LXR linux/net/core/request

   1// SPDX-License-Identifier: GPL-2.0-or-later
   2/*
   3 * NET          Generic infrastructure for Network protocols.
   4 *
   5 * Authors:     Arnaldo Carvalho de Melo <acme@conectiva.com.br>
   6 *
   7 *              From code originally in include/net/tcp.h
   8 */
   9
  10#include <linux/module.h>
  11#include <linux/random.h>
  12#include <linux/slab.h>
  13#include <linux/string.h>
  14#include <linux/tcp.h>
  15#include <linux/vmalloc.h>
  16
  17#include <net/request_sock.h>
  18
  19/*
  20 * Maximum number of SYN_RECV sockets in queue per LISTEN socket.
  21 * One SYN_RECV socket costs about 80bytes on a 32bit machine.
  22 * It would be better to replace it with a global counter for all sockets
  23 * but then some measure against one socket starving all other sockets
  24 * would be needed.
  25 *
  26 * The minimum value of it is 128. Experiments with real servers show that
  27 * it is absolutely not enough even at 100conn/sec. 256 cures most
  28 * of problems.
  29 * This value is adjusted to 128 for low memory machines,
  30 * and it will increase in proportion to the memory of machine.
  31 * Note : Dont forget somaxconn that may limit backlog too.
  32 */
  33
  34void reqsk_queue_alloc(struct request_sock_queue *queue)
  35{
  36        spin_lock_init(&queue->rskq_lock);
  37
  38        spin_lock_init(&queue->fastopenq.lock);
  39        queue->fastopenq.rskq_rst_head = NULL;
  40        queue->fastopenq.rskq_rst_tail = NULL;
  41        queue->fastopenq.qlen = 0;
  42
  43        queue->rskq_accept_head = NULL;
  44}
  45
  46/*
  47 * This function is called to set a Fast Open socket's "fastopen_rsk" field
  48 * to NULL when a TFO socket no longer needs to access the request_sock.
  49 * This happens only after 3WHS has been either completed or aborted (e.g.,
  50 * RST is received).
  51 *
  52 * Before TFO, a child socket is created only after 3WHS is completed,
  53 * hence it never needs to access the request_sock. things get a lot more
  54 * complex with TFO. A child socket, accepted or not, has to access its
  55 * request_sock for 3WHS processing, e.g., to retransmit SYN-ACK pkts,
  56 * until 3WHS is either completed or aborted. Afterwards the req will stay
  57 * until either the child socket is accepted, or in the rare case when the
  58 * listener is closed before the child is accepted.
  59 *
  60 * In short, a request socket is only freed after BOTH 3WHS has completed
  61 * (or aborted) and the child socket has been accepted (or listener closed).
  62 * When a child socket is accepted, its corresponding req->sk is set to
  63 * NULL since it's no longer needed. More importantly, "req->sk == NULL"
  64 * will be used by the code below to determine if a child socket has been
  65 * accepted or not, and the check is protected by the fastopenq->lock
  66 * described below.
  67 *
  68 * Note that fastopen_rsk is only accessed from the child socket's context
  69 * with its socket lock held. But a request_sock (req) can be accessed by
  70 * both its child socket through fastopen_rsk, and a listener socket through
  71 * icsk_accept_queue.rskq_accept_head. To protect the access a simple spin
  72 * lock per listener "icsk->icsk_accept_queue.fastopenq->lock" is created.
  73 * only in the rare case when both the listener and the child locks are held,
  74 * e.g., in inet_csk_listen_stop() do we not need to acquire the lock.
  75 * The lock also protects other fields such as fastopenq->qlen, which is
  76 * decremented by this function when fastopen_rsk is no longer needed.
  77 *
  78 * Note that another solution was to simply use the existing socket lock
  79 * from the listener. But first socket lock is difficult to use. It is not
  80 * a simple spin lock - one must consider sock_owned_by_user() and arrange
  81 * to use sk_add_backlog() stuff. But what really makes it infeasible is the
  82 * locking hierarchy violation. E.g., inet_csk_listen_stop() may try to
  83 * acquire a child's lock while holding listener's socket lock. A corner
  84 * case might also exist in tcp_v4_hnd_req() that will trigger this locking
  85 * order.
  86 *
  87 * This function also sets "treq->tfo_listener" to false.
  88 * treq->tfo_listener is used by the listener so it is protected by the
  89 * fastopenq->lock in this function.
  90 */
  91void reqsk_fastopen_remove(struct sock *sk, struct request_sock *req,
  92                           bool reset)
  93{
  94        struct sock *lsk = req->rsk_listener;
  95        struct fastopen_queue *fastopenq;
  96
  97        fastopenq = &inet_csk(lsk)->icsk_accept_queue.fastopenq;
  98
  99        RCU_INIT_POINTER(tcp_sk(sk)->fastopen_rsk, NULL);
 100        spin_lock_bh(&fastopenq->lock);
 101        fastopenq->qlen--;
 102        tcp_rsk(req)->tfo_listener = false;
 103        if (req->sk)    /* the child socket hasn't been accepted yet */
 104                goto out;
 105
 106        if (!reset || lsk->sk_state != TCP_LISTEN) {
 107                /* If the listener has been closed don't bother with the
 108                 * special RST handling below.
 109                 */
 110                spin_unlock_bh(&fastopenq->lock);
 111                reqsk_put(req);
 112                return;
 113        }
 114        /* Wait for 60secs before removing a req that has triggered RST.
 115         * This is a simple defense against TFO spoofing attack - by
 116         * counting the req against fastopen.max_qlen, and disabling
 117         * TFO when the qlen exceeds max_qlen.
 118         *
 119         * For more details see CoNext'11 "TCP Fast Open" paper.
 120         */
 121        req->rsk_timer.expires = jiffies + 60*HZ;
 122        if (fastopenq->rskq_rst_head == NULL)
 123                fastopenq->rskq_rst_head = req;
 124        else
 125                fastopenq->rskq_rst_tail->dl_next = req;
 126
 127        req->dl_next = NULL;
 128        fastopenq->rskq_rst_tail = req;
 129        fastopenq->qlen++;
 130out:
 131        spin_unlock_bh(&fastopenq->lock);
 132}
 133