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 
reqsk_queue_alloc(struct request_sock_queue * queue)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  */
reqsk_fastopen_remove(struct sock * sk,struct request_sock * req,bool reset)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 }
133