Re: [PATCH net v3] Fix race for duplicate reqsk on identical SYN

Kuniyuki Iwashima
2024-06-17T17:59:27Z

From: luoxuanqiang <luoxuanqiang@kylinos.cn>
Date: Mon, 17 Jun 2024 15:56:40 +0800
> When bonding is configured in BOND_MODE_BROADCAST mode, if two identical
> SYN packets are received at the same time and processed on different CPUs,
> it can potentially create the same sk (sock) but two different reqsk
> (request_sock) in tcp_conn_request().
>
> These two different reqsk will respond with two SYNACK packets, and since
> the generation of the seq (ISN) incorporates a timestamp, the final two
> SYNACK packets will have different seq values.
>
> The consequence is that when the Client receives and replies with an ACK
> to the earlier SYNACK packet, we will reset(RST) it.
>
> ========================================================================
>
> This behavior is consistently reproducible in my local setup,
> which comprises:
>
> | NETA1 ------ NETB1 |
> PC_A --- bond --- | | --- bond --- PC_B
> | NETA2 ------ NETB2 |
>
> - PC_A is the Server and has two network cards, NETA1 and NETA2. I have
> bonded these two cards using BOND_MODE_BROADCAST mode and configured
> them to be handled by different CPU.
>
> - PC_B is the Client, also equipped with two network cards, NETB1 and
> NETB2, which are also bonded and configured in BOND_MODE_BROADCAST mode.
>
> If the client attempts a TCP connection to the server, it might encounter
> a failure. Capturing packets from the server side reveals:
>
> 10.10.10.10.45182 > localhost: Flags [S], seq 320236027,
> 10.10.10.10.45182 > localhost: Flags [S], seq 320236027,
> localhost > 10.10.10.10.45182: Flags [S.], seq 2967855116,
> localhost > 10.10.10.10.45182: Flags [S.], seq 2967855123, <==
> 10.10.10.10.45182 > localhost: Flags [.], ack 4294967290,
> 10.10.10.10.45182 > localhost: Flags [.], ack 4294967290,
> localhost > 10.10.10.10.45182: Flags [R], seq 2967855117, <==
> localhost > 10.10.10.10.45182: Flags [R], seq 2967855117,
>
> Two SYNACKs with different seq numbers are sent by localhost,
> resulting in an anomaly.
>
> ========================================================================
>
> The attempted solution is as follows:
> In the tcp_conn_request(), while inserting reqsk into the ehash table,
> it also checks if an entry already exists. If found, it avoids
> reinsertion and releases it.
>
> Simultaneously, In the reqsk_queue_hash_req(), the start of the
> req->rsk_timer is adjusted to be after successful insertion.
>
> Signed-off-by: luoxuanqiang <luoxuanqiang@kylinos.cn>
> ---
> include/net/inet_connection_sock.h | 4 ++--
> net/dccp/ipv4.c | 2 +-
> net/dccp/ipv6.c | 2 +-
> net/ipv4/inet_connection_sock.c | 19 +++++++++++++------
> net/ipv4/tcp_input.c | 9 ++++++++-
> 5 files changed, 25 insertions(+), 11 deletions(-)
>
> diff --git a/include/net/inet_connection_sock.h b/include/net/inet_connection_sock.h
> index 7d6b1254c92d..8ebab6220dbc 100644
> --- a/include/net/inet_connection_sock.h
> +++ b/include/net/inet_connection_sock.h
> @@ -263,8 +263,8 @@ struct dst_entry *inet_csk_route_child_sock(const struct sock *sk,
> struct sock *inet_csk_reqsk_queue_add(struct sock *sk,
> struct request_sock *req,
> struct sock *child);
> -void inet_csk_reqsk_queue_hash_add(struct sock *sk, struct request_sock *req,
> - unsigned long timeout);
> +bool inet_csk_reqsk_queue_hash_add(struct sock *sk, struct request_sock *req,
> + unsigned long timeout, bool *found_dup_sk);
> struct sock *inet_csk_complete_hashdance(struct sock *sk, struct sock *child,
> struct request_sock *req,
> bool own_req);
> diff --git a/net/dccp/ipv4.c b/net/dccp/ipv4.c
> index ff41bd6f99c3..13aafdeb9205 100644
> --- a/net/dccp/ipv4.c
> +++ b/net/dccp/ipv4.c
> @@ -657,7 +657,7 @@ int dccp_v4_conn_request(struct sock *sk, struct sk_buff *skb)
> if (dccp_v4_send_response(sk, req))
> goto drop_and_free;
>
> - inet_csk_reqsk_queue_hash_add(sk, req, DCCP_TIMEOUT_INIT);
> + inet_csk_reqsk_queue_hash_add(sk, req, DCCP_TIMEOUT_INIT, NULL);
> reqsk_put(req);
> return 0;
>
> diff --git a/net/dccp/ipv6.c b/net/dccp/ipv6.c
> index 85f4b8fdbe5e..493cdb12ce2b 100644
> --- a/net/dccp/ipv6.c
> +++ b/net/dccp/ipv6.c
> @@ -400,7 +400,7 @@ static int dccp_v6_conn_request(struct sock *sk, struct sk_buff *skb)
> if (dccp_v6_send_response(sk, req))
> goto drop_and_free;
>
> - inet_csk_reqsk_queue_hash_add(sk, req, DCCP_TIMEOUT_INIT);
> + inet_csk_reqsk_queue_hash_add(sk, req, DCCP_TIMEOUT_INIT, NULL);
> reqsk_put(req);
> return 0;
>
> diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
> index d81f74ce0f02..2fa9b33ae26a 100644
> --- a/net/ipv4/inet_connection_sock.c
> +++ b/net/ipv4/inet_connection_sock.c
> @@ -1122,25 +1122,32 @@ static void reqsk_timer_handler(struct timer_list *t)
> inet_csk_reqsk_queue_drop_and_put(oreq->rsk_listener, oreq);
> }
>
> -static void reqsk_queue_hash_req(struct request_sock *req,
> - unsigned long timeout)
> +static bool reqsk_queue_hash_req(struct request_sock *req,
> + unsigned long timeout, bool *found_dup_sk)
> {

Given any changes here in reqsk_queue_hash_req() conflicts with 4.19
(oldest stable) and DCCP does not check found_dup_sk, you can define
found_dup_sk here, then you need not touch DCCP at all.

> + if (!inet_ehash_insert(req_to_sk(req), NULL, found_dup_sk))
> + return false;
> +
> + /* The timer needs to be setup after a successful insertion. */
> timer_setup(&req->rsk_timer, reqsk_timer_handler, TIMER_PINNED);
> mod_timer(&req->rsk_timer, jiffies + timeout);
>
> - inet_ehash_insert(req_to_sk(req), NULL, NULL);
> /* before letting lookups find us, make sure all req fields
> * are committed to memory and refcnt initialized.
> */
> smp_wmb();
> refcount_set(&req->rsk_refcnt, 2 + 1);
> + return true;
> }
>
> -void inet_csk_reqsk_queue_hash_add(struct sock *sk, struct request_sock *req,
> - unsigned long timeout)
> +bool inet_csk_reqsk_queue_hash_add(struct sock *sk, struct request_sock *req,
> + unsigned long timeout, bool *found_dup_sk)
> {
> - reqsk_queue_hash_req(req, timeout);
> + if (!reqsk_queue_hash_req(req, timeout, found_dup_sk))
> + return false;
> +
> inet_csk_reqsk_queue_added(sk);
> + return true;
> }
> EXPORT_SYMBOL_GPL(inet_csk_reqsk_queue_hash_add);
>
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 9c04a9c8be9d..e006c374f781 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -7255,8 +7255,15 @@ int tcp_conn_request(struct request_sock_ops *rsk_ops,
> } else {
> tcp_rsk(req)->tfo_listener = false;
> if (!want_cookie) {
> + bool found_dup_sk = false;
> +
> req->timeout = tcp_timeout_init((struct sock *)req);
> - inet_csk_reqsk_queue_hash_add(sk, req, req->timeout);
> + if (unlikely(!inet_csk_reqsk_queue_hash_add(sk, req, req->timeout,
> + &found_dup_sk))) {
> + reqsk_free(req);
> + return 0;
> + }
> +
> }
> af_ops->send_synack(sk, dst, &fl, req, &foc,
> !want_cookie ? TCP_SYNACK_NORMAL :
> --
> 2.25.1