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Commit b3d05147 authored by Eric Dumazet's avatar Eric Dumazet Committed by David S. Miller
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tcp: do not mess with listener sk_wmem_alloc 



When removing sk_refcnt manipulation on synflood, I missed that
using skb_set_owner_w() was racy, if sk->sk_wmem_alloc had already
transitioned to 0.

We should hold sk_refcnt instead, but this is a big deal under attack.
(Doing so increase performance from 3.2 Mpps to 3.8 Mpps only)

In this patch, I chose to not attach a socket to syncookies skb.

Performance is now 5 Mpps instead of 3.2 Mpps.

Following patch will remove last known false sharing in
tcp_rcv_state_process()

Fixes: 3b24d854 ("tcp/dccp: do not touch listener sk_refcnt under synflood")
Signed-off-by: default avatarEric Dumazet <edumazet@google.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent ac18dd9e

include/net/tcp.h

+7 −2
Original line number Diff line number Diff line
@@ -452,10 +452,15 @@ struct sock *tcp_v4_syn_recv_sock(const struct sock *sk, struct sk_buff *skb, 
int tcp_v4_do_rcv(struct sock *sk, struct sk_buff *skb);

int tcp_v4_connect(struct sock *sk, struct sockaddr *uaddr, int addr_len);

int tcp_connect(struct sock *sk);

enum tcp_synack_type {

	TCP_SYNACK_NORMAL,

	TCP_SYNACK_FASTOPEN,

	TCP_SYNACK_COOKIE,

};

struct sk_buff *tcp_make_synack(const struct sock *sk, struct dst_entry *dst,

				struct request_sock *req,

				struct tcp_fastopen_cookie *foc,

				bool attach_req);

				enum tcp_synack_type synack_type);

int tcp_disconnect(struct sock *sk, int flags);



void tcp_finish_connect(struct sock *sk, struct sk_buff *skb);
@@ -1728,7 +1733,7 @@ struct tcp_request_sock_ops { 
	int (*send_synack)(const struct sock *sk, struct dst_entry *dst,

			   struct flowi *fl, struct request_sock *req,

			   struct tcp_fastopen_cookie *foc,

			   bool attach_req);

			   enum tcp_synack_type synack_type);

};



#ifdef CONFIG_SYN_COOKIES

net/ipv4/tcp_input.c

+4 −3
Original line number Diff line number Diff line
@@ -6327,7 +6327,7 @@ int tcp_conn_request(struct request_sock_ops *rsk_ops, 
	}

	if (fastopen_sk) {

		af_ops->send_synack(fastopen_sk, dst, &fl, req,

				    &foc, false);

				    &foc, TCP_SYNACK_FASTOPEN);

		/* Add the child socket directly into the accept queue */

		inet_csk_reqsk_queue_add(sk, req, fastopen_sk);

		sk->sk_data_ready(sk);
@@ -6337,8 +6337,9 @@ int tcp_conn_request(struct request_sock_ops *rsk_ops, 
		tcp_rsk(req)->tfo_listener = false;

		if (!want_cookie)

			inet_csk_reqsk_queue_hash_add(sk, req, TCP_TIMEOUT_INIT);

		af_ops->send_synack(sk, dst, &fl, req,

				    &foc, !want_cookie);

		af_ops->send_synack(sk, dst, &fl, req, &foc,

				    !want_cookie ? TCP_SYNACK_NORMAL :

						   TCP_SYNACK_COOKIE);

		if (want_cookie) {

			reqsk_free(req);

			return 0;

net/ipv4/tcp_ipv4.c

+2 −2
Original line number Diff line number Diff line
@@ -830,7 +830,7 @@ static int tcp_v4_send_synack(const struct sock *sk, struct dst_entry *dst, 
			      struct flowi *fl,

			      struct request_sock *req,

			      struct tcp_fastopen_cookie *foc,

				  bool attach_req)

			      enum tcp_synack_type synack_type)

{

	const struct inet_request_sock *ireq = inet_rsk(req);

	struct flowi4 fl4;
@@ -841,7 +841,7 @@ static int tcp_v4_send_synack(const struct sock *sk, struct dst_entry *dst, 
	if (!dst && (dst = inet_csk_route_req(sk, &fl4, req)) == NULL)

		return -1;



	skb = tcp_make_synack(sk, dst, req, foc, attach_req);

	skb = tcp_make_synack(sk, dst, req, foc, synack_type);



	if (skb) {

		__tcp_v4_send_check(skb, ireq->ir_loc_addr, ireq->ir_rmt_addr);

net/ipv4/tcp_output.c

+12 −4
Original line number Diff line number Diff line
@@ -2944,7 +2944,7 @@ int tcp_send_synack(struct sock *sk) 
struct sk_buff *tcp_make_synack(const struct sock *sk, struct dst_entry *dst,

				struct request_sock *req,

				struct tcp_fastopen_cookie *foc,

				bool attach_req)

				enum tcp_synack_type synack_type)

{

	struct inet_request_sock *ireq = inet_rsk(req);

	const struct tcp_sock *tp = tcp_sk(sk);
@@ -2964,14 +2964,22 @@ struct sk_buff *tcp_make_synack(const struct sock *sk, struct dst_entry *dst, 
	/* Reserve space for headers. */

	skb_reserve(skb, MAX_TCP_HEADER);



	if (attach_req) {

	switch (synack_type) {

	case TCP_SYNACK_NORMAL:

		skb_set_owner_w(skb, req_to_sk(req));

	} else {

		break;

	case TCP_SYNACK_COOKIE:

		/* Under synflood, we do not attach skb to a socket,

		 * to avoid false sharing.

		 */

		break;

	case TCP_SYNACK_FASTOPEN:

		/* sk is a const pointer, because we want to express multiple

		 * cpu might call us concurrently.

		 * sk->sk_wmem_alloc in an atomic, we can promote to rw.

		 */

		skb_set_owner_w(skb, (struct sock *)sk);

		break;

	}

	skb_dst_set(skb, dst);
@@ -3516,7 +3524,7 @@ int tcp_rtx_synack(const struct sock *sk, struct request_sock *req) 
	int res;



	tcp_rsk(req)->txhash = net_tx_rndhash();

	res = af_ops->send_synack(sk, NULL, &fl, req, NULL, true);

	res = af_ops->send_synack(sk, NULL, &fl, req, NULL, TCP_SYNACK_NORMAL);

	if (!res) {

		TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_RETRANSSEGS);

		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPSYNRETRANS);

net/ipv6/tcp_ipv6.c

+2 −2
Original line number Diff line number Diff line
@@ -439,7 +439,7 @@ static int tcp_v6_send_synack(const struct sock *sk, struct dst_entry *dst, 
			      struct flowi *fl,

			      struct request_sock *req,

			      struct tcp_fastopen_cookie *foc,

			      bool attach_req)

			      enum tcp_synack_type synack_type)

{

	struct inet_request_sock *ireq = inet_rsk(req);

	struct ipv6_pinfo *np = inet6_sk(sk);
@@ -452,7 +452,7 @@ static int tcp_v6_send_synack(const struct sock *sk, struct dst_entry *dst, 
					       IPPROTO_TCP)) == NULL)

		goto done;



	skb = tcp_make_synack(sk, dst, req, foc, attach_req);

	skb = tcp_make_synack(sk, dst, req, foc, synack_type);



	if (skb) {

		__tcp_v6_send_check(skb, &ireq->ir_v6_loc_addr,