tcp/dccp: fix hashdance race for passive sessions

Multiple cpus can process duplicates of incoming ACK messages
matching a SYN_RECV request socket. This is a rare event under
normal operations, but definitely can happen.

Only one must win the race, otherwise corruption would occur.

To fix this without adding new atomic ops, we use logic in
inet_ehash_nolisten() to detect the request was present in the same
ehash bucket where we try to insert the new child.

If request socket was not found, we have to undo the child creation.

This actually removes a spin_lock()/spin_unlock() pair in
reqsk_queue_unlink() for the fast path.

Fixes: e994b2f0fb ("tcp: do not lock listener to process SYN packets")
Fixes: 079096f103 ("tcp/dccp: install syn_recv requests into ehash table")
Signed-off-by: Eric Dumazet <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

net/ipv4/inet_connection_sock.c

@@ -523,15 +523,15 @@ static bool reqsk_queue_unlink(struct request_sock_queue *queue,
 			       struct request_sock *req)
 {
 	struct inet_hashinfo *hashinfo = req_to_sk(req)->sk_prot->h.hashinfo;
-	spinlock_t *lock;
-	bool found;
+	bool found = false;
 
-	lock = inet_ehash_lockp(hashinfo, req->rsk_hash);
-
-	spin_lock(lock);
-	found = __sk_nulls_del_node_init_rcu(req_to_sk(req));
-	spin_unlock(lock);
+	if (sk_hashed(req_to_sk(req))) {
+		spinlock_t *lock = inet_ehash_lockp(hashinfo, req->rsk_hash);
 
+		spin_lock(lock);
+		found = __sk_nulls_del_node_init_rcu(req_to_sk(req));
+		spin_unlock(lock);
+	}
 	if (timer_pending(&req->rsk_timer) && del_timer_sync(&req->rsk_timer))
 		reqsk_put(req);
 	return found;
@@ -811,6 +811,25 @@ void inet_csk_reqsk_queue_add(struct sock *sk, struct request_sock *req,
 }
 EXPORT_SYMBOL(inet_csk_reqsk_queue_add);
 
+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);
+		inet_csk_reqsk_queue_add(sk, req, child);
+		/* Warning: caller must not call reqsk_put(req);
+		 * child stole last reference on it.
+		 */
+		return child;
+	}
+	/* Too bad, another child took ownership of the request, undo. */
+	bh_unlock_sock(child);
+	sock_put(child);
+	return NULL;
+}
+EXPORT_SYMBOL(inet_csk_complete_hashdance);
+
 /*
  *	This routine closes sockets which have been at least partially
  *	opened, but not yet accepted.

net/ipv4/inet_hashtables.c

@@ -407,13 +407,13 @@ static u32 inet_sk_port_offset(const struct sock *sk)
 /* insert a socket into ehash, and eventually remove another one
  * (The another one can be a SYN_RECV or TIMEWAIT
  */
-int inet_ehash_insert(struct sock *sk, struct sock *osk)
+bool inet_ehash_insert(struct sock *sk, struct sock *osk)
 {
 	struct inet_hashinfo *hashinfo = sk->sk_prot->h.hashinfo;
 	struct hlist_nulls_head *list;
 	struct inet_ehash_bucket *head;
 	spinlock_t *lock;
-	int ret = 0;
+	bool ret = true;
 
 	WARN_ON_ONCE(!sk_unhashed(sk));
 
@@ -423,30 +423,41 @@ int inet_ehash_insert(struct sock *sk, struct sock *osk)
 	lock = inet_ehash_lockp(hashinfo, sk->sk_hash);
 
 	spin_lock(lock);
-	__sk_nulls_add_node_rcu(sk, list);
 	if (osk) {
-		WARN_ON(sk->sk_hash != osk->sk_hash);
-		sk_nulls_del_node_init_rcu(osk);
+		WARN_ON_ONCE(sk->sk_hash != osk->sk_hash);
+		ret = sk_nulls_del_node_init_rcu(osk);
 	}
+	if (ret)
+		__sk_nulls_add_node_rcu(sk, list);
 	spin_unlock(lock);
 	return ret;
 }
 
-void __inet_hash_nolisten(struct sock *sk, struct sock *osk)
+bool inet_ehash_nolisten(struct sock *sk, struct sock *osk)
 {
-	inet_ehash_insert(sk, osk);
-	sock_prot_inuse_add(sock_net(sk), sk->sk_prot, 1);
+	bool ok = inet_ehash_insert(sk, osk);
+
+	if (ok) {
+		sock_prot_inuse_add(sock_net(sk), sk->sk_prot, 1);
+	} else {
+		percpu_counter_inc(sk->sk_prot->orphan_count);
+		sk->sk_state = TCP_CLOSE;
+		sock_set_flag(sk, SOCK_DEAD);
+		inet_csk_destroy_sock(sk);
+	}
+	return ok;
 }
-EXPORT_SYMBOL_GPL(__inet_hash_nolisten);
+EXPORT_SYMBOL_GPL(inet_ehash_nolisten);
 
 void __inet_hash(struct sock *sk, struct sock *osk)
 {
 	struct inet_hashinfo *hashinfo = sk->sk_prot->h.hashinfo;
 	struct inet_listen_hashbucket *ilb;
 
-	if (sk->sk_state != TCP_LISTEN)
-		return __inet_hash_nolisten(sk, osk);
-
+	if (sk->sk_state != TCP_LISTEN) {
+		inet_ehash_nolisten(sk, osk);
+		return;
+	}
 	WARN_ON(!sk_unhashed(sk));
 	ilb = &hashinfo->listening_hash[inet_sk_listen_hashfn(sk)];
 
@@ -567,7 +578,7 @@ ok:
 		inet_bind_hash(sk, tb, port);
 		if (sk_unhashed(sk)) {
 			inet_sk(sk)->inet_sport = htons(port);
-			__inet_hash_nolisten(sk, (struct sock *)tw);
+			inet_ehash_nolisten(sk, (struct sock *)tw);
 		}
 		if (tw)
 			inet_twsk_bind_unhash(tw, hinfo);
@@ -584,7 +595,7 @@ ok:
 	tb  = inet_csk(sk)->icsk_bind_hash;
 	spin_lock_bh(&head->lock);
 	if (sk_head(&tb->owners) == sk && !sk->sk_bind_node.next) {
-		__inet_hash_nolisten(sk, NULL);
+		inet_ehash_nolisten(sk, NULL);
 		spin_unlock_bh(&head->lock);
 		return 0;
 	} else {

net/ipv4/syncookies.c

@@ -221,8 +221,10 @@ struct sock *tcp_get_cookie_sock(struct sock *sk, struct sk_buff *skb,
 {
 	struct inet_connection_sock *icsk = inet_csk(sk);
 	struct sock *child;
+	bool own_req;
 
-	child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst);
+	child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst,
+						 NULL, &own_req);
 	if (child) {
 		atomic_set(&req->rsk_refcnt, 1);
 		sock_rps_save_rxhash(child, skb);

net/ipv4/tcp_fastopen.c

@@ -133,12 +133,14 @@ static struct sock *tcp_fastopen_create_child(struct sock *sk,
 	struct request_sock_queue *queue = &inet_csk(sk)->icsk_accept_queue;
 	struct sock *child;
 	u32 end_seq;
+	bool own_req;
 
 	req->num_retrans = 0;
 	req->num_timeout = 0;
 	req->sk = NULL;
 
-	child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, req, NULL);
+	child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, req, NULL,
+							 NULL, &own_req);
 	if (!child)
 		return NULL;
 

net/ipv4/tcp_ipv4.c

@@ -1247,7 +1247,9 @@ EXPORT_SYMBOL(tcp_v4_conn_request);
  */
 struct sock *tcp_v4_syn_recv_sock(const struct sock *sk, struct sk_buff *skb,
 				  struct request_sock *req,
-				  struct dst_entry *dst)
+				  struct dst_entry *dst,
+				  struct request_sock *req_unhash,
+				  bool *own_req)
 {
 	struct inet_request_sock *ireq;
 	struct inet_sock *newinet;
@@ -1323,7 +1325,7 @@ struct sock *tcp_v4_syn_recv_sock(const struct sock *sk, struct sk_buff *skb,
 
 	if (__inet_inherit_port(sk, newsk) < 0)
 		goto put_and_exit;
-	__inet_hash_nolisten(newsk, NULL);
+	*own_req = inet_ehash_nolisten(newsk, req_to_sk(req_unhash));
 
 	return newsk;
 

net/ipv4/tcp_minisocks.c

@@ -580,6 +580,7 @@ struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
 	const struct tcphdr *th = tcp_hdr(skb);
 	__be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK);
 	bool paws_reject = false;
+	bool own_req;
 
 	tmp_opt.saw_tstamp = 0;
 	if (th->doff > (sizeof(struct tcphdr)>>2)) {
@@ -767,18 +768,14 @@ struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
 	 * ESTABLISHED STATE. If it will be dropped after
 	 * socket is created, wait for troubles.
 	 */
-	child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, req, NULL);
+	child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, req, NULL,
+							 req, &own_req);
 	if (!child)
 		goto listen_overflow;
 
 	sock_rps_save_rxhash(child, skb);
 	tcp_synack_rtt_meas(child, req);
-	inet_csk_reqsk_queue_drop(sk, req);
-	inet_csk_reqsk_queue_add(sk, req, child);
-	/* Warning: caller must not call reqsk_put(req);
-	 * child stole last reference on it.
-	 */
-	return child;
+	return inet_csk_complete_hashdance(sk, child, req, own_req);
 
 listen_overflow:
 	if (!sysctl_tcp_abort_on_overflow) {