tcp: add rcu protection around tp->fastopen_rsk

Both tcp_v4_err() and tcp_v6_err() do the following operations
while they do not own the socket lock :

	fastopen = tp->fastopen_rsk;
 	snd_una = fastopen ? tcp_rsk(fastopen)->snt_isn : tp->snd_una;

The problem is that without appropriate barrier, the compiler
might reload tp->fastopen_rsk and trigger a NULL deref.

request sockets are protected by RCU, we can simply add
the missing annotations and barriers to solve the issue.

Fixes: 168a8f58 ("tcp: TCP Fast Open Server - main code path")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/tcp.h

@@ -393,7 +393,7 @@ struct tcp_sock {
 	/* fastopen_rsk points to request_sock that resulted in this big
 	 * socket. Used to retransmit SYNACKs etc.
 	 */
-	struct request_sock *fastopen_rsk;
+	struct request_sock __rcu *fastopen_rsk;
 	u32	*saved_syn;
 };
 
@@ -447,8 +447,8 @@ static inline struct tcp_timewait_sock *tcp_twsk(const struct sock *sk)
 
 static inline bool tcp_passive_fastopen(const struct sock *sk)
 {
-	return (sk->sk_state == TCP_SYN_RECV &&
-		tcp_sk(sk)->fastopen_rsk != NULL);
+	return sk->sk_state == TCP_SYN_RECV &&
+	       rcu_access_pointer(tcp_sk(sk)->fastopen_rsk) != NULL;
 }
 
 static inline void fastopen_queue_tune(struct sock *sk, int backlog)

net/core/request_sock.c

@@ -96,7 +96,7 @@ void reqsk_fastopen_remove(struct sock *sk, struct request_sock *req,
 
 	fastopenq = &inet_csk(lsk)->icsk_accept_queue.fastopenq;
 
-	tcp_sk(sk)->fastopen_rsk = NULL;
+	RCU_INIT_POINTER(tcp_sk(sk)->fastopen_rsk, NULL);
 	spin_lock_bh(&fastopenq->lock);
 	fastopenq->qlen--;
 	tcp_rsk(req)->tfo_listener = false;

net/ipv4/inet_connection_sock.c

@@ -906,7 +906,7 @@ static void inet_child_forget(struct sock *sk, struct request_sock *req,
 	percpu_counter_inc(sk->sk_prot->orphan_count);
 
 	if (sk->sk_protocol == IPPROTO_TCP && tcp_rsk(req)->tfo_listener) {
-		BUG_ON(tcp_sk(child)->fastopen_rsk != req);
+		BUG_ON(rcu_access_pointer(tcp_sk(child)->fastopen_rsk) != req);
 		BUG_ON(sk != req->rsk_listener);
 
 		/* Paranoid, to prevent race condition if
@@ -915,7 +915,7 @@ static void inet_child_forget(struct sock *sk, struct request_sock *req,
 		 * Also to satisfy an assertion in
 		 * tcp_v4_destroy_sock().
 		 */
-		tcp_sk(child)->fastopen_rsk = NULL;
+		RCU_INIT_POINTER(tcp_sk(child)->fastopen_rsk, NULL);
 	}
 	inet_csk_destroy_sock(child);
 }

net/ipv4/tcp.c

@@ -543,7 +543,7 @@ __poll_t tcp_poll(struct file *file, struct socket *sock, poll_table *wait)
 
 	/* Connected or passive Fast Open socket? */
 	if (state != TCP_SYN_SENT &&
-	    (state != TCP_SYN_RECV || tp->fastopen_rsk)) {
+	    (state != TCP_SYN_RECV || rcu_access_pointer(tp->fastopen_rsk))) {
 		int target = sock_rcvlowat(sk, 0, INT_MAX);
 
 		if (tp->urg_seq == tp->copied_seq &&
@@ -2487,7 +2487,10 @@ void tcp_close(struct sock *sk, long timeout)
 	}
 
 	if (sk->sk_state == TCP_CLOSE) {
-		struct request_sock *req = tcp_sk(sk)->fastopen_rsk;
+		struct request_sock *req;
+
+		req = rcu_dereference_protected(tcp_sk(sk)->fastopen_rsk,
+						lockdep_sock_is_held(sk));
 		/* We could get here with a non-NULL req if the socket is
 		 * aborted (e.g., closed with unread data) before 3WHS
 		 * finishes.
@@ -3831,8 +3834,10 @@ EXPORT_SYMBOL(tcp_md5_hash_key);
 
 void tcp_done(struct sock *sk)
 {
-	struct request_sock *req = tcp_sk(sk)->fastopen_rsk;
+	struct request_sock *req;
 
+	req = rcu_dereference_protected(tcp_sk(sk)->fastopen_rsk,
+					lockdep_sock_is_held(sk));
 	if (sk->sk_state == TCP_SYN_SENT || sk->sk_state == TCP_SYN_RECV)
 		TCP_INC_STATS(sock_net(sk), TCP_MIB_ATTEMPTFAILS);
 

net/ipv4/tcp_fastopen.c

@@ -253,7 +253,7 @@ static struct sock *tcp_fastopen_create_child(struct sock *sk,
 	 */
 	tp = tcp_sk(child);
 
-	tp->fastopen_rsk = req;
+	rcu_assign_pointer(tp->fastopen_rsk, req);
 	tcp_rsk(req)->tfo_listener = true;
 
 	/* RFC1323: The window in SYN & SYN/ACK segments is never

net/ipv4/tcp_input.c

@@ -2666,7 +2666,7 @@ static void tcp_process_loss(struct sock *sk, int flag, int num_dupack,
 	struct tcp_sock *tp = tcp_sk(sk);
 	bool recovered = !before(tp->snd_una, tp->high_seq);
 
-	if ((flag & FLAG_SND_UNA_ADVANCED || tp->fastopen_rsk) &&
+	if ((flag & FLAG_SND_UNA_ADVANCED || rcu_access_pointer(tp->fastopen_rsk)) &&
 	    tcp_try_undo_loss(sk, false))
 		return;
 
@@ -2990,7 +2990,7 @@ void tcp_rearm_rto(struct sock *sk)
 	/* If the retrans timer is currently being used by Fast Open
 	 * for SYN-ACK retrans purpose, stay put.
 	 */
-	if (tp->fastopen_rsk)
+	if (rcu_access_pointer(tp->fastopen_rsk))
 		return;
 
 	if (!tp->packets_out) {
@@ -6087,6 +6087,8 @@ static int tcp_rcv_synsent_state_process(struct sock *sk, struct sk_buff *skb,
 
 static void tcp_rcv_synrecv_state_fastopen(struct sock *sk)
 {
+	struct request_sock *req;
+
 	tcp_try_undo_loss(sk, false);
 
 	/* Reset rtx states to prevent spurious retransmits_timed_out() */
@@ -6096,7 +6098,9 @@ static void tcp_rcv_synrecv_state_fastopen(struct sock *sk)
 	/* Once we leave TCP_SYN_RECV or TCP_FIN_WAIT_1,
 	 * we no longer need req so release it.
 	 */
-	reqsk_fastopen_remove(sk, tcp_sk(sk)->fastopen_rsk, false);
+	req = rcu_dereference_protected(tcp_sk(sk)->fastopen_rsk,
+					lockdep_sock_is_held(sk));
+	reqsk_fastopen_remove(sk, req, false);
 
 	/* Re-arm the timer because data may have been sent out.
 	 * This is similar to the regular data transmission case
@@ -6171,7 +6175,8 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb)
 
 	tcp_mstamp_refresh(tp);
 	tp->rx_opt.saw_tstamp = 0;
-	req = tp->fastopen_rsk;
+	req = rcu_dereference_protected(tp->fastopen_rsk,
+					lockdep_sock_is_held(sk));
 	if (req) {
 		bool req_stolen;
 

net/ipv4/tcp_ipv4.c

@@ -478,7 +478,7 @@ int tcp_v4_err(struct sk_buff *icmp_skb, u32 info)
 	icsk = inet_csk(sk);
 	tp = tcp_sk(sk);
 	/* XXX (TFO) - tp->snd_una should be ISN (tcp_create_openreq_child() */
-	fastopen = tp->fastopen_rsk;
+	fastopen = rcu_dereference(tp->fastopen_rsk);
 	snd_una = fastopen ? tcp_rsk(fastopen)->snt_isn : tp->snd_una;
 	if (sk->sk_state != TCP_LISTEN &&
 	    !between(seq, snd_una, tp->snd_nxt)) {
@@ -2121,7 +2121,7 @@ void tcp_v4_destroy_sock(struct sock *sk)
 	if (inet_csk(sk)->icsk_bind_hash)
 		inet_put_port(sk);
 
-	BUG_ON(tp->fastopen_rsk);
+	BUG_ON(rcu_access_pointer(tp->fastopen_rsk));
 
 	/* If socket is aborted during connect operation */
 	tcp_free_fastopen_req(tp);

net/ipv4/tcp_minisocks.c

@@ -541,7 +541,7 @@ struct sock *tcp_create_openreq_child(const struct sock *sk,
 	newtp->rx_opt.mss_clamp = req->mss;
 	tcp_ecn_openreq_child(newtp, req);
 	newtp->fastopen_req = NULL;
-	newtp->fastopen_rsk = NULL;
+	RCU_INIT_POINTER(newtp->fastopen_rsk, NULL);
 
 	__TCP_INC_STATS(sock_net(sk), TCP_MIB_PASSIVEOPENS);
 

net/ipv4/tcp_output.c

@@ -2482,7 +2482,7 @@ bool tcp_schedule_loss_probe(struct sock *sk, bool advancing_rto)
 	/* Don't do any loss probe on a Fast Open connection before 3WHS
 	 * finishes.
 	 */
-	if (tp->fastopen_rsk)
+	if (rcu_access_pointer(tp->fastopen_rsk))
 		return false;
 
 	early_retrans = sock_net(sk)->ipv4.sysctl_tcp_early_retrans;

net/ipv4/tcp_timer.c

@@ -386,15 +386,13 @@ abort:		tcp_write_err(sk);
  *	Timer for Fast Open socket to retransmit SYNACK. Note that the
  *	sk here is the child socket, not the parent (listener) socket.
  */
-static void tcp_fastopen_synack_timer(struct sock *sk)
+static void tcp_fastopen_synack_timer(struct sock *sk, struct request_sock *req)
 {
 	struct inet_connection_sock *icsk = inet_csk(sk);
 	int max_retries = icsk->icsk_syn_retries ? :
 	    sock_net(sk)->ipv4.sysctl_tcp_synack_retries + 1; /* add one more retry for fastopen */
 	struct tcp_sock *tp = tcp_sk(sk);
-	struct request_sock *req;
 
-	req = tcp_sk(sk)->fastopen_rsk;
 	req->rsk_ops->syn_ack_timeout(req);
 
 	if (req->num_timeout >= max_retries) {
@@ -435,11 +433,14 @@ void tcp_retransmit_timer(struct sock *sk)
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct net *net = sock_net(sk);
 	struct inet_connection_sock *icsk = inet_csk(sk);
+	struct request_sock *req;
 
-	if (tp->fastopen_rsk) {
+	req = rcu_dereference_protected(tp->fastopen_rsk,
+					lockdep_sock_is_held(sk));
+	if (req) {
 		WARN_ON_ONCE(sk->sk_state != TCP_SYN_RECV &&
 			     sk->sk_state != TCP_FIN_WAIT1);
-		tcp_fastopen_synack_timer(sk);
+		tcp_fastopen_synack_timer(sk, req);
 		/* Before we receive ACK to our SYN-ACK don't retransmit
 		 * anything else (e.g., data or FIN segments).
 		 */

net/ipv6/tcp_ipv6.c

@@ -406,7 +406,7 @@ static int tcp_v6_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
 
 	tp = tcp_sk(sk);
 	/* XXX (TFO) - tp->snd_una should be ISN (tcp_create_openreq_child() */
-	fastopen = tp->fastopen_rsk;
+	fastopen = rcu_dereference(tp->fastopen_rsk);
 	snd_una = fastopen ? tcp_rsk(fastopen)->snt_isn : tp->snd_una;
 	if (sk->sk_state != TCP_LISTEN &&
 	    !between(seq, snd_una, tp->snd_nxt)) {