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 = 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 }