Amazon Linux AMI: CVE-2025-21920: Security patch for kernel (ALAS-2025-1977)

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.