Re: [PATCH v7 bpf-next 07/11] tcp: Migrate TCP_NEW_SYN_RECV requests at receiving the final ACK.

[Eric Dumazet]

On 5/21/21 8:21 PM, Kuniyuki Iwashima wrote:
> This patch also changes the code to call reuseport_migrate_sock() and
> inet_reqsk_clone(), but unlike the other cases, we do not call
> inet_reqsk_clone() right after reuseport_migrate_sock().
> 
> Currently, in the receive path for TCP_NEW_SYN_RECV sockets, its listener
> has three kinds of refcnt:
> 
>   (A) for listener itself
>   (B) carried by reuqest_sock
>   (C) sock_hold() in tcp_v[46]_rcv()
> 
> While processing the req, (A) may disappear by close(listener). Also, (B)
> can disappear by accept(listener) once we put the req into the accept
> queue. So, we have to hold another refcnt (C) for the listener to prevent
> use-after-free.
> 
> For socket migration, we call reuseport_migrate_sock() to select a listener
> with (A) and to increment the new listener's refcnt in tcp_v[46]_rcv().
> This refcnt corresponds to (C) and is cleaned up later in tcp_v[46]_rcv().
> Thus we have to take another refcnt (B) for the newly cloned request_sock.
> 
> In inet_csk_complete_hashdance(), we hold the count (B), clone the req, and
> try to put the new req into the accept queue. By migrating req after
> winning the "own_req" race, we can avoid such a worst situation:
> 
>   CPU 1 looks up req1
>   CPU 2 looks up req1, unhashes it, then CPU 1 loses the race
>   CPU 3 looks up req2, unhashes it, then CPU 2 loses the race
>   ...
> 
> Signed-off-by: Kuniyuki Iwashima <kuniyu@xxxxxxxxxxxx>
> Acked-by: Martin KaFai Lau <kafai@xxxxxx>
> ---
>  net/ipv4/inet_connection_sock.c | 34 ++++++++++++++++++++++++++++++---
>  net/ipv4/tcp_ipv4.c             | 20 +++++++++++++------
>  net/ipv4/tcp_minisocks.c        |  4 ++--
>  net/ipv6/tcp_ipv6.c             | 14 +++++++++++---
>  4 files changed, 58 insertions(+), 14 deletions(-)
> 
> diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
> index c1f068464363..b795198f919a 100644
> --- a/net/ipv4/inet_connection_sock.c
> +++ b/net/ipv4/inet_connection_sock.c
> @@ -1113,12 +1113,40 @@ struct sock *inet_csk_complete_hashdance(struct sock *sk, struct sock *child,
>  					 struct request_sock *req, bool own_req)
>  {
>  	if (own_req) {
> -		inet_csk_reqsk_queue_drop(sk, req);
> -		reqsk_queue_removed(&inet_csk(sk)->icsk_accept_queue, req);
> -		if (inet_csk_reqsk_queue_add(sk, req, child))
> +		inet_csk_reqsk_queue_drop(req->rsk_listener, req);
> +		reqsk_queue_removed(&inet_csk(req->rsk_listener)->icsk_accept_queue, req);
> +
> +		if (sk != req->rsk_listener) {
> +			/* another listening sk has been selected,
> +			 * migrate the req to it.
> +			 */
> +			struct request_sock *nreq;
> +
> +			/* hold a refcnt for the nreq->rsk_listener
> +			 * which is assigned in inet_reqsk_clone()
> +			 */
> +			sock_hold(sk);
> +			nreq = inet_reqsk_clone(req, sk);
> +			if (!nreq) {
> +				inet_child_forget(sk, req, child);

Don't you need a sock_put(sk) here ?

\
> +				goto child_put;
> +			}
> +
> +			refcount_set(&nreq->rsk_refcnt, 1);
> +			if (inet_csk_reqsk_queue_add(sk, nreq, child)) {
> +				reqsk_migrate_reset(req);
> +				reqsk_put(req);
> +				return child;
> +			}
> +
> +			reqsk_migrate_reset(nreq);
> +			__reqsk_free(nreq);
> +		} else if (inet_csk_reqsk_queue_add(sk, req, child)) {
>  			return child;
> +		}
>