TCPCT part 1d: define TCP cookie option, extend existing struct's

Data structures are carefully composed to require minimal additions.
For example, the struct tcp_options_received cookie_plus variable fits
between existing 16-bit and 8-bit variables, requiring no additional
space (taking alignment into consideration).  There are no additions to
tcp_request_sock, and only 1 pointer in tcp_sock.

This is a significantly revised implementation of an earlier (year-old)
patch that no longer applies cleanly, with permission of the original
author (Adam Langley):

    http://thread.gmane.org/gmane.linux.network/102586

The principle difference is using a TCP option to carry the cookie nonce,
instead of a user configured offset in the data.  This is more flexible and
less subject to user configuration error.  Such a cookie option has been
suggested for many years, and is also useful without SYN data, allowing
several related concepts to use the same extension option.

    "Re: SYN floods (was: does history repeat itself?)", September 9, 1996.
    http://www.merit.net/mail.archives/nanog/1996-09/msg00235.html

    "Re: what a new TCP header might look like", May 12, 1998.
    ftp://ftp.isi.edu/end2end/end2end-interest-1998.mail

These functions will also be used in subsequent patches that implement
additional features.

Requires:
   TCPCT part 1a: add request_values parameter for sending SYNACK
   TCPCT part 1b: generate Responder Cookie secret
   TCPCT part 1c: sysctl_tcp_cookie_size, socket option TCP_COOKIE_TRANSACTIONS

Signed-off-by: William.Allen.Simpson@gmail.com
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 13 insertions, 0 deletions

diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index da6e24416d75..f2ec38289a4a 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -1864,6 +1864,19 @@ static int tcp_v6_init_sock(struct sock *sk)
         tp->af_specific = &tcp_sock_ipv6_specific;
 #endif
 
+        /* TCP Cookie Transactions */
+        if (sysctl_tcp_cookie_size > 0) {
+                /* Default, cookies without s_data_payload. */
+                tp->cookie_values =
+                        kzalloc(sizeof(*tp->cookie_values),
+                                sk->sk_allocation);
+                if (tp->cookie_values != NULL)
+                        kref_init(&tp->cookie_values->kref);
+        }
+        /* Presumed zeroed, in order of appearance:
+         *      cookie_in_always, cookie_out_never,
+         *      s_data_constant, s_data_in, s_data_out
+         */
         sk->sk_sndbuf = sysctl_tcp_wmem[1];
         sk->sk_rcvbuf = sysctl_tcp_rmem[1];