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>

2 files changed, 106 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)
diff --git a/include/net/tcp.h b/include/net/tcp.h
index 738b65f01e26..f9abd9becabd 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -30,6 +30,7 @@
 #include <linux/dmaengine.h>
 #include <linux/crypto.h>
 #include <linux/cryptohash.h>
+#include <linux/kref.h>
 
 #include <net/inet_connection_sock.h>
 #include <net/inet_timewait_sock.h>
@@ -164,6 +165,7 @@ extern void tcp_time_wait(struct sock *sk, int state, int timeo);
 #define TCPOPT_SACK             5       /* SACK Block */
 #define TCPOPT_TIMESTAMP        8       /* Better RTT estimations/PAWS */
 #define TCPOPT_MD5SIG           19      /* MD5 Signature (RFC2385) */
+#define TCPOPT_COOKIE           253     /* Cookie extension (experimental) */
 
 /*
  *     TCP option lengths
@@ -174,6 +176,10 @@ extern void tcp_time_wait(struct sock *sk, int state, int timeo);
 #define TCPOLEN_SACK_PERM      2
 #define TCPOLEN_TIMESTAMP      10
 #define TCPOLEN_MD5SIG         18
+#define TCPOLEN_COOKIE_BASE    2        /* Cookie-less header extension */
+#define TCPOLEN_COOKIE_PAIR    3        /* Cookie pair header extension */
+#define TCPOLEN_COOKIE_MIN     (TCPOLEN_COOKIE_BASE+TCP_COOKIE_MIN)
+#define TCPOLEN_COOKIE_MAX     (TCPOLEN_COOKIE_BASE+TCP_COOKIE_MAX)
 
 /* But this is what stacks really send out. */
 #define TCPOLEN_TSTAMP_ALIGNED          12
@@ -1482,6 +1488,83 @@ struct tcp_request_sock_ops {
 
 extern int tcp_cookie_generator(u32 *bakery);
 
+/**
+ *      struct tcp_cookie_values - each socket needs extra space for the
+ *      cookies, together with (optional) space for any SYN data.
+ *
+ *      A tcp_sock contains a pointer to the current value, and this is
+ *      cloned to the tcp_timewait_sock.
+ *
+ * @cookie_pair:        variable data from the option exchange.
+ *
+ * @cookie_desired:     user specified tcpct_cookie_desired.  Zero
+ *                      indicates default (sysctl_tcp_cookie_size).
+ *                      After cookie sent, remembers size of cookie.
+ *                      Range 0, TCP_COOKIE_MIN to TCP_COOKIE_MAX.
+ *
+ * @s_data_desired:     user specified tcpct_s_data_desired.  When the
+ *                      constant payload is specified (@s_data_constant),
+ *                      holds its length instead.
+ *                      Range 0 to TCP_MSS_DESIRED.
+ *
+ * @s_data_payload:     constant data that is to be included in the
+ *                      payload of SYN or SYNACK segments when the
+ *                      cookie option is present.
+ */
+struct tcp_cookie_values {
+        struct kref     kref;
+        u8              cookie_pair[TCP_COOKIE_PAIR_SIZE];
+        u8              cookie_pair_size;
+        u8              cookie_desired;
+        u16             s_data_desired:11,
+                        s_data_constant:1,
+                        s_data_in:1,
+                        s_data_out:1,
+                        s_data_unused:2;
+        u8              s_data_payload[0];
+};
+
+static inline void tcp_cookie_values_release(struct kref *kref)
+{
+        kfree(container_of(kref, struct tcp_cookie_values, kref));
+}
+
+/* The length of constant payload data.  Note that s_data_desired is
+ * overloaded, depending on s_data_constant: either the length of constant
+ * data (returned here) or the limit on variable data.
+ */
+static inline int tcp_s_data_size(const struct tcp_sock *tp)
+{
+        return (tp->cookie_values != NULL && tp->cookie_values->s_data_constant)
+                ? tp->cookie_values->s_data_desired
+                : 0;
+}
+
+/**
+ *      struct tcp_extend_values - tcp_ipv?.c to tcp_output.c workspace.
+ *
+ *      As tcp_request_sock has already been extended in other places, the
+ *      only remaining method is to pass stack values along as function
+ *      parameters.  These parameters are not needed after sending SYNACK.
+ *
+ * @cookie_bakery:      cryptographic secret and message workspace.
+ *
+ * @cookie_plus:        bytes in authenticator/cookie option, copied from
+ *                      struct tcp_options_received (above).
+ */
+struct tcp_extend_values {
+        struct request_values           rv;
+        u32                             cookie_bakery[COOKIE_WORKSPACE_WORDS];
+        u8                              cookie_plus:6,
+                                        cookie_out_never:1,
+                                        cookie_in_always:1;
+};
+
+static inline struct tcp_extend_values *tcp_xv(struct request_values *rvp)
+{
+        return (struct tcp_extend_values *)rvp;
+}
+
 extern void tcp_v4_init(void);
 extern void tcp_init(void);