[PATCH 3/3] mptcp: fix syncookie process if mptcp can not_accept new subflow

[Jianguo Wu <wujianguo106@163.com>]

From: Jianguo Wu <wujianguo@chinatelecom.cn>

Lots of "TCP: tcp_fin: Impossible, sk->sk_state=7" in client side when doing stress testing.
There are at least two cases may trigger this warning:
1. mptcp is in syncookie, and server recv MP_JOIN SYN request, in subflow_check_req(),
   the mptcp_can_accept_new_subflow() return false, so subflow_init_req_cookie_join_save()
   isn't called, i.e. not store the data present in the MP_JOIN syn request and the random
   nonce in hash table - join_entries[], but still send synack. When recv 3rd-ack,
   mptcp_token_join_cookie_init_state() will return false, and 3rd-ack is dropped, then if mptcp
   conn is closed by client, client will send a DATA_FIN and a MPTCP FIN, the DATA_FIN doesn't have
   MP_CAPABLE or MP_JOIN, so mptcp_subflow_init_cookie_req() will return 0, and pass the cookie check,
   MP_JOIN request is fallback to normal TCP. Server will send a TCP FIN if closed, in client side,
   when process TCP FIN, it will do reset, the code path is:
     tcp_data_queue()->mptcp_incoming_options()->check_fully_established()->mptcp_subflow_reset().
   mptcp_subflow_reset() will set sock state to TCP_CLOSE, so tcp_fin will hit TCP_CLOSE, and print the warning.
2. mptcp is in syncookie, and server recv 3rd-ack, in mptcp_subflow_init_cookie_req(), mptcp_can_accept_new_subflow()
   return false, and subflow_req->mp_join is not set to 1, so in subflow_syn_recv_sock() will not reset the MP_JOIN
   subflow, but fallback to normal TCP, and then the same thing happens when server will send a TCP FIN if closed.

For case1, subflow_check_req() return -EPERM, then tcp_conn_request() will drop MP_JOIN SYN.
For case2, let subflow_syn_recv_sock() do mptcp_can_accept_new_subflow() check, and do fatal fallback, send reset.
And do sanity check in tcp_data_queue().

Fixes: 9466a1ccebbe("mptcp: enable JOIN requests even if cookies are in use")
Signed-off-by: Jianguo Wu <wujianguo@chinatelecom.cn>
---
 net/ipv4/tcp_input.c | 7 ++++++-
 net/mptcp/subflow.c  | 6 +++---
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 7d5e59f..537f24a 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -4941,8 +4941,13 @@ static void tcp_data_queue(struct sock *sk, struct sk_buff *skb)
 	bool fragstolen;
 	int eaten;

-	if (sk_is_mptcp(sk))
+	if (sk_is_mptcp(sk)) {
 		mptcp_incoming_options(sk, skb);
+		if (sk->sk_state == TCP_CLOSE) {
+			__kfree_skb(skb);
+			return;
+		}
+	}

 	if (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq) {
 		__kfree_skb(skb);
diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index 75ed530..6d98e19 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -224,6 +224,8 @@ static int subflow_check_req(struct request_sock *req,
 		if (unlikely(req->syncookie)) {
 			if (mptcp_can_accept_new_subflow(subflow_req->msk))
 				subflow_init_req_cookie_join_save(subflow_req, skb);
+			else
+				return -EPERM;
 		}

 		pr_debug("token=%u, remote_nonce=%u msk=%p", subflow_req->token,
@@ -263,9 +265,7 @@ int mptcp_subflow_init_cookie_req(struct request_sock *req,
 		if (!mptcp_token_join_cookie_init_state(subflow_req, skb))
 			return -EINVAL;

-		if (mptcp_can_accept_new_subflow(subflow_req->msk))
-			subflow_req->mp_join = 1;
-
+		subflow_req->mp_join = 1;
 		subflow_req->ssn_offset = TCP_SKB_CB(skb)->seq - 1;
 	}

-- 
1.8.3.1