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Subject[PATCH 4.4 028/131] tcp/dccp: drop SYN packets if accept queue is full
Date
4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Eric Dumazet <edumazet@google.com>

commit 5ea8ea2cb7f1d0db15762c9b0bb9e7330425a071 upstream.

Per listen(fd, backlog) rules, there is really no point accepting a SYN,
sending a SYNACK, and dropping the following ACK packet if accept queue
is full, because application is not draining accept queue fast enough.

This behavior is fooling TCP clients that believe they established a
flow, while there is nothing at server side. They might then send about
10 MSS (if using IW10) that will be dropped anyway while server is under
stress.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Acked-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>


---
include/net/inet_connection_sock.h | 5 -----
net/dccp/ipv4.c | 8 +-------
net/dccp/ipv6.c | 2 +-
net/ipv4/tcp_input.c | 8 +-------
4 files changed, 3 insertions(+), 20 deletions(-)

--- a/include/net/inet_connection_sock.h
+++ b/include/net/inet_connection_sock.h
@@ -289,11 +289,6 @@ static inline int inet_csk_reqsk_queue_l
return reqsk_queue_len(&inet_csk(sk)->icsk_accept_queue);
}

-static inline int inet_csk_reqsk_queue_young(const struct sock *sk)
-{
- return reqsk_queue_len_young(&inet_csk(sk)->icsk_accept_queue);
-}
-
static inline int inet_csk_reqsk_queue_is_full(const struct sock *sk)
{
return inet_csk_reqsk_queue_len(sk) >= sk->sk_max_ack_backlog;
--- a/net/dccp/ipv4.c
+++ b/net/dccp/ipv4.c
@@ -592,13 +592,7 @@ int dccp_v4_conn_request(struct sock *sk
if (inet_csk_reqsk_queue_is_full(sk))
goto 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.
- */
- if (sk_acceptq_is_full(sk) && inet_csk_reqsk_queue_young(sk) > 1)
+ if (sk_acceptq_is_full(sk))
goto drop;

req = inet_reqsk_alloc(&dccp_request_sock_ops, sk, true);
--- a/net/dccp/ipv6.c
+++ b/net/dccp/ipv6.c
@@ -324,7 +324,7 @@ static int dccp_v6_conn_request(struct s
if (inet_csk_reqsk_queue_is_full(sk))
goto drop;

- if (sk_acceptq_is_full(sk) && inet_csk_reqsk_queue_young(sk) > 1)
+ if (sk_acceptq_is_full(sk))
goto drop;

req = inet_reqsk_alloc(&dccp6_request_sock_ops, sk, true);
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -6305,13 +6305,7 @@ int tcp_conn_request(struct request_sock
goto 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.
- */
- if (sk_acceptq_is_full(sk) && inet_csk_reqsk_queue_young(sk) > 1) {
+ if (sk_acceptq_is_full(sk)) {
NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_LISTENOVERFLOWS);
goto drop;
}

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