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
34 void 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 */
91 void 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++;
130 out:
131 spin_unlock_bh(&fastopenq->lock);
132 }