tcp: fix race condition when creating child sockets from syncookies

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>

diff --git a/include/net/inet_hashtables.h b/include/net/inet_hashtables.h
index 92560974ea67e6e3de6adad9dd9a75c6de9386fb..ca6a3ea9057ec431b2dd30726c2e6d06be94fe72 100644 (file)
--- a/include/net/inet_hashtables.h
+++ b/include/net/inet_hashtables.h
@@ -247,8 +247,9 @@ void inet_hashinfo2_init(struct inet_hashinfo *h, const char *name,
                         unsigned long high_limit);
 int inet_hashinfo2_init_mod(struct inet_hashinfo *h);
 
-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);

diff --git a/net/dccp/ipv4.c b/net/dccp/ipv4.c
index bb3d70664dde0521091147bc40baf546b2b80c94..b0b6e6a4784e5f503bd6e34dbd650d15129ec7cf 100644 (file)
--- a/net/dccp/ipv4.c
+++ b/net/dccp/ipv4.c
@@ -427,7 +427,7 @@ struct sock *dccp_v4_request_recv_sock(const struct sock *sk,
 
        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

diff --git a/net/dccp/ipv6.c b/net/dccp/ipv6.c
index ef4ab28cfde0e3c1ce9c18550599c7b00bb6de8d..78ee1b5acf1f1c18289817f1771588c892c41166 100644 (file)
--- a/net/dccp/ipv6.c
+++ b/net/dccp/ipv6.c
@@ -533,7 +533,7 @@ static struct sock *dccp_v6_request_recv_sock(const struct sock *sk,
                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);

diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
index 4148f5f78f313cde1e0596b9eb3696df16e3f990..f60869acbef0291b08e5cd27ea47a51b4fdefa05 100644 (file)
--- a/net/ipv4/inet_connection_sock.c
+++ b/net/ipv4/inet_connection_sock.c
@@ -787,7 +787,7 @@ static void reqsk_queue_hash_req(struct request_sock *req,
        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.
         */

diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c
index 8cbe74313f38268a3cf2fecb36eb2d1d2dd60ac0..45fb450b45227ce05cc5a2fd8b8a32089ad5c476 100644 (file)
--- a/net/ipv4/inet_hashtables.c
+++ b/net/ipv4/inet_hashtables.c
@@ -20,6 +20,9 @@
 #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>
@@ -508,10 +511,52 @@ static u32 inet_sk_port_offset(const struct sock *sk)
                                          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;
@@ -530,16 +575,23 @@ bool inet_ehash_insert(struct sock *sk, struct sock *osk)
        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);
@@ -583,7 +635,7 @@ int __inet_hash(struct sock *sk, struct sock *osk)
        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));
@@ -679,7 +731,7 @@ int __inet_hash_connect(struct inet_timewait_death_row *death_row,
                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;
                }
@@ -758,7 +810,7 @@ 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);

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 25d35a56ceba0582da560b239f8c51f135d9a751..c95fcdfeed42909d7ba88cd2dc5570e95d0e4651 100644 (file)
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -1499,6 +1499,7 @@ struct sock *tcp_v4_syn_recv_sock(const struct sock *sk, struct sk_buff *skb,
                                  bool *own_req)
 {
        struct inet_request_sock *ireq;
+       bool found_dup_sk = false;
        struct inet_sock *newinet;
        struct tcp_sock *newtp;
        struct sock *newsk;
@@ -1576,12 +1577,22 @@ 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;
-       *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;
 

diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index 3d49e8d0afeebb1cb7133932dca9a1ac6d6884ab..d2502911b7fac5c6864aedebb37616b7873e1753 100644 (file)
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -1193,6 +1193,7 @@ static struct sock *tcp_v6_syn_recv_sock(const struct sock *sk, struct sk_buff *
        const struct ipv6_pinfo *np = tcp_inet6_sk(sk);
        struct ipv6_txoptions *opt;
        struct inet_sock *newinet;
+       bool found_dup_sk = false;
        struct tcp_sock *newtp;
        struct sock *newsk;
 #ifdef CONFIG_TCP_MD5SIG
@@ -1368,7 +1369,8 @@ static struct sock *tcp_v6_syn_recv_sock(const struct sock *sk, struct sk_buff *
                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);
 
@@ -1383,6 +1385,15 @@ static struct sock *tcp_v6_syn_recv_sock(const struct sock *sk, struct sk_buff *
                                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;