tcp: fix race condition when creating child sockets from syncookies

[ 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>

diff --git a/include/net/inet_hashtables.h b/include/net/inet_hashtables.h
index 573ab110c9ec8920a5c94e596b905a9e9aadaa99..e5f4d27114048f645e7c486d46c487b4af36fb82 100644 (file)
--- a/include/net/inet_hashtables.h
+++ b/include/net/inet_hashtables.h
@@ -215,8 +215,9 @@ void inet_put_port(struct sock *sk);
 
 void inet_hashinfo_init(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 176bddacc16eb7a51b3cd20c207d146a22cb8942..7e93087d13667400129a41e48b163fbd32b8175d 100644 (file)
--- a/net/dccp/ipv4.c
+++ b/net/dccp/ipv4.c
@@ -428,7 +428,7 @@ 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 2cd3508a378692db5f4f23ae3cd32c68821d4d30..ae4851fdbe9e52a9b5becc98116b64181e4e0906 100644 (file)
--- a/net/dccp/ipv6.c
+++ b/net/dccp/ipv6.c
@@ -538,7 +538,7 @@ 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 08ba0f91f2aba626ac1e42bcf4543efd710f3ae2..44b1d630c40b1a44d7b8a07921c74ee6e44698d0 100644 (file)
--- a/net/ipv4/inet_connection_sock.c
+++ b/net/ipv4/inet_connection_sock.c
@@ -784,7 +784,7 @@ static void reqsk_queue_hash_req(struct request_sock *req,
                            (unsigned long)req);
        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 8a54babf5c9034cfe1350997d1e39c7d79e9d441..1346e45cf8d131a6547f5a11fbdcacf6991b0f38 100644 (file)
--- a/net/ipv4/inet_hashtables.c
+++ b/net/ipv4/inet_hashtables.c
@@ -23,6 +23,9 @@
 #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>
@@ -395,10 +398,52 @@ 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;
@@ -417,16 +462,23 @@ 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);
@@ -469,7 +521,7 @@ 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));
@@ -556,7 +608,7 @@ 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;
                }
@@ -632,7 +684,7 @@ 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 744479c8b91f3b4c8af73aa41175997aea181eb0..a598c515343442208da56252c3c7f5e823c93d6c 100644 (file)
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -1344,6 +1344,7 @@ 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;
@@ -1414,12 +1415,22 @@ 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 0f0772c48bf0838b22c79017d2f7b046d9164cd1..21637031fbab712e428e12e0ac9433220c77b287 100644 (file)
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -1064,6 +1064,7 @@ static struct sock *tcp_v6_syn_recv_sock(const struct sock *sk, struct sk_buff *
        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
@@ -1232,7 +1233,8 @@ 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);
 
@@ -1247,6 +1249,15 @@ 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;