CVE-2025-21920

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 (garp_init_applicant) and MRP (mrp_init_applicant) for the underlying device.

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

__dev_mc_add uses dev->addr_len to determine the length of the new multicast address.

This causes an out-of-bounds read if dev->addr_len 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 garp_pdu_rcv:

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.

This vulnerability is rated as Moderate severity for Red Hat Enterprise Linux because exploitation requires local access with administrative networking privileges (CAP_NET_ADMIN) to establish virtual interfaces, restricting unauthorized use under standard configurations. While unprivileged user namespaces could permit local users to instantiate these network devices, the resulting out-of-bounds read primarily exposes kernel addresses to user space or causes a denial of service rather than enabling direct privilege escalation. Additionally, creating VLAN interfaces on top of non-Ethernet devices represents an uncommon operational scenario, further limiting real-world exposure.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.