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 rwlock_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 write_lock_bh(&queue->syn_wait_lock); 67 queue->listen_opt = lopt; 68 write_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 write_lock_bh(&queue->syn_wait_lock); 85 lopt = queue->listen_opt; 86 queue->listen_opt = NULL; 87 write_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 (lopt->qlen != 0) { 98 unsigned int i; 99 100 for (i = 0; i < lopt->nr_table_entries; i++) { 101 struct request_sock *req; 102 103 while ((req = lopt->syn_table[i]) != NULL) { 104 lopt->syn_table[i] = req->dl_next; 105 lopt->qlen--; 106 reqsk_free(req); 107 } 108 } 109 } 110 111 WARN_ON(lopt->qlen != 0); 112 kvfree(lopt); 113 } 114 115 /* 116 * This function is called to set a Fast Open socket's "fastopen_rsk" field 117 * to NULL when a TFO socket no longer needs to access the request_sock. 118 * This happens only after 3WHS has been either completed or aborted (e.g., 119 * RST is received). 120 * 121 * Before TFO, a child socket is created only after 3WHS is completed, 122 * hence it never needs to access the request_sock. things get a lot more 123 * complex with TFO. A child socket, accepted or not, has to access its 124 * request_sock for 3WHS processing, e.g., to retransmit SYN-ACK pkts, 125 * until 3WHS is either completed or aborted. Afterwards the req will stay 126 * until either the child socket is accepted, or in the rare case when the 127 * listener is closed before the child is accepted. 128 * 129 * In short, a request socket is only freed after BOTH 3WHS has completed 130 * (or aborted) and the child socket has been accepted (or listener closed). 131 * When a child socket is accepted, its corresponding req->sk is set to 132 * NULL since it's no longer needed. More importantly, "req->sk == NULL" 133 * will be used by the code below to determine if a child socket has been 134 * accepted or not, and the check is protected by the fastopenq->lock 135 * described below. 136 * 137 * Note that fastopen_rsk is only accessed from the child socket's context 138 * with its socket lock held. But a request_sock (req) can be accessed by 139 * both its child socket through fastopen_rsk, and a listener socket through 140 * icsk_accept_queue.rskq_accept_head. To protect the access a simple spin 141 * lock per listener "icsk->icsk_accept_queue.fastopenq->lock" is created. 142 * only in the rare case when both the listener and the child locks are held, 143 * e.g., in inet_csk_listen_stop() do we not need to acquire the lock. 144 * The lock also protects other fields such as fastopenq->qlen, which is 145 * decremented by this function when fastopen_rsk is no longer needed. 146 * 147 * Note that another solution was to simply use the existing socket lock 148 * from the listener. But first socket lock is difficult to use. It is not 149 * a simple spin lock - one must consider sock_owned_by_user() and arrange 150 * to use sk_add_backlog() stuff. But what really makes it infeasible is the 151 * locking hierarchy violation. E.g., inet_csk_listen_stop() may try to 152 * acquire a child's lock while holding listener's socket lock. A corner 153 * case might also exist in tcp_v4_hnd_req() that will trigger this locking 154 * order. 155 * 156 * When a TFO req is created, it needs to sock_hold its listener to prevent 157 * the latter data structure from going away. 158 * 159 * This function also sets "treq->listener" to NULL and unreference listener 160 * socket. treq->listener is used by the listener so it is protected by the 161 * fastopenq->lock in this function. 162 */ 163 void reqsk_fastopen_remove(struct sock *sk, struct request_sock *req, 164 bool reset) 165 { 166 struct sock *lsk = tcp_rsk(req)->listener; 167 struct fastopen_queue *fastopenq = 168 inet_csk(lsk)->icsk_accept_queue.fastopenq; 169 170 tcp_sk(sk)->fastopen_rsk = NULL; 171 spin_lock_bh(&fastopenq->lock); 172 fastopenq->qlen--; 173 tcp_rsk(req)->listener = NULL; 174 if (req->sk) /* the child socket hasn't been accepted yet */ 175 goto out; 176 177 if (!reset || lsk->sk_state != TCP_LISTEN) { 178 /* If the listener has been closed don't bother with the 179 * special RST handling below. 180 */ 181 spin_unlock_bh(&fastopenq->lock); 182 sock_put(lsk); 183 reqsk_free(req); 184 return; 185 } 186 /* Wait for 60secs before removing a req that has triggered RST. 187 * This is a simple defense against TFO spoofing attack - by 188 * counting the req against fastopen.max_qlen, and disabling 189 * TFO when the qlen exceeds max_qlen. 190 * 191 * For more details see CoNext'11 "TCP Fast Open" paper. 192 */ 193 req->expires = jiffies + 60*HZ; 194 if (fastopenq->rskq_rst_head == NULL) 195 fastopenq->rskq_rst_head = req; 196 else 197 fastopenq->rskq_rst_tail->dl_next = req; 198 199 req->dl_next = NULL; 200 fastopenq->rskq_rst_tail = req; 201 fastopenq->qlen++; 202 out: 203 spin_unlock_bh(&fastopenq->lock); 204 sock_put(lsk); 205 } 206