net/core/request_sock.c

   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;
  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         if (lopt_size > PAGE_SIZE)
  51                 lopt = vzalloc(lopt_size);
  52         else
  53                 lopt = kzalloc(lopt_size, GFP_KERNEL);
  54         if (lopt == NULL)
  55                 return -ENOMEM;
  56
  57         for (lopt->max_qlen_log = 3;
  58              (1 << lopt->max_qlen_log) < nr_table_entries;
  59              lopt->max_qlen_log++);
  60
  61         get_random_bytes(&lopt->hash_rnd, sizeof(lopt->hash_rnd));
  62         rwlock_init(&queue->syn_wait_lock);
  63         queue->rskq_accept_head = NULL;
  64         lopt->nr_table_entries = 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         struct listen_sock *lopt;
  76         size_t lopt_size;
  77
  78         /*
  79          * this is an error recovery path only
  80          * no locking needed and the lopt is not NULL
  81          */
  82
  83         lopt = queue->listen_opt;
  84         lopt_size = sizeof(struct listen_sock) +
  85                 lopt->nr_table_entries * sizeof(struct request_sock *);
  86
  87         if (lopt_size > PAGE_SIZE)
  88                 vfree(lopt);
  89         else
  90                 kfree(lopt);
  91 }
  92
  93 static inline struct listen_sock *reqsk_queue_yank_listen_sk(
  94                 struct request_sock_queue *queue)
  95 {
  96         struct listen_sock *lopt;
  97
  98         write_lock_bh(&queue->syn_wait_lock);
  99         lopt = queue->listen_opt;
 100         queue->listen_opt = NULL;
 101         write_unlock_bh(&queue->syn_wait_lock);
 102
 103         return lopt;
 104 }
 105
 106 void reqsk_queue_destroy(struct request_sock_queue *queue)
 107 {
 108         /* make all the listen_opt local to us */
 109         struct listen_sock *lopt = reqsk_queue_yank_listen_sk(queue);
 110         size_t lopt_size = sizeof(struct listen_sock) +
 111                 lopt->nr_table_entries * sizeof(struct request_sock *);
 112
 113         if (lopt->qlen != 0) {
 114                 unsigned int i;
 115
 116                 for (i = 0; i < lopt->nr_table_entries; i++) {
 117                         struct request_sock *req;
 118
 119                         while ((req = lopt->syn_table[i]) != NULL) {
 120                                 lopt->syn_table[i] = req->dl_next;
 121                                 lopt->qlen--;
 122                                 reqsk_free(req);
 123                         }
 124                 }
 125         }
 126
 127         WARN_ON(lopt->qlen != 0);
 128         if (lopt_size > PAGE_SIZE)
 129                 vfree(lopt);
 130         else
 131                 kfree(lopt);
 132 }
 133
 134 /*
 135  * This function is called to set a Fast Open socket's "fastopen_rsk" field
 136  * to NULL when a TFO socket no longer needs to access the request_sock.
 137  * This happens only after 3WHS has been either completed or aborted (e.g.,
 138  * RST is received).
 139  *
 140  * Before TFO, a child socket is created only after 3WHS is completed,
 141  * hence it never needs to access the request_sock. things get a lot more
 142  * complex with TFO. A child socket, accepted or not, has to access its
 143  * request_sock for 3WHS processing, e.g., to retransmit SYN-ACK pkts,
 144  * until 3WHS is either completed or aborted. Afterwards the req will stay
 145  * until either the child socket is accepted, or in the rare case when the
 146  * listener is closed before the child is accepted.
 147  *
 148  * In short, a request socket is only freed after BOTH 3WHS has completed
 149  * (or aborted) and the child socket has been accepted (or listener closed).
 150  * When a child socket is accepted, its corresponding req->sk is set to
 151  * NULL since it's no longer needed. More importantly, "req->sk == NULL"
 152  * will be used by the code below to determine if a child socket has been
 153  * accepted or not, and the check is protected by the fastopenq->lock
 154  * described below.
 155  *
 156  * Note that fastopen_rsk is only accessed from the child socket's context
 157  * with its socket lock held. But a request_sock (req) can be accessed by
 158  * both its child socket through fastopen_rsk, and a listener socket through
 159  * icsk_accept_queue.rskq_accept_head. To protect the access a simple spin
 160  * lock per listener "icsk->icsk_accept_queue.fastopenq->lock" is created.
 161  * only in the rare case when both the listener and the child locks are held,
 162  * e.g., in inet_csk_listen_stop() do we not need to acquire the lock.
 163  * The lock also protects other fields such as fastopenq->qlen, which is
 164  * decremented by this function when fastopen_rsk is no longer needed.
 165  *
 166  * Note that another solution was to simply use the existing socket lock
 167  * from the listener. But first socket lock is difficult to use. It is not
 168  * a simple spin lock - one must consider sock_owned_by_user() and arrange
 169  * to use sk_add_backlog() stuff. But what really makes it infeasible is the
 170  * locking hierarchy violation. E.g., inet_csk_listen_stop() may try to
 171  * acquire a child's lock while holding listener's socket lock. A corner
 172  * case might also exist in tcp_v4_hnd_req() that will trigger this locking
 173  * order.
 174  *
 175  * When a TFO req is created, it needs to sock_hold its listener to prevent
 176  * the latter data structure from going away.
 177  *
 178  * This function also sets "treq->listener" to NULL and unreference listener
 179  * socket. treq->listener is used by the listener so it is protected by the
 180  * fastopenq->lock in this function.
 181  */
 182 void reqsk_fastopen_remove(struct sock *sk, struct request_sock *req,
 183                            bool reset)
 184 {
 185         struct sock *lsk = tcp_rsk(req)->listener;
 186         struct fastopen_queue *fastopenq =
 187             inet_csk(lsk)->icsk_accept_queue.fastopenq;
 188
 189         tcp_sk(sk)->fastopen_rsk = NULL;
 190         spin_lock_bh(&fastopenq->lock);
 191         fastopenq->qlen--;
 192         tcp_rsk(req)->listener = NULL;
 193         if (req->sk)    /* the child socket hasn't been accepted yet */
 194                 goto out;
 195
 196         if (!reset || lsk->sk_state != TCP_LISTEN) {
 197                 /* If the listener has been closed don't bother with the
 198                  * special RST handling below.
 199                  */
 200                 spin_unlock_bh(&fastopenq->lock);
 201                 sock_put(lsk);
 202                 reqsk_free(req);
 203                 return;
 204         }
 205         /* Wait for 60secs before removing a req that has triggered RST.
 206          * This is a simple defense against TFO spoofing attack - by
 207          * counting the req against fastopen.max_qlen, and disabling
 208          * TFO when the qlen exceeds max_qlen.
 209          *
 210          * For more details see CoNext'11 "TCP Fast Open" paper.
 211          */
 212         req->expires = jiffies + 60*HZ;
 213         if (fastopenq->rskq_rst_head == NULL)
 214                 fastopenq->rskq_rst_head = req;
 215         else
 216                 fastopenq->rskq_rst_tail->dl_next = req;
 217
 218         req->dl_next = NULL;
 219         fastopenq->rskq_rst_tail = req;
 220         fastopenq->qlen++;
 221 out:
 222         spin_unlock_bh(&fastopenq->lock);
 223         sock_put(lsk);
 224 }