Re: [PATCH v3] tcp: fix race condition when creating child sockets from syncookies

[Eric Dumazet]

On Wed, Nov 11, 2020 at 8:35 AM Ricardo Dias <rdias@xxxxxxxxxxxxxxx> wrote:
>
> 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 return that socket and use it to process the TCP
> packet received, and discard the second child socket to the same client.
>
> Signed-off-by: Ricardo Dias <rdias@xxxxxxxxxxxxxxx>
> ---
> v3 (2020-11-11):
>   * Fixed IPv6 handling in inet_ehash_insert
>   * Removed unecessary comparison while traversing the ehash bucket
>     list.
>
> v2 (2020-11-09):
>   * Changed the author's email domain.
>   * Removed the helper function inet_ehash_insert_chk_dup and moved the
>     logic to the existing inet_ehash_insert.
>   * Updated the callers of iner_ehash_nolisten to deal with the new
>     logic.
>
>  include/net/inet_hashtables.h |  6 +--
>  net/dccp/ipv4.c               |  4 +-
>  net/dccp/ipv6.c               |  4 +-
>  net/ipv4/inet_hashtables.c    | 69 ++++++++++++++++++++++++++++++-----
>  net/ipv4/syncookies.c         |  5 ++-
>  net/ipv4/tcp_ipv4.c           | 10 ++++-
>  net/ipv6/tcp_ipv6.c           | 17 ++++++++-
>  7 files changed, 97 insertions(+), 18 deletions(-)
>
> diff --git a/include/net/inet_hashtables.h b/include/net/inet_hashtables.h
> index 92560974ea67..dffa345d52a7 100644
> --- a/include/net/inet_hashtables.h
> +++ b/include/net/inet_hashtables.h
> @@ -247,9 +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);
> -int __inet_hash(struct sock *sk, struct sock *osk);
> +bool inet_ehash_insert(struct sock *sk, struct sock **osk);
> +bool inet_ehash_nolisten(struct sock *sk, struct sock **osk);
> +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 9c28c8251125..99bbba478991 100644
> --- a/net/dccp/ipv4.c
> +++ b/net/dccp/ipv4.c
> @@ -400,6 +400,7 @@ struct sock *dccp_v4_request_recv_sock(const struct sock *sk,
>         struct inet_request_sock *ireq;
>         struct inet_sock *newinet;
>         struct sock *newsk;
> +       struct sock *osk;
>
>         if (sk_acceptq_is_full(sk))
>                 goto exit_overflow;
> @@ -427,7 +428,8 @@ 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));
> +       osk = req_to_sk(req_unhash);
> +       *own_req = inet_ehash_nolisten(newsk, &osk);
>         if (*own_req)
>                 ireq->ireq_opt = NULL;
>         else
> diff --git a/net/dccp/ipv6.c b/net/dccp/ipv6.c
> index ef4ab28cfde0..91a825c00a97 100644
> --- a/net/dccp/ipv6.c
> +++ b/net/dccp/ipv6.c
> @@ -407,6 +407,7 @@ static struct sock *dccp_v6_request_recv_sock(const struct sock *sk,
>         struct inet_sock *newinet;
>         struct dccp6_sock *newdp6;
>         struct sock *newsk;
> +       struct sock *osk;
>
>         if (skb->protocol == htons(ETH_P_IP)) {
>                 /*
> @@ -533,7 +534,8 @@ 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));
> +       osk = req_to_sk(req_unhash);
> +       *own_req = inet_ehash_nolisten(newsk, &osk);
>         /* 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_hashtables.c b/net/ipv4/inet_hashtables.c
> index 239e54474b65..1fce64f7f0dc 100644
> --- 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>
> @@ -510,17 +513,27 @@ 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
> +/* 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, it returns that socket
> + * through the osk parameter.


I think this deserves a third parameter, to avoid confusion.


>   */
> -bool 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;
> +       const struct hlist_nulls_node *node;
> +       struct sock *esk;
> +       spinlock_t *lock; /* protects hashinfo socket entry */
> +       struct net *net = sock_net(sk);
> +       const int dif = sk->sk_bound_dev_if;
> +       const int sdif = sk->sk_bound_dev_if;
>         bool ret = true;
>
> +       INET_ADDR_COOKIE(acookie, sk->sk_daddr, sk->sk_rcv_saddr);
> +       const __portpair ports = INET_COMBINED_PORTS(sk->sk_dport, sk->sk_num);
> +
>         WARN_ON_ONCE(!sk_unhashed(sk));
>
>         sk->sk_hash = sk_ehashfn(sk);
> @@ -529,17 +542,53 @@ bool inet_ehash_insert(struct sock *sk, struct sock *osk)
>         lock = inet_ehash_lockp(hashinfo, sk->sk_hash);
>
>         spin_lock(lock);
> -       if (osk) {
> -               WARN_ON_ONCE(sk->sk_hash != osk->sk_hash);
> -               ret = sk_nulls_del_node_init_rcu(osk);
> +       if (osk && *osk) {
> +               WARN_ON_ONCE(sk->sk_hash != (*osk)->sk_hash);
> +               ret = sk_nulls_del_node_init_rcu(*osk);
> +       } else if (osk && !*osk) {
> +               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))) {
> +                                       if (unlikely(!refcount_inc_not_zero(&esk->sk_refcnt)))

Can you explain how this could happen ?
Again, we own the lock here, finding a socket in ehash, with a zero
refcount can not happen.

Only a true rcu lookup could see a zero refcount.

> +                                               goto out;
> +                                       goto found;
> +                               }
> +                       }
> +#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))) {
> +                                       if (unlikely(!refcount_inc_not_zero(&esk->sk_refcnt)))
> +                                               goto out;
> +                                       goto found;
> +                               }
> +                       }
> +#endif
> +               }
> +
>         }
> +out:
> +       esk = NULL;
>         if (ret)
>                 __sk_nulls_add_node_rcu(sk, list);
> +
> +found:
>         spin_unlock(lock);
> +       if (esk) {
> +               *osk = esk;
> +               ret = false;
> +       }
>         return ret;
>  }
>
> -bool inet_ehash_nolisten(struct sock *sk, struct sock *osk)
> +bool inet_ehash_nolisten(struct sock *sk, struct sock **osk)
>  {
>         bool ok = inet_ehash_insert(sk, osk);
>
> @@ -578,7 +627,7 @@ static int inet_reuseport_add_sock(struct sock *sk,
>         return reuseport_alloc(sk, inet_rcv_saddr_any(sk));
>  }
>
> -int __inet_hash(struct sock *sk, struct sock *osk)
> +int __inet_hash(struct sock *sk, struct sock **osk)
>  {
>         struct inet_hashinfo *hashinfo = sk->sk_prot->h.hashinfo;
>         struct inet_listen_hashbucket *ilb;
> @@ -760,7 +809,7 @@ int __inet_hash_connect(struct inet_timewait_death_row *death_row,
>         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);

Why could not we 'find' a prior socket, and since you ignore the result,
leave a refcount increment, and a socket leak ?

Same remark for all callers.

>         }
>         if (tw)

See the problem here ?

If tw was initially NULL, then inet_ehash_nolisten() could have overwritten tw
with another socket.

Surely calling inet_twsk_bind_unhash() could be disastrous.

>                 inet_twsk_bind_unhash(tw, hinfo);
> diff --git a/net/ipv4/syncookies.c b/net/ipv4/syncookies.c
> index e03756631541..c4bb895085f0 100644
> --- a/net/ipv4/syncookies.c
> +++ b/net/ipv4/syncookies.c
> @@ -208,7 +208,7 @@ struct sock *tcp_get_cookie_sock(struct sock *sk, struct sk_buff *skb,
>
>         child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst,
>                                                  NULL, &own_req);
> -       if (child) {
> +       if (child && own_req) {
>                 refcount_set(&req->rsk_refcnt, 1);
>                 tcp_sk(child)->tsoffset = tsoff;
>                 sock_rps_save_rxhash(child, skb);
> @@ -223,6 +223,9 @@ struct sock *tcp_get_cookie_sock(struct sock *sk, struct sk_buff *skb,
>
>                 bh_unlock_sock(child);
>                 sock_put(child);
> +       }  else if (child && !own_req) {
> +               __reqsk_free(req);
> +               return child;
>         }
>         __reqsk_free(req);
>
> diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
> index 592c73962723..7daaea30fc30 100644
> --- a/net/ipv4/tcp_ipv4.c
> +++ b/net/ipv4/tcp_ipv4.c
> @@ -1501,6 +1501,7 @@ struct sock *tcp_v4_syn_recv_sock(const struct sock *sk, struct sk_buff *skb,
>         int l3index;
>  #endif
>         struct ip_options_rcu *inet_opt;
> +       struct sock *osk;
>
>         if (sk_acceptq_is_full(sk))
>                 goto exit_overflow;
> @@ -1565,11 +1566,18 @@ 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));
> +       osk = req_to_sk(req_unhash);
> +       *own_req = inet_ehash_nolisten(newsk, &osk);
>         if (likely(*own_req)) {
>                 tcp_move_syn(newtp, req);
>                 ireq->ireq_opt = NULL;
>         } else {
> +               if (!req_unhash && osk) {
> +                       /* This code path should only be executed in the
> +                        * syncookie case only
> +                        */
> +                       newsk = osk;
> +               }
>                 newinet->inet_opt = NULL;
>         }
>         return newsk;
> diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
> index 305870a72352..376dc75395c5 100644
> --- a/net/ipv6/tcp_ipv6.c
> +++ b/net/ipv6/tcp_ipv6.c
> @@ -1190,6 +1190,7 @@ static struct sock *tcp_v6_syn_recv_sock(const struct sock *sk, struct sk_buff *
>         struct inet_sock *newinet;
>         struct tcp_sock *newtp;
>         struct sock *newsk;
> +       struct sock *osk;
>  #ifdef CONFIG_TCP_MD5SIG
>         struct tcp_md5sig_key *key;
>         int l3index;
> @@ -1206,6 +1207,12 @@ static struct sock *tcp_v6_syn_recv_sock(const struct sock *sk, struct sk_buff *
>
>                 if (!newsk)
>                         return NULL;
> +               else if (!own_req) {
> +                       /* We're returning an existing child socket, probably
> +                        * created by a previous syncookie ACK.
> +                        */
> +                       return newsk;
> +               }
>
>                 inet_sk(newsk)->pinet6 = tcp_inet6_sk(newsk);
>
> @@ -1359,7 +1366,8 @@ 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));
> +       osk = req_to_sk(req_unhash);
> +       *own_req = inet_ehash_nolisten(newsk, &osk);
>         if (*own_req) {
>                 tcp_move_syn(newtp, req);
>
> @@ -1374,6 +1382,13 @@ 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 && osk) {
> +                       /* This code path should only be executed in the
> +                        * syncookie case only
> +                        */
> +                       newsk = osk;
> +               }
>         }
>
>         return newsk;
> --
> 2.25.1
>