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       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                 :          0 : void reqsk_queue_alloc(struct request_sock_queue *queue)
      35                 :            : {
      36                 :          0 :         spin_lock_init(&queue->rskq_lock);
      37                 :            : 
      38                 :          0 :         spin_lock_init(&queue->fastopenq.lock);
      39                 :          0 :         queue->fastopenq.rskq_rst_head = NULL;
      40                 :          0 :         queue->fastopenq.rskq_rst_tail = NULL;
      41                 :          0 :         queue->fastopenq.qlen = 0;
      42                 :            : 
      43                 :          0 :         queue->rskq_accept_head = NULL;
      44                 :          0 : }
      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                 :          0 : void reqsk_fastopen_remove(struct sock *sk, struct request_sock *req,
      92                 :            :                            bool reset)
      93                 :            : {
      94                 :          0 :         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                 :          0 :         fastopenq->qlen--;
     102                 :          0 :         tcp_rsk(req)->tfo_listener = false;
     103         [ #  # ]:          0 :         if (req->sk) /* the child socket hasn't been accepted yet */
     104                 :            :                 goto out;
     105                 :            : 
     106   [ #  #  #  # ]:          0 :         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                 :          0 :                 reqsk_put(req);
     112                 :          0 :                 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                 :          0 :         req->rsk_timer.expires = jiffies + 60*HZ;
     122         [ #  # ]:          0 :         if (fastopenq->rskq_rst_head == NULL)
     123                 :          0 :                 fastopenq->rskq_rst_head = req;
     124                 :            :         else
     125                 :          0 :                 fastopenq->rskq_rst_tail->dl_next = req;
     126                 :            : 
     127                 :          0 :         req->dl_next = NULL;
     128                 :          0 :         fastopenq->rskq_rst_tail = req;
     129                 :          0 :         fastopenq->qlen++;
     130                 :            : out:
     131                 :            :         spin_unlock_bh(&fastopenq->lock);
     132                 :            : }