1 /* 2 * NET Generic infrastructure for Network protocols. 3 * 4 * Authors: Arnaldo Carvalho de Melo <acme@conectiva.com.br> 5 * 6 * From code originally in include/net/tcp.h 7 * 8 * This program is free software; you can redistribute it and/or 9 * modify it under the terms of the GNU General Public License 10 * as published by the Free Software Foundation; either version 11 * 2 of the License, or (at your option) any later version. 12 */ 13 14 #include <linux/module.h> 15 #include <linux/random.h> 16 #include <linux/slab.h> 17 #include <linux/string.h> 18 #include <linux/tcp.h> 19 #include <linux/vmalloc.h> 20 21 #include <net/request_sock.h> 22 23 /* 24 * Maximum number of SYN_RECV sockets in queue per LISTEN socket. 25 * One SYN_RECV socket costs about 80bytes on a 32bit machine. 26 * It would be better to replace it with a global counter for all sockets 27 * but then some measure against one socket starving all other sockets 28 * would be needed. 29 * 30 * The minimum value of it is 128. Experiments with real servers show that 31 * it is absolutely not enough even at 100conn/sec. 256 cures most 32 * of problems. 33 * This value is adjusted to 128 for low memory machines, 34 * and it will increase in proportion to the memory of machine. 35 * Note : Dont forget somaxconn that may limit backlog too. 36 */ 37 int sysctl_max_syn_backlog = 256; 38 EXPORT_SYMBOL(sysctl_max_syn_backlog); 39 40 void reqsk_queue_alloc(struct request_sock_queue *queue) 41 { 42 spin_lock_init(&queue->rskq_lock); 43 44 spin_lock_init(&queue->fastopenq.lock); 45 queue->fastopenq.rskq_rst_head = NULL; 46 queue->fastopenq.rskq_rst_tail = NULL; 47 queue->fastopenq.qlen = 0; 48 49 queue->rskq_accept_head = NULL; 50 } 51 52 /* 53 * This function is called to set a Fast Open socket's "fastopen_rsk" field 54 * to NULL when a TFO socket no longer needs to access the request_sock. 55 * This happens only after 3WHS has been either completed or aborted (e.g., 56 * RST is received). 57 * 58 * Before TFO, a child socket is created only after 3WHS is completed, 59 * hence it never needs to access the request_sock. things get a lot more 60 * complex with TFO. A child socket, accepted or not, has to access its 61 * request_sock for 3WHS processing, e.g., to retransmit SYN-ACK pkts, 62 * until 3WHS is either completed or aborted. Afterwards the req will stay 63 * until either the child socket is accepted, or in the rare case when the 64 * listener is closed before the child is accepted. 65 * 66 * In short, a request socket is only freed after BOTH 3WHS has completed 67 * (or aborted) and the child socket has been accepted (or listener closed). 68 * When a child socket is accepted, its corresponding req->sk is set to 69 * NULL since it's no longer needed. More importantly, "req->sk == NULL" 70 * will be used by the code below to determine if a child socket has been 71 * accepted or not, and the check is protected by the fastopenq->lock 72 * described below. 73 * 74 * Note that fastopen_rsk is only accessed from the child socket's context 75 * with its socket lock held. But a request_sock (req) can be accessed by 76 * both its child socket through fastopen_rsk, and a listener socket through 77 * icsk_accept_queue.rskq_accept_head. To protect the access a simple spin 78 * lock per listener "icsk->icsk_accept_queue.fastopenq->lock" is created. 79 * only in the rare case when both the listener and the child locks are held, 80 * e.g., in inet_csk_listen_stop() do we not need to acquire the lock. 81 * The lock also protects other fields such as fastopenq->qlen, which is 82 * decremented by this function when fastopen_rsk is no longer needed. 83 * 84 * Note that another solution was to simply use the existing socket lock 85 * from the listener. But first socket lock is difficult to use. It is not 86 * a simple spin lock - one must consider sock_owned_by_user() and arrange 87 * to use sk_add_backlog() stuff. But what really makes it infeasible is the 88 * locking hierarchy violation. E.g., inet_csk_listen_stop() may try to 89 * acquire a child's lock while holding listener's socket lock. A corner 90 * case might also exist in tcp_v4_hnd_req() that will trigger this locking 91 * order. 92 * 93 * This function also sets "treq->tfo_listener" to false. 94 * treq->tfo_listener is used by the listener so it is protected by the 95 * fastopenq->lock in this function. 96 */ 97 void reqsk_fastopen_remove(struct sock *sk, struct request_sock *req, 98 bool reset) 99 { 100 struct sock *lsk = req->rsk_listener; 101 struct fastopen_queue *fastopenq; 102 103 fastopenq = &inet_csk(lsk)->icsk_accept_queue.fastopenq; 104 105 tcp_sk(sk)->fastopen_rsk = NULL; 106 spin_lock_bh(&fastopenq->lock); 107 fastopenq->qlen--; 108 tcp_rsk(req)->tfo_listener = false; 109 if (req->sk) /* the child socket hasn't been accepted yet */ 110 goto out; 111 112 if (!reset || lsk->sk_state != TCP_LISTEN) { 113 /* If the listener has been closed don't bother with the 114 * special RST handling below. 115 */ 116 spin_unlock_bh(&fastopenq->lock); 117 reqsk_put(req); 118 return; 119 } 120 /* Wait for 60secs before removing a req that has triggered RST. 121 * This is a simple defense against TFO spoofing attack - by 122 * counting the req against fastopen.max_qlen, and disabling 123 * TFO when the qlen exceeds max_qlen. 124 * 125 * For more details see CoNext'11 "TCP Fast Open" paper. 126 */ 127 req->rsk_timer.expires = jiffies + 60*HZ; 128 if (fastopenq->rskq_rst_head == NULL) 129 fastopenq->rskq_rst_head = req; 130 else 131 fastopenq->rskq_rst_tail->dl_next = req; 132 133 req->dl_next = NULL; 134 fastopenq->rskq_rst_tail = req; 135 fastopenq->qlen++; 136 out: 137 spin_unlock_bh(&fastopenq->lock); 138 } 139