Re: [PATCH v1 bpf-next 04/11] tcp: Migrate TFO requests causing RST during TCP_SYN_RECV.

[Eric Dumazet]

On 12/1/20 3:44 PM, Kuniyuki Iwashima wrote:
> A TFO request socket is only freed after BOTH 3WHS has completed (or
> aborted) and the child socket has been accepted (or its listener has been
> closed). Hence, depending on the order, there can be two kinds of request
> sockets in the accept queue.
> 
>   3WHS -> accept : TCP_ESTABLISHED
>   accept -> 3WHS : TCP_SYN_RECV
> 
> Unlike TCP_ESTABLISHED socket, accept() does not free the request socket
> for TCP_SYN_RECV socket. It is freed later at reqsk_fastopen_remove().
> Also, it accesses request_sock.rsk_listener. So, in order to complete TFO
> socket migration, we have to set the current listener to it at accept()
> before reqsk_fastopen_remove().
> 
> Moreover, if TFO request caused RST before 3WHS has completed, it is held
> in the listener's TFO queue to prevent DDoS attack. Thus, we also have to
> migrate the requests in TFO queue.
> 
> Reviewed-by: Benjamin Herrenschmidt <benh@xxxxxxxxxx>
> Signed-off-by: Kuniyuki Iwashima <kuniyu@xxxxxxxxxxxx>
> ---
>  net/ipv4/inet_connection_sock.c | 35 ++++++++++++++++++++++++++++++++-
>  1 file changed, 34 insertions(+), 1 deletion(-)
> 
> diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
> index b27241ea96bd..361efe55b1ad 100644
> --- a/net/ipv4/inet_connection_sock.c
> +++ b/net/ipv4/inet_connection_sock.c
> @@ -500,6 +500,16 @@ struct sock *inet_csk_accept(struct sock *sk, int flags, int *err, bool kern)
>  	    tcp_rsk(req)->tfo_listener) {
>  		spin_lock_bh(&queue->fastopenq.lock);
>  		if (tcp_rsk(req)->tfo_listener) {
> +			if (req->rsk_listener != sk) {
> +				/* TFO request was migrated to another listener so
> +				 * the new listener must be used in reqsk_fastopen_remove()
> +				 * to hold requests which cause RST.
> +				 */
> +				sock_put(req->rsk_listener);
> +				sock_hold(sk);
> +				req->rsk_listener = sk;
> +			}
> +
>  			/* We are still waiting for the final ACK from 3WHS
>  			 * so can't free req now. Instead, we set req->sk to
>  			 * NULL to signify that the child socket is taken
> @@ -954,7 +964,6 @@ static void inet_child_forget(struct sock *sk, struct request_sock *req,
>  
>  	if (sk->sk_protocol == IPPROTO_TCP && tcp_rsk(req)->tfo_listener) {
>  		BUG_ON(rcu_access_pointer(tcp_sk(child)->fastopen_rsk) != req);
> -		BUG_ON(sk != req->rsk_listener);

>  
>  		/* Paranoid, to prevent race condition if
>  		 * an inbound pkt destined for child is
> @@ -995,6 +1004,7 @@ EXPORT_SYMBOL(inet_csk_reqsk_queue_add);
>  void inet_csk_reqsk_queue_migrate(struct sock *sk, struct sock *nsk)
>  {
>  	struct request_sock_queue *old_accept_queue, *new_accept_queue;
> +	struct fastopen_queue *old_fastopenq, *new_fastopenq;
>  
>  	old_accept_queue = &inet_csk(sk)->icsk_accept_queue;
>  	new_accept_queue = &inet_csk(nsk)->icsk_accept_queue;
> @@ -1019,6 +1029,29 @@ void inet_csk_reqsk_queue_migrate(struct sock *sk, struct sock *nsk)
>  
>  	spin_unlock(&new_accept_queue->rskq_lock);
>  	spin_unlock(&old_accept_queue->rskq_lock);
> +
> +	old_fastopenq = &old_accept_queue->fastopenq;
> +	new_fastopenq = &new_accept_queue->fastopenq;
> +
> +	spin_lock_bh(&old_fastopenq->lock);
> +	spin_lock_bh(&new_fastopenq->lock);


Same remark about lockdep being not happy with this (I guess)

> +
> +	new_fastopenq->qlen += old_fastopenq->qlen;
> +	old_fastopenq->qlen = 0;
> +
> +	if (old_fastopenq->rskq_rst_head) {
> +		if (new_fastopenq->rskq_rst_head)
> +			old_fastopenq->rskq_rst_tail->dl_next = new_fastopenq->rskq_rst_head;
> +		else
> +			old_fastopenq->rskq_rst_tail = new_fastopenq->rskq_rst_tail;
> +
> +		new_fastopenq->rskq_rst_head = old_fastopenq->rskq_rst_head;
> +		old_fastopenq->rskq_rst_head = NULL;
> +		old_fastopenq->rskq_rst_tail = NULL;
> +	}
> +
> +	spin_unlock_bh(&new_fastopenq->lock);
> +	spin_unlock_bh(&old_fastopenq->lock);
>  }
>  EXPORT_SYMBOL(inet_csk_reqsk_queue_migrate);
>  
>