1 // SPDX-License-Identifier: GPL-2.0-or-later 2 /* 3 * NET Generic infrastructure for Network protocols. 4 * 5 * Authors: Arnaldo Carvalho de Melo <acme@conectiva.com.br> 6 * 7 * From code originally in include/net/tcp.h 8 */ 9 10 #include <linux/module.h> 11 #include <linux/random.h> 12 #include <linux/slab.h> 13 #include <linux/string.h> 14 #include <linux/tcp.h> 15 #include <linux/vmalloc.h> 16 17 #include <net/request_sock.h> 18 19 /* 20 * Maximum number of SYN_RECV sockets in queue per LISTEN socket. 21 * One SYN_RECV socket costs about 80bytes on a 32bit machine. 22 * It would be better to replace it with a global counter for all sockets 23 * but then some measure against one socket starving all other sockets 24 * would be needed. 25 * 26 * The minimum value of it is 128. Experiments with real servers show that 27 * it is absolutely not enough even at 100conn/sec. 256 cures most 28 * of problems. 29 * This value is adjusted to 128 for low memory machines, 30 * and it will increase in proportion to the memory of machine. 31 * Note : Dont forget somaxconn that may limit backlog too. 32 */ 33 34 void reqsk_queue_alloc(struct request_sock_queue *queue) 35 { 36 queue->fastopenq.rskq_rst_head = NULL; 37 queue->fastopenq.rskq_rst_tail = NULL; 38 queue->fastopenq.qlen = 0; 39 40 queue->rskq_accept_head = NULL; 41 } 42 43 /* 44 * This function is called to set a Fast Open socket's "fastopen_rsk" field 45 * to NULL when a TFO socket no longer needs to access the request_sock. 46 * This happens only after 3WHS has been either completed or aborted (e.g., 47 * RST is received). 48 * 49 * Before TFO, a child socket is created only after 3WHS is completed, 50 * hence it never needs to access the request_sock. things get a lot more 51 * complex with TFO. A child socket, accepted or not, has to access its 52 * request_sock for 3WHS processing, e.g., to retransmit SYN-ACK pkts, 53 * until 3WHS is either completed or aborted. Afterwards the req will stay 54 * until either the child socket is accepted, or in the rare case when the 55 * listener is closed before the child is accepted. 56 * 57 * In short, a request socket is only freed after BOTH 3WHS has completed 58 * (or aborted) and the child socket has been accepted (or listener closed). 59 * When a child socket is accepted, its corresponding req->sk is set to 60 * NULL since it's no longer needed. More importantly, "req->sk == NULL" 61 * will be used by the code below to determine if a child socket has been 62 * accepted or not, and the check is protected by the fastopenq->lock 63 * described below. 64 * 65 * Note that fastopen_rsk is only accessed from the child socket's context 66 * with its socket lock held. But a request_sock (req) can be accessed by 67 * both its child socket through fastopen_rsk, and a listener socket through 68 * icsk_accept_queue.rskq_accept_head. To protect the access a simple spin 69 * lock per listener "icsk->icsk_accept_queue.fastopenq->lock" is created. 70 * only in the rare case when both the listener and the child locks are held, 71 * e.g., in inet_csk_listen_stop() do we not need to acquire the lock. 72 * The lock also protects other fields such as fastopenq->qlen, which is 73 * decremented by this function when fastopen_rsk is no longer needed. 74 * 75 * Note that another solution was to simply use the existing socket lock 76 * from the listener. But first socket lock is difficult to use. It is not 77 * a simple spin lock - one must consider sock_owned_by_user() and arrange 78 * to use sk_add_backlog() stuff. But what really makes it infeasible is the 79 * locking hierarchy violation. E.g., inet_csk_listen_stop() may try to 80 * acquire a child's lock while holding listener's socket lock. A corner 81 * case might also exist in tcp_v4_hnd_req() that will trigger this locking 82 * order. 83 * 84 * This function also sets "treq->tfo_listener" to false. 85 * treq->tfo_listener is used by the listener so it is protected by the 86 * fastopenq->lock in this function. 87 */ 88 void reqsk_fastopen_remove(struct sock *sk, struct request_sock *req, 89 bool reset) 90 { 91 struct sock *lsk = req->rsk_listener; 92 struct fastopen_queue *fastopenq; 93 94 fastopenq = &inet_csk(lsk)->icsk_accept_queue.fastopenq; 95 96 RCU_INIT_POINTER(tcp_sk(sk)->fastopen_rsk, NULL); 97 spin_lock_bh(&fastopenq->lock); 98 fastopenq->qlen--; 99 tcp_rsk(req)->tfo_listener = false; 100 if (req->sk) /* the child socket hasn't been accepted yet */ 101 goto out; 102 103 if (!reset || lsk->sk_state != TCP_LISTEN) { 104 /* If the listener has been closed don't bother with the 105 * special RST handling below. 106 */ 107 spin_unlock_bh(&fastopenq->lock); 108 reqsk_put(req); 109 return; 110 } 111 /* Wait for 60secs before removing a req that has triggered RST. 112 * This is a simple defense against TFO spoofing attack - by 113 * counting the req against fastopen.max_qlen, and disabling 114 * TFO when the qlen exceeds max_qlen. 115 * 116 * For more details see CoNext'11 "TCP Fast Open" paper. 117 */ 118 req->rsk_timer.expires = jiffies + 60*HZ; 119 if (fastopenq->rskq_rst_head == NULL) 120 fastopenq->rskq_rst_head = req; 121 else 122 fastopenq->rskq_rst_tail->dl_next = req; 123 124 req->dl_next = NULL; 125 fastopenq->rskq_rst_tail = req; 126 fastopenq->qlen++; 127 out: 128 spin_unlock_bh(&fastopenq->lock); 129 } 130