From da76459dc21b5af2449af2d36eb95226cb186ce2 Mon Sep 17 00:00:00 2001 From: Daniel Baumann Date: Sun, 28 Apr 2024 11:35:11 +0200 Subject: Adding upstream version 2.6.12. Signed-off-by: Daniel Baumann --- doc/linux-syn-cookies.txt | 106 ++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 106 insertions(+) create mode 100644 doc/linux-syn-cookies.txt (limited to 'doc/linux-syn-cookies.txt') diff --git a/doc/linux-syn-cookies.txt b/doc/linux-syn-cookies.txt new file mode 100644 index 0000000..ca13066 --- /dev/null +++ b/doc/linux-syn-cookies.txt @@ -0,0 +1,106 @@ +SYN cookie analysis on 3.10 + +include/net/request_sock.h: + +static inline int reqsk_queue_is_full(const struct request_sock_queue *queue) +{ + return queue->listen_opt->qlen >> queue->listen_opt->max_qlen_log; +} + +include/net/inet_connection_sock.h: + +static inline int inet_csk_reqsk_queue_is_full(const struct sock *sk) +{ + return reqsk_queue_is_full(&inet_csk(sk)->icsk_accept_queue); +} + +max_qlen_log is computed to equal log2(min(min(listen_backlog,somaxconn), sysctl_max_syn_backlog), +and this is done this way following this path : + + socket.c:listen(fd, backlog) : + + backlog = min(backlog, somaxconn) + => af_inet.c:inet_listen(sock, backlog) + + => inet_connection_sock.c:inet_csk_listen_start(sk, backlog) + + sk_max_ack_backlog = backlog + => request_sock.c:reqsk_queue_alloc(sk, backlog (=nr_table_entries)) + + nr_table_entries = min_t(u32, nr_table_entries, sysctl_max_syn_backlog); + nr_table_entries = max_t(u32, nr_table_entries, 8); + nr_table_entries = roundup_pow_of_two(nr_table_entries + 1); + for (lopt->max_qlen_log = 3; + (1 << lopt->max_qlen_log) < nr_table_entries; + lopt->max_qlen_log++); + + +tcp_ipv4.c:tcp_v4_conn_request() + - inet_csk_reqsk_queue_is_full() returns true when the listening socket's + qlen is larger than 1 << max_qlen_log, so basically qlen >= min(backlog,max_backlog) + + - tcp_syn_flood_action() returns true when sysctl_tcp_syncookies is set. It + also emits a warning once per listening socket when activating the feature. + + if (inet_csk_reqsk_queue_is_full(sk) && !isn) { + want_cookie = tcp_syn_flood_action(sk, skb, "TCP"); + if (!want_cookie) + goto drop; + } + + => when the socket's current backlog is >= min(backlog,max_backlog), + either tcp_syn_cookies is set so we set want_cookie to 1, or we drop. + + + /* Accept backlog is full. If we have already queued enough + * of warm entries in syn queue, drop request. It is better than + * clogging syn queue with openreqs with exponentially increasing + * timeout. + */ + +sock.h:sk_acceptq_is_full() = sk_ack_backlog > sk_max_ack_backlog + = sk_ack_backlog > min(somaxconn, listen_backlog) + + if (sk_acceptq_is_full(sk) && inet_csk_reqsk_queue_young(sk) > 1) { + NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_LISTENOVERFLOWS); + goto drop; + } + +====> the following algorithm is applied in the reverse order but with these + priorities : + + 1) IF socket's accept queue >= min(somaxconn, listen_backlog) THEN drop + + 2) IF socket's SYN backlog < min(somaxconn, listen_backlog, tcp_max_syn_backlog) THEN accept + + 3) IF tcp_syn_cookies THEN send_syn_cookie + + 4) otherwise drop + +====> the problem is the accept queue being filled, but it's supposed to be + filled only with validated client requests (step 1). + + + + req = inet_reqsk_alloc(&tcp_request_sock_ops); + if (!req) + goto drop; + + ... + if (!sysctl_tcp_syncookies && + (sysctl_max_syn_backlog - inet_csk_reqsk_queue_len(sk) < + (sysctl_max_syn_backlog >> 2)) && + !tcp_peer_is_proven(req, dst, false)) { + /* Without syncookies last quarter of + * backlog is filled with destinations, + * proven to be alive. + * It means that we continue to communicate + * to destinations, already remembered + * to the moment of synflood. + */ + LIMIT_NETDEBUG(KERN_DEBUG pr_fmt("drop open request from %pI4/%u\n"), + &saddr, ntohs(tcp_hdr(skb)->source)); + goto drop_and_release; + } + + -- cgit v1.2.3