From: Eric Dumazet <eric.dumazet@gmail.com>
To: "Jason A. Donenfeld" <Jason@zx2c4.com>,
davem@davemloft.net, netdev@vger.kernel.org
Subject: Re: [PATCH net 3/3] wireguard: send: account for mtu=0 devices
Date: Fri, 14 Feb 2020 07:18:28 -0800 [thread overview]
Message-ID: <ba6b4c66-3c15-cdbc-7d0e-eaf307c5904c@gmail.com> (raw)
In-Reply-To: <20200214133404.30643-4-Jason@zx2c4.com>
On 2/14/20 5:34 AM, 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.
>
If mtu is set to 0, the device must not send any payload.
>
> diff --git a/drivers/net/wireguard/send.c b/drivers/net/wireguard/send.c
> index c13260563446..ae77474dadeb 100644
> --- a/drivers/net/wireguard/send.c
> +++ b/drivers/net/wireguard/send.c
> @@ -148,7 +148,8 @@ static unsigned int calculate_skb_padding(struct sk_buff *skb)
> * wouldn't want the final subtraction to overflow in the case of the
> * padded_size being clamped.
> */
> - unsigned int last_unit = skb->len % PACKET_CB(skb)->mtu;
> + unsigned int last_unit = PACKET_CB(skb)->mtu ?
> + skb->len % PACKET_CB(skb)->mtu : skb->len;
> unsigned int padded_size = ALIGN(last_unit, MESSAGE_PADDING_MULTIPLE);
>
> if (padded_size > PACKET_CB(skb)->mtu)
>
Are you sure this works ?
Last statement :
return padded_size - last_unit;
will return a a ' negative number'
next prev parent reply other threads:[~2020-02-14 15:18 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-14 13:34 [PATCH net 0/3] wireguard fixes for 5.6-rc2 Jason A. Donenfeld
2020-02-14 13:34 ` [PATCH net 1/3] wireguard: selftests: reduce complexity and fix make races Jason A. Donenfeld
2020-02-14 13:34 ` [PATCH net 2/3] wireguard: receive: reset last_under_load to zero Jason A. Donenfeld
2020-02-14 13:34 ` [PATCH net 3/3] wireguard: send: account for mtu=0 devices Jason A. Donenfeld
2020-02-14 15:18 ` Eric Dumazet [this message]
2020-02-14 15:56 ` Jason A. Donenfeld
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