tcp: TCP Fast Open Server - header & support functions

This patch adds all the necessary data structure and support
functions to implement TFO server side. It also documents a number
of flags for the sysctl_tcp_fastopen knob, and adds a few Linux
extension MIBs.

In addition, it includes the following:

1. a new TCP_FASTOPEN socket option an application must call to
supply a max backlog allowed in order to enable TFO on its listener.

2. A number of key data structures:
"fastopen_rsk" in tcp_sock - for a big socket to access its
request_sock for retransmission and ack processing purpose. It is
non-NULL iff 3WHS not completed.

"fastopenq" in request_sock_queue - points to a per Fast Open
listener data structure "fastopen_queue" to keep track of qlen (# of
outstanding Fast Open requests) and max_qlen, among other things.

"listener" in tcp_request_sock - to point to the original listener
for book-keeping purpose, i.e., to maintain qlen against max_qlen
as part of defense against IP spoofing attack.

3. various data structure and functions, many in tcp_fastopen.c, to
support server side Fast Open cookie operations, including
/proc/sys/net/ipv4/tcp_fastopen_key to allow manual rekeying.

Signed-off-by: H.K. Jerry Chu <hkchu@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

4 files changed, 121 insertions, 10 deletions

diff --git a/include/linux/snmp.h b/include/linux/snmp.h
index ad6e3a6bf9fb..fdfba235f9f1 100644
--- a/include/linux/snmp.h
+++ b/include/linux/snmp.h
@@ -241,6 +241,10 @@ enum
         LINUX_MIB_TCPCHALLENGEACK,              /* TCPChallengeACK */
         LINUX_MIB_TCPSYNCHALLENGE,              /* TCPSYNChallenge */
         LINUX_MIB_TCPFASTOPENACTIVE,            /* TCPFastOpenActive */
+        LINUX_MIB_TCPFASTOPENPASSIVE,           /* TCPFastOpenPassive*/
+        LINUX_MIB_TCPFASTOPENPASSIVEFAIL,       /* TCPFastOpenPassiveFail */
+        LINUX_MIB_TCPFASTOPENLISTENOVERFLOW,    /* TCPFastOpenListenOverflow */
+        LINUX_MIB_TCPFASTOPENCOOKIEREQD,        /* TCPFastOpenCookieReqd */
         __LINUX_MIB_MAX
 };
 
diff --git a/include/linux/tcp.h b/include/linux/tcp.h
index eb125a4c30b3..ae46df590629 100644
--- a/include/linux/tcp.h
+++ b/include/linux/tcp.h
@@ -110,6 +110,7 @@ enum {
 #define TCP_REPAIR_QUEUE        20
 #define TCP_QUEUE_SEQ           21
 #define TCP_REPAIR_OPTIONS      22
+#define TCP_FASTOPEN            23      /* Enable FastOpen on listeners */
 
 struct tcp_repair_opt {
         __u32   opt_code;
@@ -246,6 +247,7 @@ static inline unsigned int tcp_optlen(const struct sk_buff *skb)
 /* TCP Fast Open */
 #define TCP_FASTOPEN_COOKIE_MIN 4       /* Min Fast Open Cookie size in bytes */
 #define TCP_FASTOPEN_COOKIE_MAX 16      /* Max Fast Open Cookie size in bytes */
+#define TCP_FASTOPEN_COOKIE_SIZE 8      /* the size employed by this impl. */
 
 /* TCP Fast Open Cookie as stored in memory */
 struct tcp_fastopen_cookie {
@@ -312,9 +314,14 @@ struct tcp_request_sock {
         /* Only used by TCP MD5 Signature so far. */
         const struct tcp_request_sock_ops *af_specific;
 #endif
+        struct sock                     *listener; /* needed for TFO */
         u32                             rcv_isn;
         u32                             snt_isn;
         u32                             snt_synack; /* synack sent time */
+        u32                             rcv_nxt; /* the ack # by SYNACK. For
+                                                  * FastOpen it's the seq#
+                                                  * after data-in-SYN.
+                                                  */
 };
 
 static inline struct tcp_request_sock *tcp_rsk(const struct request_sock *req)
@@ -505,14 +512,18 @@ struct tcp_sock {
         struct tcp_md5sig_info  __rcu *md5sig_info;
 #endif
 
-/* TCP fastopen related information */
-        struct tcp_fastopen_request *fastopen_req;
-
         /* 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;
+
+/* TCP fastopen related information */
+        struct tcp_fastopen_request *fastopen_req;
+        /* fastopen_rsk points to request_sock that resulted in this big
+         * socket. Used to retransmit SYNACKs etc.
+         */
+        struct request_sock *fastopen_rsk;
 };
 
 enum tsq_flags {
@@ -552,6 +563,34 @@ static inline struct tcp_timewait_sock *tcp_twsk(const struct sock *sk)
         return (struct tcp_timewait_sock *)sk;
 }
 
+static inline bool tcp_passive_fastopen(const struct sock *sk)
+{
+        return (sk->sk_state == TCP_SYN_RECV &&
+                tcp_sk(sk)->fastopen_rsk != NULL);
+}
+
+static inline bool fastopen_cookie_present(struct tcp_fastopen_cookie *foc)
+{
+        return foc->len != -1;
+}
+
+static inline int fastopen_init_queue(struct sock *sk, int backlog)
+{
+        struct request_sock_queue *queue =
+            &inet_csk(sk)->icsk_accept_queue;
+
+        if (queue->fastopenq == NULL) {
+                queue->fastopenq = kzalloc(
+                    sizeof(struct fastopen_queue),
+                    sk->sk_allocation);
+                if (queue->fastopenq == NULL)
+                        return -ENOMEM;
+                spin_lock_init(&queue->fastopenq->lock);
+        }
+        queue->fastopenq->max_qlen = backlog;
+        return 0;
+}
+
 #endif  /* __KERNEL__ */
 
 #endif  /* _LINUX_TCP_H */
diff --git a/include/net/request_sock.h b/include/net/request_sock.h
index 4c0766e201e3..c3cdd6c9f448 100644
--- a/include/net/request_sock.h
+++ b/include/net/request_sock.h
@@ -106,6 +106,34 @@ struct listen_sock {
         struct request_sock     *syn_table[0];
 };
 
+/*
+ * For a TCP Fast Open listener -
+ *      lock - protects the access to all the reqsk, which is co-owned by
+ *              the listener and the child socket.
+ *      qlen - pending TFO requests (still in TCP_SYN_RECV).
+ *      max_qlen - max TFO reqs allowed before TFO is disabled.
+ *
+ *      XXX (TFO) - ideally these fields can be made as part of "listen_sock"
+ *      structure above. But there is some implementation difficulty due to
+ *      listen_sock being part of request_sock_queue hence will be freed when
+ *      a listener is stopped. But TFO related fields may continue to be
+ *      accessed even after a listener is closed, until its sk_refcnt drops
+ *      to 0 implying no more outstanding TFO reqs. One solution is to keep
+ *      listen_opt around until sk_refcnt drops to 0. But there is some other
+ *      complexity that needs to be resolved. E.g., a listener can be disabled
+ *      temporarily through shutdown()->tcp_disconnect(), and re-enabled later.
+ */
+struct fastopen_queue {
+        struct request_sock     *rskq_rst_head; /* Keep track of past TFO */
+        struct request_sock     *rskq_rst_tail; /* requests that caused RST.
+                                                 * This is part of the defense
+                                                 * against spoofing attack.
+                                                 */
+        spinlock_t      lock;
+        int             qlen;           /* # of pending (TCP_SYN_RECV) reqs */
+        int             max_qlen;       /* != 0 iff TFO is currently enabled */
+};
+
 /** struct request_sock_queue - queue of request_socks
  *
  * @rskq_accept_head - FIFO head of established children
@@ -129,6 +157,12 @@ struct request_sock_queue {
         u8                      rskq_defer_accept;
         /* 3 bytes hole, try to pack */
         struct listen_sock      *listen_opt;
+        struct fastopen_queue   *fastopenq; /* This is non-NULL iff TFO has been
+                                             * enabled on this listener. Check
+                                             * max_qlen != 0 in fastopen_queue
+                                             * to determine if TFO is enabled
+                                             * right at this moment.
+                                             */
 };
 
 extern int reqsk_queue_alloc(struct request_sock_queue *queue,
@@ -136,6 +170,8 @@ extern int reqsk_queue_alloc(struct request_sock_queue *queue,
 
 extern void __reqsk_queue_destroy(struct request_sock_queue *queue);
 extern void reqsk_queue_destroy(struct request_sock_queue *queue);
+extern void reqsk_fastopen_remove(struct sock *sk,
+