107 lines
3.4 KiB
Plaintext
107 lines
3.4 KiB
Plaintext
SYN cookie analysis on 3.10
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include/net/request_sock.h:
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static inline int reqsk_queue_is_full(const struct request_sock_queue *queue)
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{
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return queue->listen_opt->qlen >> queue->listen_opt->max_qlen_log;
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}
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include/net/inet_connection_sock.h:
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static inline int inet_csk_reqsk_queue_is_full(const struct sock *sk)
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{
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return reqsk_queue_is_full(&inet_csk(sk)->icsk_accept_queue);
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}
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max_qlen_log is computed to equal log2(min(min(listen_backlog,somaxconn), sysctl_max_syn_backlog),
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and this is done this way following this path :
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socket.c:listen(fd, backlog) :
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backlog = min(backlog, somaxconn)
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=> af_inet.c:inet_listen(sock, backlog)
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=> inet_connection_sock.c:inet_csk_listen_start(sk, backlog)
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sk_max_ack_backlog = backlog
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=> request_sock.c:reqsk_queue_alloc(sk, backlog (=nr_table_entries))
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nr_table_entries = min_t(u32, nr_table_entries, sysctl_max_syn_backlog);
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nr_table_entries = max_t(u32, nr_table_entries, 8);
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nr_table_entries = roundup_pow_of_two(nr_table_entries + 1);
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for (lopt->max_qlen_log = 3;
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(1 << lopt->max_qlen_log) < nr_table_entries;
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lopt->max_qlen_log++);
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tcp_ipv4.c:tcp_v4_conn_request()
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- inet_csk_reqsk_queue_is_full() returns true when the listening socket's
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qlen is larger than 1 << max_qlen_log, so basically qlen >= min(backlog,max_backlog)
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- tcp_syn_flood_action() returns true when sysctl_tcp_syncookies is set. It
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also emits a warning once per listening socket when activating the feature.
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if (inet_csk_reqsk_queue_is_full(sk) && !isn) {
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want_cookie = tcp_syn_flood_action(sk, skb, "TCP");
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if (!want_cookie)
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goto drop;
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}
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=> when the socket's current backlog is >= min(backlog,max_backlog),
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either tcp_syn_cookies is set so we set want_cookie to 1, or we drop.
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/* Accept backlog is full. If we have already queued enough
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* of warm entries in syn queue, drop request. It is better than
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* clogging syn queue with openreqs with exponentially increasing
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* timeout.
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*/
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sock.h:sk_acceptq_is_full() = sk_ack_backlog > sk_max_ack_backlog
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= sk_ack_backlog > min(somaxconn, listen_backlog)
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if (sk_acceptq_is_full(sk) && inet_csk_reqsk_queue_young(sk) > 1) {
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NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_LISTENOVERFLOWS);
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goto drop;
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}
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====> the following algorithm is applied in the reverse order but with these
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priorities :
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1) IF socket's accept queue >= min(somaxconn, listen_backlog) THEN drop
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2) IF socket's SYN backlog < min(somaxconn, listen_backlog, tcp_max_syn_backlog) THEN accept
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3) IF tcp_syn_cookies THEN send_syn_cookie
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4) otherwise drop
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====> the problem is the accept queue being filled, but it's supposed to be
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filled only with validated client requests (step 1).
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req = inet_reqsk_alloc(&tcp_request_sock_ops);
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if (!req)
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goto drop;
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...
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if (!sysctl_tcp_syncookies &&
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(sysctl_max_syn_backlog - inet_csk_reqsk_queue_len(sk) <
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(sysctl_max_syn_backlog >> 2)) &&
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!tcp_peer_is_proven(req, dst, false)) {
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/* Without syncookies last quarter of
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* backlog is filled with destinations,
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* proven to be alive.
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* It means that we continue to communicate
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* to destinations, already remembered
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* to the moment of synflood.
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*/
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LIMIT_NETDEBUG(KERN_DEBUG pr_fmt("drop open request from %pI4/%u\n"),
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&saddr, ntohs(tcp_hdr(skb)->source));
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goto drop_and_release;
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}
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