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 | 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. A corner |
81 | * case might also exist in tcp_v4_hnd_req() that will trigger this locking |
82 | * order. |
83 | * |
84 | * This function also sets "treq->tfo_listener" to false. |
85 | * treq->tfo_listener is used by the listener so it is protected by the |
86 | * fastopenq->lock in this function. |
87 | */ |
88 | void reqsk_fastopen_remove(struct sock *sk, struct request_sock *req, |
89 | bool reset) |
90 | { |
91 | struct sock *lsk = req->rsk_listener; |
92 | struct fastopen_queue *fastopenq; |
93 | |
94 | fastopenq = &inet_csk(sk: lsk)->icsk_accept_queue.fastopenq; |
95 | |
96 | RCU_INIT_POINTER(tcp_sk(sk)->fastopen_rsk, NULL); |
97 | spin_lock_bh(lock: &fastopenq->lock); |
98 | fastopenq->qlen--; |
99 | tcp_rsk(req)->tfo_listener = false; |
100 | if (req->sk) /* the child socket hasn't been accepted yet */ |
101 | goto out; |
102 | |
103 | if (!reset || lsk->sk_state != TCP_LISTEN) { |
104 | /* If the listener has been closed don't bother with the |
105 | * special RST handling below. |
106 | */ |
107 | spin_unlock_bh(lock: &fastopenq->lock); |
108 | reqsk_put(req); |
109 | return; |
110 | } |
111 | /* Wait for 60secs before removing a req that has triggered RST. |
112 | * This is a simple defense against TFO spoofing attack - by |
113 | * counting the req against fastopen.max_qlen, and disabling |
114 | * TFO when the qlen exceeds max_qlen. |
115 | * |
116 | * For more details see CoNext'11 "TCP Fast Open" paper. |
117 | */ |
118 | req->rsk_timer.expires = jiffies + 60*HZ; |
119 | if (fastopenq->rskq_rst_head == NULL) |
120 | fastopenq->rskq_rst_head = req; |
121 | else |
122 | fastopenq->rskq_rst_tail->dl_next = req; |
123 | |
124 | req->dl_next = NULL; |
125 | fastopenq->rskq_rst_tail = req; |
126 | fastopenq->qlen++; |
127 | out: |
128 | spin_unlock_bh(lock: &fastopenq->lock); |
129 | } |
130 | |