Re: [PATCH v3 net 3/4] wireguard: send: account for mtu=0 devices

[Eric Dumazet <eric.dumazet@gmail.com>]

On 2/14/20 2:57 PM, Jason A. Donenfeld wrote:
> It turns out there's an easy way to get packets queued up while still
> having an MTU of zero, and that's via persistent keep alive. This commit
> makes sure that in whatever condition, we don't wind up dividing by
> zero. Note that an MTU of zero for a wireguard interface is something
> quasi-valid, so I don't think the correct fix is to limit it via
> min_mtu. This can be reproduced easily with:
> 
> ip link add wg0 type wireguard
> ip link add wg1 type wireguard
> ip link set wg0 up mtu 0
> ip link set wg1 up
> wg set wg0 private-key <(wg genkey)
> wg set wg1 listen-port 1 private-key <(wg genkey) peer $(wg show wg0 public-key)
> wg set wg0 peer $(wg show wg1 public-key) persistent-keepalive 1 endpoint 127.0.0.1:1
> 
> However, while min_mtu=0 seems fine, it makes sense to restrict the
> max_mtu. This commit also restricts the maximum MTU to the greatest
> number for which rounding up to the padding multiple won't overflow a
> signed integer. Packets this large were always rejected anyway
> eventually, due to checks deeper in, but it seems more sound not to even
> let the administrator configure something that won't work anyway.
> 
> We use this opportunity to clean up this function a bit so that it's
> clear which paths we're expecting.
> 
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> Cc: Eric Dumazet <eric.dumazet@gmail.com>

Reviewed-by: Eric Dumazet <edumazet@google.com>

Thanks !