tcp: fix fastopen races vs lockless listener

There are multiple races that need fixes :

1) skb_get() + queue skb + kfree_skb() is racy

An accept() can be done on another cpu, data consumed immediately.
tcp_recvmsg() uses __kfree_skb() as it is assumed all skb found in
socket receive queue are private.

Then the kfree_skb() in tcp_rcv_state_process() uses an already freed skb

2) tcp_reqsk_record_syn() needs to be done before tcp_try_fastopen()
for the same reasons.

3) We want to send the SYNACK before queueing child into accept queue,
otherwise we might reintroduce the ooo issue fixed in
commit 7c85af881044 ("tcp: avoid reorders for TFO passive connections")

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/ipv4/tcp_fastopen.c b/net/ipv4/tcp_fastopen.c
index 410ac481fda03cb7dc4250e736fc4523e16d53cb..93396bf7b475f3972d247d282faa406d962696dd 100644 (file)
--- a/net/ipv4/tcp_fastopen.c
+++ b/net/ipv4/tcp_fastopen.c
@@ -168,8 +168,6 @@ static struct sock *tcp_fastopen_create_child(struct sock *sk,
                                  TCP_TIMEOUT_INIT, TCP_RTO_MAX);
 
        atomic_set(&req->rsk_refcnt, 2);
-       /* Add the child socket directly into the accept queue */
-       inet_csk_reqsk_queue_add(sk, req, child);
 
        /* Now finish processing the fastopen child socket. */
        inet_csk(child)->icsk_af_ops->rebuild_header(child);
@@ -178,12 +176,10 @@ static struct sock *tcp_fastopen_create_child(struct sock *sk,
        tcp_init_metrics(child);
        tcp_init_buffer_space(child);
 
-       /* Queue the data carried in the SYN packet. We need to first
-        * bump skb's refcnt because the caller will attempt to free it.
-        * Note that IPv6 might also have used skb_get() trick
-        * in tcp_v6_conn_request() to keep this SYN around (treq->pktopts)
-        * So we need to eventually get a clone of the packet,
-        * before inserting it in sk_receive_queue.
+       /* Queue the data carried in the SYN packet.
+        * We used to play tricky games with skb_get().
+        * With lockless listener, it is a dead end.
+        * Do not think about it.
         *
         * XXX (TFO) - we honor a zero-payload TFO request for now,
         * (any reason not to?) but no need to queue the skb since
@@ -191,12 +187,7 @@ static struct sock *tcp_fastopen_create_child(struct sock *sk,
         */
        end_seq = TCP_SKB_CB(skb)->end_seq;
        if (end_seq != TCP_SKB_CB(skb)->seq + 1) {
-               struct sk_buff *skb2;
-
-               if (unlikely(skb_shared(skb)))
-                       skb2 = skb_clone(skb, GFP_ATOMIC);
-               else
-                       skb2 = skb_get(skb);
+               struct sk_buff *skb2 = skb_clone(skb, GFP_ATOMIC);
 
                if (likely(skb2)) {
                        skb_dst_drop(skb2);
@@ -214,12 +205,9 @@ static struct sock *tcp_fastopen_create_child(struct sock *sk,
                }
        }
        tcp_rsk(req)->rcv_nxt = tp->rcv_nxt = end_seq;
-       sk->sk_data_ready(sk);
-       bh_unlock_sock(child);
-       /* Note: sock_put(child) will be done by tcp_conn_request()
-        * after SYNACK packet is sent.
+       /* tcp_conn_request() is sending the SYNACK,
+        * and queues the child into listener accept queue.
         */
-       WARN_ON(!req->sk);
        return child;
 }
 

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 27108757c310db853525b4189ad06af556741ec7..a95c8eb04ff77883b2b7d27b20df5279ff9f3b77 100644 (file)
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -6229,12 +6229,16 @@ int tcp_conn_request(struct request_sock_ops *rsk_ops,
        tcp_rsk(req)->txhash = net_tx_rndhash();
        tcp_openreq_init_rwin(req, sk, dst);
        if (!want_cookie) {
-               fastopen_sk = tcp_try_fastopen(sk, skb, req, &foc, dst);
                tcp_reqsk_record_syn(sk, req, skb);
+               fastopen_sk = tcp_try_fastopen(sk, skb, req, &foc, dst);
        }
        if (fastopen_sk) {
                af_ops->send_synack(fastopen_sk, dst, &fl, req,
                                    skb_get_queue_mapping(skb), &foc, false);
+               /* Add the child socket directly into the accept queue */
+               inet_csk_reqsk_queue_add(sk, req, fastopen_sk);
+               sk->sk_data_ready(sk);
+               bh_unlock_sock(fastopen_sk);
                sock_put(fastopen_sk);
        } else {
                tcp_rsk(req)->tfo_listener = false;