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 queue->fastopenq.rskq_rst_head = NULL;
37 queue->fastopenq.rskq_rst_tail = NULL;
38 queue->fastopenq.qlen = 0;
39
40 queue->rskq_accept_head = NULL;
41}
42
43/*
44 * This function is called to set a Fast Open socket's "fastopen_rsk" field
45 * to NULL when a TFO socket no longer needs to access the request_sock.
46 * This happens only after 3WHS has been either completed or aborted (e.g.,
47 * RST is received).
48 *
49 * Before TFO, a child socket is created only after 3WHS is completed,
50 * hence it never needs to access the request_sock. things get a lot more
51 * complex with TFO. A child socket, accepted or not, has to access its
52 * request_sock for 3WHS processing, e.g., to retransmit SYN-ACK pkts,
53 * until 3WHS is either completed or aborted. Afterwards the req will stay
54 * until either the child socket is accepted, or in the rare case when the
55 * listener is closed before the child is accepted.
56 *
57 * In short, a request socket is only freed after BOTH 3WHS has completed
58 * (or aborted) and the child socket has been accepted (or listener closed).
59 * When a child socket is accepted, its corresponding req->sk is set to
60 * NULL since it's no longer needed. More importantly, "req->sk == NULL"
61 * will be used by the code below to determine if a child socket has been
62 * accepted or not, and the check is protected by the fastopenq->lock
63 * described below.
64 *
65 * Note that fastopen_rsk is only accessed from the child socket's context
66 * with its socket lock held. But a request_sock (req) can be accessed by
67 * both its child socket through fastopen_rsk, and a listener socket through
68 * icsk_accept_queue.rskq_accept_head. To protect the access a simple spin
69 * lock per listener "icsk->icsk_accept_queue.fastopenq->lock" is created.
70 * only in the rare case when both the listener and the child locks are held,
71 * e.g., in inet_csk_listen_stop() do we not need to acquire the lock.
72 * The lock also protects other fields such as fastopenq->qlen, which is
73 * decremented by this function when fastopen_rsk is no longer needed.
74 *
75 * Note that another solution was to simply use the existing socket lock
76 * from the listener. But first socket lock is difficult to use. It is not
77 * a simple spin lock - one must consider sock_owned_by_user() and arrange
78 * to use sk_add_backlog() stuff. But what really makes it infeasible is the
79 * locking hierarchy violation. E.g., inet_csk_listen_stop() may try to
80 * acquire a child's lock while holding listener's socket lock.
81 *
82 * This function also sets "treq->tfo_listener" to false.
83 * treq->tfo_listener is used by the listener so it is protected by the
84 * fastopenq->lock in this function.
85 */
86void reqsk_fastopen_remove(struct sock *sk, struct request_sock *req,
87 bool reset)
88{
89 struct sock *lsk = req->rsk_listener;
90 struct fastopen_queue *fastopenq;
91
92 fastopenq = &inet_csk(lsk)->icsk_accept_queue.fastopenq;
93
94 RCU_INIT_POINTER(tcp_sk(sk)->fastopen_rsk, NULL);
95 spin_lock_bh(lock: &fastopenq->lock);
96 fastopenq->qlen--;
97 tcp_rsk(req)->tfo_listener = false;
98 if (req->sk) /* the child socket hasn't been accepted yet */
99 goto out;
100
101 if (!reset || lsk->sk_state != TCP_LISTEN) {
102 /* If the listener has been closed don't bother with the
103 * special RST handling below.
104 */
105 spin_unlock_bh(lock: &fastopenq->lock);
106 reqsk_put(req);
107 return;
108 }
109 /* Wait for 60secs before removing a req that has triggered RST.
110 * This is a simple defense against TFO spoofing attack - by
111 * counting the req against fastopen.max_qlen, and disabling
112 * TFO when the qlen exceeds max_qlen.
113 *
114 * For more details see CoNext'11 "TCP Fast Open" paper.
115 */
116 req->rsk_timer.expires = jiffies + 60*HZ;
117 if (fastopenq->rskq_rst_head == NULL)
118 fastopenq->rskq_rst_head = req;
119 else
120 fastopenq->rskq_rst_tail->dl_next = req;
121
122 req->dl_next = NULL;
123 fastopenq->rskq_rst_tail = req;
124 fastopenq->qlen++;
125out:
126 spin_unlock_bh(lock: &fastopenq->lock);
127}
128