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 void reqsk_queue_alloc(struct request_sock_queue *queue)
  41 {
  42         spin_lock_init(&queue->rskq_lock);
  43
  44         spin_lock_init(&queue->fastopenq.lock);
  45         queue->fastopenq.rskq_rst_head = NULL;
  46         queue->fastopenq.rskq_rst_tail = NULL;
  47         queue->fastopenq.qlen = 0;
  48
  49         queue->rskq_accept_head = NULL;
  50 }
  51
  52 /*
  53  * This function is called to set a Fast Open socket's "fastopen_rsk" field
  54  * to NULL when a TFO socket no longer needs to access the request_sock.
  55  * This happens only after 3WHS has been either completed or aborted (e.g.,
  56  * RST is received).
  57  *
  58  * Before TFO, a child socket is created only after 3WHS is completed,
  59  * hence it never needs to access the request_sock. things get a lot more
  60  * complex with TFO. A child socket, accepted or not, has to access its
  61  * request_sock for 3WHS processing, e.g., to retransmit SYN-ACK pkts,
  62  * until 3WHS is either completed or aborted. Afterwards the req will stay
  63  * until either the child socket is accepted, or in the rare case when the
  64  * listener is closed before the child is accepted.
  65  *
  66  * In short, a request socket is only freed after BOTH 3WHS has completed
  67  * (or aborted) and the child socket has been accepted (or listener closed).
  68  * When a child socket is accepted, its corresponding req->sk is set to
  69  * NULL since it's no longer needed. More importantly, "req->sk == NULL"
  70  * will be used by the code below to determine if a child socket has been
  71  * accepted or not, and the check is protected by the fastopenq->lock
  72  * described below.
  73  *
  74  * Note that fastopen_rsk is only accessed from the child socket's context
  75  * with its socket lock held. But a request_sock (req) can be accessed by
  76  * both its child socket through fastopen_rsk, and a listener socket through
  77  * icsk_accept_queue.rskq_accept_head. To protect the access a simple spin
  78  * lock per listener "icsk->icsk_accept_queue.fastopenq->lock" is created.
  79  * only in the rare case when both the listener and the child locks are held,
  80  * e.g., in inet_csk_listen_stop() do we not need to acquire the lock.
  81  * The lock also protects other fields such as fastopenq->qlen, which is
  82  * decremented by this function when fastopen_rsk is no longer needed.
  83  *
  84  * Note that another solution was to simply use the existing socket lock
  85  * from the listener. But first socket lock is difficult to use. It is not
  86  * a simple spin lock - one must consider sock_owned_by_user() and arrange
  87  * to use sk_add_backlog() stuff. But what really makes it infeasible is the
  88  * locking hierarchy violation. E.g., inet_csk_listen_stop() may try to
  89  * acquire a child's lock while holding listener's socket lock. A corner
  90  * case might also exist in tcp_v4_hnd_req() that will trigger this locking
  91  * order.
  92  *
  93  * This function also sets "treq->tfo_listener" to false.
  94  * treq->tfo_listener is used by the listener so it is protected by the
  95  * fastopenq->lock in this function.
  96  */
  97 void reqsk_fastopen_remove(struct sock *sk, struct request_sock *req,
  98                            bool reset)
  99 {
 100         struct sock *lsk = req->rsk_listener;
 101         struct fastopen_queue *fastopenq;
 102
 103         fastopenq = &inet_csk(lsk)->icsk_accept_queue.fastopenq;
 104
 105         tcp_sk(sk)->fastopen_rsk = NULL;
 106         spin_lock_bh(&fastopenq->lock);
 107         fastopenq->qlen--;
 108         tcp_rsk(req)->tfo_listener = false;
 109         if (req->sk)    /* the child socket hasn't been accepted yet */
 110                 goto out;
 111
 112         if (!reset || lsk->sk_state != TCP_LISTEN) {
 113                 /* If the listener has been closed don't bother with the
 114                  * special RST handling below.
 115                  */
 116                 spin_unlock_bh(&fastopenq->lock);
 117                 reqsk_put(req);
 118                 return;
 119         }
 120         /* Wait for 60secs before removing a req that has triggered RST.
 121          * This is a simple defense against TFO spoofing attack - by
 122          * counting the req against fastopen.max_qlen, and disabling
 123          * TFO when the qlen exceeds max_qlen.
 124          *
 125          * For more details see CoNext'11 "TCP Fast Open" paper.
 126          */
 127         req->rsk_timer.expires = jiffies + 60*HZ;
 128         if (fastopenq->rskq_rst_head == NULL)
 129                 fastopenq->rskq_rst_head = req;
 130         else
 131                 fastopenq->rskq_rst_tail->dl_next = req;
 132
 133         req->dl_next = NULL;
 134         fastopenq->rskq_rst_tail = req;
 135         fastopenq->qlen++;
 136 out:
 137         spin_unlock_bh(&fastopenq->lock);
 138 }