Re: [PATCH 2/2] ipv6: don't deliver packets with zero length to raw sockets

[Sabrina Dubroca <sd@queasysnail.net>]

2017-04-21, 21:18:00 +1000, Jamie Bainbridge wrote:
> Hi Sabrina,
> 
> On Fri, Apr 21, 2017 at 8:01 PM, Sabrina Dubroca <sd@queasysnail.net> wrote:
> > Hi Jamie,
> >
> > 2017-04-21, 13:58:44 +1000, Jamie Bainbridge wrote:
> >> IPv6 assumes there is data after the network header and blindly delivers
> >> skbs to raw sockets without checking the presence of data.
> >>
> >> With an application in a common loop where it checks select/poll/epoll
> >> then ioctl(SIOCINQ/FIONREAD) is positive before continuing to
> >> recvfrom(), this behaviour can cause the application to loop forever
> >> on ioctl() because there is a zero-length skb to receive.
> >>
> >> With this, it is very easy to make a Denial of Service attack by
> >> crafting a packet which declares a Next Header in the IPv6 header but
> >> does not actually supply a transport header and/or payload.
> >>
> >> skb->len is already correctly set in ip6_input_finish() with pskb_pull()
> >> so check this length before delivering zero data to raw sockets.
> >
> > Isn't that changing behavior? recv() currently returns 0 when a packet
> > that stops right after the IP header arrives. After this, the userspace
> > program won't receive anything in this case?
> 
> The recv() never occurs. The skb will not be cloned or passed into
> rawv6_rcv(), the socket notification method (select/poll/epoll) will
> not trigger, and the userspace program won't be informed the packet
> has arrived. The behaviour is the same as if there was no raw socket,
> or as if the Next Header did not match the raw socket's protocol.
> 
> As you know, IPv6 raw sockets do not offer access to the network
> header by design (RFC3542). An IPv6 raw socket only receives data
> after the network header. It's not like IPv4 where the raw socket
> would still get the network header in the same situation.
> 
> If the raw socket is watching for data with valid transport headers,
> or the user has implemented their own transport protocol, or the user
> is sending raw data with no transport header, those are still
> correctly cloned and passed to rawv6_rcv() to be received by the raw
> socket. Nothing is broken for cases where there is data after the
> network header, I tested both paged and unpaged skbs and both worked
> properly.
> 
> I cannot see the use in delivering a skb with zero bytes after the
> network header to a raw socket.

Knowing that a message was received, even if it's malformed. I don't
see a fundamental difference between NextHeader == UDP without any
payload at all and NextHeader == UDP with 1B payload.
Also, with recvmsg, you would get back msg_name.

> That is like suggesting a TCP ACK with
> no data payload should result in a 0-byte skb being delivered to a
> stream socket

Not really. IMHO that's a difference between datagram and stream. An
empty message is different from no message at all.

> which is obviously wrong and would result in many
> notification-ioctl loops just like it has here.


-- 
Sabrina