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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Ricardo Dias <rdias@singlestore.com>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Sasha Levin <sashal@kernel.org>
Subject: [PATCH 4.19 08/53] tcp: fix race condition when creating child sockets from syncookies
Date:   Tue, 26 Apr 2022 10:20:48 +0200
Message-Id: <20220426081735.900497168@linuxfoundation.org>
In-Reply-To: <20220426081735.651926456@linuxfoundation.org>
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From: Ricardo Dias <rdias@singlestore.com>

[ Upstream commit 01770a166165738a6e05c3d911fb4609cc4eb416 ]

When the TCP stack is in SYN flood mode, the server child socket is
created from the SYN cookie received in a TCP packet with the ACK flag
set.

The child socket is created when the server receives the first TCP
packet with a valid SYN cookie from the client. Usually, this packet
corresponds to the final step of the TCP 3-way handshake, the ACK
packet. But is also possible to receive a valid SYN cookie from the
first TCP data packet sent by the client, and thus create a child socket
from that SYN cookie.

Since a client socket is ready to send data as soon as it receives the
SYN+ACK packet from the server, the client can send the ACK packet (sent
by the TCP stack code), and the first data packet (sent by the userspace
program) almost at the same time, and thus the server will equally
receive the two TCP packets with valid SYN cookies almost at the same
instant.

When such event happens, the TCP stack code has a race condition that
occurs between the momement a lookup is done to the established
connections hashtable to check for the existence of a connection for the
same client, and the moment that the child socket is added to the
established connections hashtable. As a consequence, this race condition
can lead to a situation where we add two child sockets to the
established connections hashtable and deliver two sockets to the
userspace program to the same client.

This patch fixes the race condition by checking if an existing child
socket exists for the same client when we are adding the second child
socket to the established connections socket. If an existing child
socket exists, we drop the packet and discard the second child socket
to the same client.

Signed-off-by: Ricardo Dias <rdias@singlestore.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20201120111133.GA67501@rdias-suse-pc.lan
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 include/net/inet_hashtables.h   |    5 +-
 net/dccp/ipv4.c                 |    2 -
 net/dccp/ipv6.c                 |    2 -
 net/ipv4/inet_connection_sock.c |    2 -
 net/ipv4/inet_hashtables.c      |   68 +++++++++++++++++++++++++++++++++++-----
 net/ipv4/tcp_ipv4.c             |   15 +++++++-
 net/ipv6/tcp_ipv6.c             |   13 +++++++
 7 files changed, 91 insertions(+), 16 deletions(-)

--- a/include/net/inet_hashtables.h
+++ b/include/net/inet_hashtables.h
@@ -232,8 +232,9 @@ void inet_hashinfo2_init(struct inet_has
 			 unsigned long low_limit,
 			 unsigned long high_limit);
 
-bool inet_ehash_insert(struct sock *sk, struct sock *osk);
-bool inet_ehash_nolisten(struct sock *sk, struct sock *osk);
+bool inet_ehash_insert(struct sock *sk, struct sock *osk, bool *found_dup_sk);
+bool inet_ehash_nolisten(struct sock *sk, struct sock *osk,
+			 bool *found_dup_sk);
 int __inet_hash(struct sock *sk, struct sock *osk);
 int inet_hash(struct sock *sk);
 void inet_unhash(struct sock *sk);
--- a/net/dccp/ipv4.c
+++ b/net/dccp/ipv4.c
@@ -428,7 +428,7 @@ struct sock *dccp_v4_request_recv_sock(c
 
 	if (__inet_inherit_port(sk, newsk) < 0)
 		goto put_and_exit;
-	*own_req = inet_ehash_nolisten(newsk, req_to_sk(req_unhash));
+	*own_req = inet_ehash_nolisten(newsk, req_to_sk(req_unhash), NULL);
 	if (*own_req)
 		ireq->ireq_opt = NULL;
 	else
--- a/net/dccp/ipv6.c
+++ b/net/dccp/ipv6.c
@@ -538,7 +538,7 @@ static struct sock *dccp_v6_request_recv
 		dccp_done(newsk);
 		goto out;
 	}
-	*own_req = inet_ehash_nolisten(newsk, req_to_sk(req_unhash));
+	*own_req = inet_ehash_nolisten(newsk, req_to_sk(req_unhash), NULL);
 	/* Clone pktoptions received with SYN, if we own the req */
 	if (*own_req && ireq->pktopts) {
 		newnp->pktoptions = skb_clone(ireq->pktopts, GFP_ATOMIC);
--- a/net/ipv4/inet_connection_sock.c
+++ b/net/ipv4/inet_connection_sock.c
@@ -793,7 +793,7 @@ static void reqsk_queue_hash_req(struct
 	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);
+	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.
 	 */
--- a/net/ipv4/inet_hashtables.c
+++ b/net/ipv4/inet_hashtables.c
@@ -24,6 +24,9 @@
 #include <net/addrconf.h>
 #include <net/inet_connection_sock.h>
 #include <net/inet_hashtables.h>
+#if IS_ENABLED(CONFIG_IPV6)
+#include <net/inet6_hashtables.h>
+#endif
 #include <net/secure_seq.h>
 #include <net/ip.h>
 #include <net/tcp.h>
@@ -513,10 +516,52 @@ static u32 inet_sk_port_offset(const str
 					  inet->inet_dport);
 }
 
-/* insert a socket into ehash, and eventually remove another one
- * (The another one can be a SYN_RECV or TIMEWAIT
+/* Searches for an exsiting socket in the ehash bucket list.
+ * Returns true if found, false otherwise.
  */
-bool inet_ehash_insert(struct sock *sk, struct sock *osk)
+static bool inet_ehash_lookup_by_sk(struct sock *sk,
+				    struct hlist_nulls_head *list)
+{
+	const __portpair ports = INET_COMBINED_PORTS(sk->sk_dport, sk->sk_num);
+	const int sdif = sk->sk_bound_dev_if;
+	const int dif = sk->sk_bound_dev_if;
+	const struct hlist_nulls_node *node;
+	struct net *net = sock_net(sk);
+	struct sock *esk;
+
+	INET_ADDR_COOKIE(acookie, sk->sk_daddr, sk->sk_rcv_saddr);
+
+	sk_nulls_for_each_rcu(esk, node, list) {
+		if (esk->sk_hash != sk->sk_hash)
+			continue;
+		if (sk->sk_family == AF_INET) {
+			if (unlikely(INET_MATCH(esk, net, acookie,
+						sk->sk_daddr,
+						sk->sk_rcv_saddr,
+						ports, dif, sdif))) {
+				return true;
+			}
+		}
+#if IS_ENABLED(CONFIG_IPV6)
+		else if (sk->sk_family == AF_INET6) {
+			if (unlikely(INET6_MATCH(esk, net,
+						 &sk->sk_v6_daddr,
+						 &sk->sk_v6_rcv_saddr,
+						 ports, dif, sdif))) {
+				return true;
+			}
+		}
+#endif
+	}
+	return false;
+}
+
+/* Insert a socket into ehash, and eventually remove another one
+ * (The another one can be a SYN_RECV or TIMEWAIT)
+ * If an existing socket already exists, socket sk is not inserted,
+ * and sets found_dup_sk parameter to true.
+ */
+bool inet_ehash_insert(struct sock *sk, struct sock *osk, bool *found_dup_sk)
 {
 	struct inet_hashinfo *hashinfo = sk->sk_prot->h.hashinfo;
 	struct hlist_nulls_head *list;
@@ -535,16 +580,23 @@ bool inet_ehash_insert(struct sock *sk,
 	if (osk) {
 		WARN_ON_ONCE(sk->sk_hash != osk->sk_hash);
 		ret = sk_nulls_del_node_init_rcu(osk);
+	} else if (found_dup_sk) {
+		*found_dup_sk = inet_ehash_lookup_by_sk(sk, list);
+		if (*found_dup_sk)
+			ret = false;
 	}
+
 	if (ret)
 		__sk_nulls_add_node_rcu(sk, list);
+
 	spin_unlock(lock);
+
 	return ret;
 }
 
-bool inet_ehash_nolisten(struct sock *sk, struct sock *osk)
+bool inet_ehash_nolisten(struct sock *sk, struct sock *osk, bool *found_dup_sk)
 {
-	bool ok = inet_ehash_insert(sk, osk);
+	bool ok = inet_ehash_insert(sk, osk, found_dup_sk);
 
 	if (ok) {
 		sock_prot_inuse_add(sock_net(sk), sk->sk_prot, 1);
@@ -588,7 +640,7 @@ int __inet_hash(struct sock *sk, struct
 	int err = 0;
 
 	if (sk->sk_state != TCP_LISTEN) {
-		inet_ehash_nolisten(sk, osk);
+		inet_ehash_nolisten(sk, osk, NULL);
 		return 0;
 	}
 	WARN_ON(!sk_unhashed(sk));
@@ -683,7 +735,7 @@ int __inet_hash_connect(struct inet_time
 		tb = inet_csk(sk)->icsk_bind_hash;
 		spin_lock_bh(&head->lock);
 		if (sk_head(&tb->owners) == sk && !sk->sk_bind_node.next) {
-			inet_ehash_nolisten(sk, NULL);
+			inet_ehash_nolisten(sk, NULL, NULL);
 			spin_unlock_bh(&head->lock);
 			return 0;
 		}
@@ -759,7 +811,7 @@ ok:
 	inet_bind_hash(sk, tb, port);
 	if (sk_unhashed(sk)) {
 		inet_sk(sk)->inet_sport = htons(port);
-		inet_ehash_nolisten(sk, (struct sock *)tw);
+		inet_ehash_nolisten(sk, (struct sock *)tw, NULL);
 	}
 	if (tw)
 		inet_twsk_bind_unhash(tw, hinfo);
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -1415,6 +1415,7 @@ struct sock *tcp_v4_syn_recv_sock(const
 				  bool *own_req)
 {
 	struct inet_request_sock *ireq;
+	bool found_dup_sk = false;
 	struct inet_sock *newinet;
 	struct tcp_sock *newtp;
 	struct sock *newsk;
@@ -1485,12 +1486,22 @@ struct sock *tcp_v4_syn_recv_sock(const
 
 	if (__inet_inherit_port(sk, newsk) < 0)
 		goto put_and_exit;
-	*own_req = inet_ehash_nolisten(newsk, req_to_sk(req_unhash));
+	*own_req = inet_ehash_nolisten(newsk, req_to_sk(req_unhash),
+				       &found_dup_sk);
 	if (likely(*own_req)) {
 		tcp_move_syn(newtp, req);
 		ireq->ireq_opt = NULL;
 	} else {
-		newinet->inet_opt = NULL;
+		if (!req_unhash && found_dup_sk) {
+			/* This code path should only be executed in the
+			 * syncookie case only
+			 */
+			bh_unlock_sock(newsk);
+			sock_put(newsk);
+			newsk = NULL;
+		} else {
+			newinet->inet_opt = NULL;
+		}
 	}
 	return newsk;
 
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -1090,6 +1090,7 @@ static struct sock *tcp_v6_syn_recv_sock
 	struct ipv6_txoptions *opt;
 	struct tcp6_sock *newtcp6sk;
 	struct inet_sock *newinet;
+	bool found_dup_sk = false;
 	struct tcp_sock *newtp;
 	struct sock *newsk;
 #ifdef CONFIG_TCP_MD5SIG
@@ -1258,7 +1259,8 @@ static struct sock *tcp_v6_syn_recv_sock
 		tcp_done(newsk);
 		goto out;
 	}
-	*own_req = inet_ehash_nolisten(newsk, req_to_sk(req_unhash));
+	*own_req = inet_ehash_nolisten(newsk, req_to_sk(req_unhash),
+				       &found_dup_sk);
 	if (*own_req) {
 		tcp_move_syn(newtp, req);
 
@@ -1273,6 +1275,15 @@ static struct sock *tcp_v6_syn_recv_sock
 				skb_set_owner_r(newnp->pktoptions, newsk);
 			}
 		}
+	} else {
+		if (!req_unhash && found_dup_sk) {
+			/* This code path should only be executed in the
+			 * syncookie case only
+			 */
+			bh_unlock_sock(newsk);
+			sock_put(newsk);
+			newsk = NULL;
+		}
 	}
 
 	return newsk;