Re: [PATCH net v2] net: 802: enforce underlying device type for GARP and MRP

[Paolo Abeni]

On 2/25/25 3:17 PM, Oscar Maes wrote:
> 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 this by enforcing the type of the underlying device during GARP
> and MRP initialization.
> 
> Fixes: 22bedad3ce11 ("net: convert multicast list to list_head")
> Reported-by: syzbot <syzbot+91161fe81857b396c8a0@xxxxxxxxxxxxxxxxxxxxxxxxx>
> Closes: https://lore.kernel.org/netdev/000000000000ca9a81061a01ec20@xxxxxxxxxx/
> Signed-off-by: Oscar Maes <oscmaes92@xxxxxxxxx>
> ---
>  net/802/garp.c | 5 +++++
>  net/802/mrp.c  | 5 +++++
>  2 files changed, 10 insertions(+)
> 
> diff --git a/net/802/garp.c b/net/802/garp.c
> index 27f0ab146..32ab7df0e 100644
> --- a/net/802/garp.c
> +++ b/net/802/garp.c
> @@ -9,6 +9,7 @@
>  #include <linux/skbuff.h>
>  #include <linux/netdevice.h>
>  #include <linux/etherdevice.h>
> +#include <linux/if_arp.h>
>  #include <linux/rtnetlink.h>
>  #include <linux/llc.h>
>  #include <linux/slab.h>
> @@ -574,6 +575,10 @@ int garp_init_applicant(struct net_device *dev, struct garp_application *appl)
>  
>  	ASSERT_RTNL();
>  
> +	err = -EINVAL;
> +	if (dev->type != ARPHRD_ETHER)
> +		goto err1;

I agree with Ido's comment on v1, I think this belongs to
vlan_check_real_dev(). The fact that you have to write the same check
multiple times is a IMHO good argument against placing the check here.

Please move the check in vlan_check_real_dev().

Thanks,

Paolo