

## 1. ActionableScore

* Conservative score: 0
* Paranoid score: 3
* Final recommended bucket: **Ordinary Moderate / Low-like, not Actionable Moderate**::

## 2. Signal breakdown

Conservative signals:

* Reliable kernel crash / strong DoS: +1. The commit describes a concrete kernel paging request crash in `can_rx_unregister()` after invalid CAN enslaving into bonding.
* Requires admin/root/CAP_NET_ADMIN in typical deployments: -2. Creating a bond and enslaving an interface normally requires `CAP_NET_ADMIN`.
* Rare AND difficult-to-reach subsystem/configuration: -1. The trigger requires an unusual invalid combination of bonding plus CAN/vxcan device handling.
* Effective conservative score is floored at 0.

Paranoid signals:

* Local unprivileged trigger: +1. In some deployments, untrusted users may obtain `CAP_NET_ADMIN` inside user namespaces or containers and create local network devices.
* Reliable kernel crash / strong DoS: +1. The crash path is concrete and syzbot-reported.
* Availability impact realistic: +1. The result is a host kernel crash or serious kernel fault from local control-plane operations.
* No memory-corruption bonus. The demonstrated primitive is a NULL pointer dereference, not UAF, OOB write, type confusion, arbitrary write, or refcount takeover.
* No LPE bonus. The patch and description do not support privilege escalation beyond DoS.
* Call-site confidence: high. The commit directly names the downstream crash path through `can_rx_unregister()` and `isotp_release()`.

## 3. Reachability analysis

The bug is triggered locally through network device control-plane operations: creating or using a bonding master and enslaving a CAN or vxcan device. In normal host deployments this requires `CAP_NET_ADMIN`, so the conservative interpretation treats it as privileged local DoS. In containerized or user-namespace-enabled environments, the practical privilege requirement may be lower if untrusted users can create bonds and CAN/vxcan devices inside a namespace, but it is still not remotely reachable through packet traffic. The path is not a common default network data path and requires a specific invalid device combination.

## 4. Severity interpretation

This behaves like an ordinary Moderate or Low-like local DoS, not an Important-class vulnerability. Theoretical kernel memory safety concern is limited because the observed failure is a NULL pointer dereference caused by invalid device-type mixing. There is no evidence of attacker-controlled reclaim, stale object reuse, write-after-free, arbitrary write, or sensitive data exposure. The paranoid score is only raised for namespace-delegated `CAP_NET_ADMIN` exposure and reliable host availability impact.

## 5. One-sentence report phrase

Bonding CAN or vxcan devices can place the interface into an unsupported half-enslaved state and later trigger a NULL pointer dereference in CAN socket cleanup, causing a local denial of service when an attacker can perform network device administration.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This can be auto-closed for standard deployments where `CAP_NET_ADMIN` is trusted and unprivileged users cannot create bond or CAN/vxcan devices. Manual review is only useful for products that deliberately delegate network administration to untrusted containers or user namespaces.

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

Patch: bonding: refuse to enslave CAN devices [ Upstream
Commit: 69b78b5f3033272e53a2dc2dad675962654a5b38
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=69b78b5f3033272e53a2dc2dad675962654a5b38

Changed files:
  net/can/af_can.c
  net/core/dev.c
  drivers/net/bonding/bond_main.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=69b78b5f3033272e53a2dc2dad675962654a5b38

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

The original Linux kernel CVE announcement for CVE-2026-63990 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63990

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:L/PR:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: 0
  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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).