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, 23 insertions, 6 deletions

diff --git a/include/linux/tcp.h b/include/linux/tcp.h
index eaa3113b3786..7fee8a4df931 100644
--- a/include/linux/tcp.h
+++ b/include/linux/tcp.h
@@ -247,31 +247,38 @@ struct tcp_options_received {
                 sack_ok : 4,    /* SACK seen on SYN packet              */
                 snd_wscale : 4, /* Window scaling received from sender  */
                 rcv_wscale : 4; /* Window scaling to send to receiver   */
-/*      SACKs data      */
+        u8      cookie_plus:6,  /* bytes in authenticator/cookie option */
+                cookie_out_never:1,
+                cookie_in_always:1;
         u8      num_sacks;      /* Number of SACK blocks                */
-        u16     user_mss;       /* mss requested by user in ioctl */
+        u16     user_mss;       /* mss requested by user in ioctl       */
         u16     mss_clamp;      /* Maximal mss, negotiated at connection setup */
 };
 
 static inline void tcp_clear_options(struct tcp_options_received *rx_opt)
 {
-        rx_opt->tstamp_ok = rx_opt->sack_ok = rx_opt->wscale_ok = rx_opt->snd_wscale = 0;
+        rx_opt->tstamp_ok = rx_opt->sack_ok = 0;
+        rx_opt->wscale_ok = rx_opt->snd_wscale = 0;
+        rx_opt->cookie_plus = 0;
 }
 
 /* This is the max number of SACKS that we'll generate and process. It's safe
- * to increse this, although since:
+ * to increase this, although since:
  *   size = TCPOLEN_SACK_BASE_ALIGNED (4) + n * TCPOLEN_SACK_PERBLOCK (8)
  * only four options will fit in a standard TCP header */
 #define TCP_NUM_SACKS 4
 
+struct tcp_cookie_values;
+struct tcp_request_sock_ops;
+
 struct tcp_request_sock {
         struct inet_request_sock        req;
 #ifdef CONFIG_TCP_MD5SIG
         /* Only used by TCP MD5 Signature so far. */
         const struct tcp_request_sock_ops *af_specific;
 #endif
-        u32                             rcv_isn;
-        u32                             snt_isn;
+        u32                             rcv_isn;
+        u32                             snt_isn;
 };
 
 static inline struct tcp_request_sock *tcp_rsk(const struct request_sock *req)
@@ -441,6 +448,12 @@ struct tcp_sock {
 /* TCP MD5 Signature Option information */
         struct tcp_md5sig_info  *md5sig_info;
 #endif
+
+        /* When the cookie options are generated and exchanged, then this
+         * object holds a reference to them (cookie_values->kref).  Also
+         * contains related tcp_cookie_transactions fields.
+         */
+        struct tcp_cookie_values  *cookie_values;
 };
 
 static inline struct tcp_sock *tcp_sk(const struct sock *sk)
@@ -459,6 +472,10 @@ struct tcp_timewait_sock {
         u16                       tw_md5_keylen;
         u8                        tw_md5_key[TCP_MD5SIG_MAXKEYLEN];
 #endif
+        /* Few sockets in timewait have cookies; in that case, then this
+         * object holds a reference to them (tw_cookie_values->kref).
+         */
+        struct tcp_cookie_values  *tw_cookie_values;
 };
 
 static inline struct tcp_timewait_sock *tcp_twsk(const struct sock *sk)