Re: [PATCH net v3] vlan: enforce underlying device type

[Jiri Pirko]

Mon, Mar 03, 2025 at 04:56:19PM +0100, oscmaes92@xxxxxxxxx wrote:
>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.
>
>Fixes: 22bedad3ce11 ("net: convert multicast list to list_head")
>Reported-by: syzbot+91161fe81857b396c8a0@xxxxxxxxxxxxxxxxxxxxxxxxx
>Closes: https://lore.kernel.org/netdev/000000000000ca9a81061a01ec20@xxxxxxxxxx/
>Signed-off-by: Oscar Maes <oscmaes92@xxxxxxxxx>


Reviewed-by: Jiri Pirko <jiri@xxxxxxxxxx>