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[73.241.150.58]) by smtp.gmail.com with ESMTPSA id o19sm13769868pjr.2.2020.02.14.07.18.29 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 14 Feb 2020 07:18:29 -0800 (PST) Subject: Re: [PATCH net 3/3] wireguard: send: account for mtu=0 devices To: "Jason A. Donenfeld" , davem@davemloft.net, netdev@vger.kernel.org References: <20200214133404.30643-1-Jason@zx2c4.com> <20200214133404.30643-4-Jason@zx2c4.com> From: Eric Dumazet Message-ID: Date: Fri, 14 Feb 2020 07:18:28 -0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1 MIME-Version: 1.0 In-Reply-To: <20200214133404.30643-4-Jason@zx2c4.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org 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'