CVE-2025-21920: vlan: enforce underlying device type

In the Linux kernel, the following vulnerability has been resolved:

vlan: enforce underlying device type

Currently, VLAN devices can be created on top of non-ethernet devices.

Besides the fact that it doesn't make much sense, this also causes a bug which leaks the address of a kernel function to usermode.

When creating a VLAN device, we initialize GARP (garpinitapplicant) and MRP (mrpinitapplicant) for the underlying device.

As part of the initialization process, we add the multicast address of each applicant to the underlying device, by calling devmcadd.

devmcadd uses dev->addrlen to determine the length of the new multicast address.

This causes an out-of-bounds read if dev->addrlen is greater than 6, since the multicast addresses provided by GARP and MRP are only 6 bytes long.

This behaviour can be reproduced using the following commands:

ip tunnel add gretest mode ip6gre local ::1 remote ::2 dev lo ip l set up dev gretest ip link add link gretest name vlantest type vlan id 100

Then, the following command will display the address of garppdurcv:

ip maddr show | grep 01:80:c2:00:00:21

Fix the bug by enforcing the type of the underlying device during VLAN device initialization.