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 int reqsk_queue_alloc(struct request_sock_queue *queue, 41 unsigned int nr_table_entries) 42 { 43 size_t lopt_size = sizeof(struct listen_sock); 44 struct listen_sock *lopt = NULL; 45 46 nr_table_entries = min_t(u32, nr_table_entries, sysctl_max_syn_backlog); 47 nr_table_entries = max_t(u32, nr_table_entries, 8); 48 nr_table_entries = roundup_pow_of_two(nr_table_entries + 1); 49 lopt_size += nr_table_entries * sizeof(struct request_sock *); 50 51 if (lopt_size <= (PAGE_SIZE << PAGE_ALLOC_COSTLY_ORDER)) 52 lopt = kzalloc(lopt_size, GFP_KERNEL | 53 __GFP_NOWARN | 54 __GFP_NORETRY); 55 if (!lopt) 56 lopt = vzalloc(lopt_size); 57 if (!lopt) 58 return -ENOMEM; 59 60 get_random_bytes(&lopt->hash_rnd, sizeof(lopt->hash_rnd)); 61 spin_lock_init(&queue->syn_wait_lock); 62 queue->rskq_accept_head = NULL; 63 lopt->nr_table_entries = nr_table_entries; 64 lopt->max_qlen_log = ilog2(nr_table_entries); 65 66 spin_lock_bh(&queue->syn_wait_lock); 67 queue->listen_opt = lopt; 68 spin_unlock_bh(&queue->syn_wait_lock); 69 70 return 0; 71 } 72 73 void __reqsk_queue_destroy(struct request_sock_queue *queue) 74 { 75 /* This is an error recovery path only, no locking needed */ 76 kvfree(queue->listen_opt); 77 } 78 79 static inline struct listen_sock *reqsk_queue_yank_listen_sk( 80 struct request_sock_queue *queue) 81 { 82 struct listen_sock *lopt; 83 84 spin_lock_bh(&queue->syn_wait_lock); 85 lopt = queue->listen_opt; 86 queue->listen_opt = NULL; 87 spin_unlock_bh(&queue->syn_wait_lock); 88 89 return lopt; 90 } 91 92 void reqsk_queue_destroy(struct request_sock_queue *queue) 93 { 94 /* make all the listen_opt local to us */ 95 struct listen_sock *lopt = reqsk_queue_yank_listen_sk(queue); 96 97 if (listen_sock_qlen(lopt) != 0) { 98 unsigned int i; 99 100 for (i = 0; i < lopt->nr_table_entries; i++) { 101 struct request_sock *req; 102 103 spin_lock_bh(&queue->syn_wait_lock); 104 while ((req = lopt->syn_table[i]) != NULL) { 105 lopt->syn_table[i] = req->dl_next; 106 atomic_inc(&lopt->qlen_dec); 107 if (del_timer(&req->rsk_timer)) 108 reqsk_put(req); 109 reqsk_put(req); 110 } 111 spin_unlock_bh(&queue->syn_wait_lock); 112 } 113 } 114 115 if (WARN_ON(listen_sock_qlen(lopt) != 0)) 116 pr_err("qlen %u\n", listen_sock_qlen(lopt)); 117 kvfree(lopt); 118 } 119 120 /* 121 * This function is called to set a Fast Open socket's "fastopen_rsk" field 122 * to NULL when a TFO socket no longer needs to access the request_sock. 123 * This happens only after 3WHS has been either completed or aborted (e.g., 124 * RST is received). 125 * 126 * Before TFO, a child socket is created only after 3WHS is completed, 127 * hence it never needs to access the request_sock. things get a lot more 128 * complex with TFO. A child socket, accepted or not, has to access its 129 * request_sock for 3WHS processing, e.g., to retransmit SYN-ACK pkts, 130 * until 3WHS is either completed or aborted. Afterwards the req will stay 131 * until either the child socket is accepted, or in the rare case when the 132 * listener is closed before the child is accepted. 133 * 134 * In short, a request socket is only freed after BOTH 3WHS has completed 135 * (or aborted) and the child socket has been accepted (or listener closed). 136 * When a child socket is accepted, its corresponding req->sk is set to 137 * NULL since it's no longer needed. More importantly, "req->sk == NULL" 138 * will be used by the code below to determine if a child socket has been 139 * accepted or not, and the check is protected by the fastopenq->lock 140 * described below. 141 * 142 * Note that fastopen_rsk is only accessed from the child socket's context 143 * with its socket lock held. But a request_sock (req) can be accessed by 144 * both its child socket through fastopen_rsk, and a listener socket through 145 * icsk_accept_queue.rskq_accept_head. To protect the access a simple spin 146 * lock per listener "icsk->icsk_accept_queue.fastopenq->lock" is created. 147 * only in the rare case when both the listener and the child locks are held, 148 * e.g., in inet_csk_listen_stop() do we not need to acquire the lock. 149 * The lock also protects other fields such as fastopenq->qlen, which is 150 * decremented by this function when fastopen_rsk is no longer needed. 151 * 152 * Note that another solution was to simply use the existing socket lock 153 * from the listener. But first socket lock is difficult to use. It is not 154 * a simple spin lock - one must consider sock_owned_by_user() and arrange 155 * to use sk_add_backlog() stuff. But what really makes it infeasible is the 156 * locking hierarchy violation. E.g., inet_csk_listen_stop() may try to 157 * acquire a child's lock while holding listener's socket lock. A corner 158 * case might also exist in tcp_v4_hnd_req() that will trigger this locking 159 * order. 160 * 161 * This function also sets "treq->tfo_listener" to false. 162 * treq->tfo_listener is used by the listener so it is protected by the 163 * fastopenq->lock in this function. 164 */ 165 void reqsk_fastopen_remove(struct sock *sk, struct request_sock *req, 166 bool reset) 167 { 168 struct sock *lsk = req->rsk_listener; 169 struct fastopen_queue *fastopenq; 170 171 fastopenq = inet_csk(lsk)->icsk_accept_queue.fastopenq; 172 173 tcp_sk(sk)->fastopen_rsk = NULL; 174 spin_lock_bh(&fastopenq->lock); 175 fastopenq->qlen--; 176 tcp_rsk(req)->tfo_listener = false; 177 if (req->sk) /* the child socket hasn't been accepted yet */ 178 goto out; 179 180 if (!reset || lsk->sk_state != TCP_LISTEN) { 181 /* If the listener has been closed don't bother with the 182 * special RST handling below. 183 */ 184 spin_unlock_bh(&fastopenq->lock); 185 reqsk_put(req); 186 return; 187 } 188 /* Wait for 60secs before removing a req that has triggered RST. 189 * This is a simple defense against TFO spoofing attack - by 190 * counting the req against fastopen.max_qlen, and disabling 191 * TFO when the qlen exceeds max_qlen. 192 * 193 * For more details see CoNext'11 "TCP Fast Open" paper. 194 */ 195 req->rsk_timer.expires = jiffies + 60*HZ; 196 if (fastopenq->rskq_rst_head == NULL) 197 fastopenq->rskq_rst_head = req; 198 else 199 fastopenq->rskq_rst_tail->dl_next = req; 200 201 req->dl_next = NULL; 202 fastopenq->rskq_rst_tail = req; 203 fastopenq->qlen++; 204 out: 205 spin_unlock_bh(&fastopenq->lock); 206 } 207