Subject: Re: [PATCH v7 bpf-next 5/6] bpf: tcp: Support arbitrary SYN Cookie.

From: Martin KaFai Lau <[email-masked]>
Date: Thu, 11 Jan 2024 17:44:55 -0800

On 12/20/23 5:28 PM, Kuniyuki Iwashima wrote:
> This patch adds a new kfunc available at TC hook to support arbitrary
> SYN Cookie.
> 
> The basic usage is as follows:
> 
>      struct bpf_tcp_req_attrs attrs = {
>          .mss = mss,
>          .wscale_ok = wscale_ok,
>          .rcv_wscale = rcv_wscale, /* Server's WScale < 15 */
>          .snd_wscale = snd_wscale, /* Client's WScale < 15 */
>          .tstamp_ok = tstamp_ok,
>          .rcv_tsval = tsval,
>          .rcv_tsecr = tsecr, /* Server's Initial TSval */
>          .usec_ts_ok = usec_ts_ok,
>          .sack_ok = sack_ok,
>          .ecn_ok = ecn_ok,
>      }
> 
>      skc = bpf_skc_lookup_tcp(...);
>      sk = (struct sock *)bpf_skc_to_tcp_sock(skc);
>      bpf_sk_assign_tcp_reqsk(skb, sk, attrs, sizeof(attrs));
>      bpf_sk_release(skc);
> 
> bpf_sk_assign_tcp_reqsk() takes skb, a listener sk, and struct
> bpf_tcp_req_attrs and allocates reqsk and configures it.  Then,
> bpf_sk_assign_tcp_reqsk() links reqsk with skb and the listener.
> 
> The notable thing here is that we do not hold refcnt for both reqsk
> and listener.  To differentiate that, we mark reqsk->syncookie, which
> is only used in TX for now.  So, if reqsk->syncookie is 1 in RX, it
> means that the reqsk is allocated by kfunc.
> 
> When skb is freed, sock_pfree() checks if reqsk->syncookie is 1,
> and in that case, we set NULL to reqsk->rsk_listener before calling
> reqsk_free() as reqsk does not hold a refcnt of the listener.
> 
> When the TCP stack looks up a socket from the skb, we steal the
> listener from the reqsk in skb_steal_sock() and create a full sk
> in cookie_v[46]_check().
> 
> The refcnt of reqsk will finally be set to 1 in tcp_get_cookie_sock()
> after creating a full sk.
> 
> Note that we can extend struct bpf_tcp_req_attrs in the future when
> we add a new attribute that is determined in 3WHS.

Notice a few final details.

> 
> Signed-off-by: Kuniyuki Iwashima <[email-masked]>
> ---
>   include/net/tcp.h |  13 ++++++
>   net/core/filter.c | 113 +++++++++++++++++++++++++++++++++++++++++++++-
>   net/core/sock.c   |  14 +++++-
>   3 files changed, 136 insertions(+), 4 deletions(-)
> 
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index a63916f41f77..20619df8819e 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -600,6 +600,19 @@ static inline bool cookie_ecn_ok(const struct net *net, const struct dst_entry *
>   }
>   
>   #if IS_ENABLED(CONFIG_BPF)
> +struct bpf_tcp_req_attrs {
> +	u32 rcv_tsval;
> +	u32 rcv_tsecr;
> +	u16 mss;
> +	u8 rcv_wscale;
> +	u8 snd_wscale;
> +	u8 ecn_ok;
> +	u8 wscale_ok;
> +	u8 sack_ok;
> +	u8 tstamp_ok;
> +	u8 usec_ts_ok;

Add "u8 reserved[3];" for the 3 bytes tail padding.

> +};
> +
>   static inline bool cookie_bpf_ok(struct sk_buff *skb)
>   {
>   	return skb->sk;
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 24061f29c9dd..961c2d30bd72 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -11837,6 +11837,105 @@ __bpf_kfunc int bpf_sock_addr_set_sun_path(struct bpf_sock_addr_kern *sa_kern,
>   
>   	return 0;
>   }
> +
> +__bpf_kfunc int bpf_sk_assign_tcp_reqsk(struct sk_buff *skb, struct sock *sk,
> +					struct bpf_tcp_req_attrs *attrs, int attrs__sz)
> +{
> +#if IS_ENABLED(CONFIG_SYN_COOKIES)
> +	const struct request_sock_ops *ops;
> +	struct inet_request_sock *ireq;
> +	struct tcp_request_sock *treq;
> +	struct request_sock *req;
> +	struct net *net;
> +	__u16 min_mss;
> +	u32 tsoff = 0;
> +
> +	if (attrs__sz != sizeof(*attrs))
> +		return -EINVAL;
> +
> +	if (!sk)
> +		return -EINVAL;
> +
> +	if (!skb_at_tc_ingress(skb))
> +		return -EINVAL;
> +
> +	net = dev_net(skb->dev);
> +	if (net != sock_net(sk))
> +		return -ENETUNREACH;
> +
> +	switch (skb->protocol) {
> +	case htons(ETH_P_IP):
> +		ops = &tcp_request_sock_ops;
> +		min_mss = 536;
> +		break;
> +#if IS_BUILTIN(CONFIG_IPV6)
> +	case htons(ETH_P_IPV6):
> +		ops = &tcp6_request_sock_ops;
> +		min_mss = IPV6_MIN_MTU - 60;
> +		break;
> +#endif
> +	default:
> +		return -EINVAL;
> +	}
> +
> +	if (sk->sk_type != SOCK_STREAM || sk->sk_state != TCP_LISTEN ||
> +	    sk_is_mptcp(sk))
> +		return -EINVAL;
> +

and check for:

	if (attrs->reserved[0] || attrs->reserved[1] || attrs->reserved[2])
		return -EINVAL;

It will be safer if it needs to extend "struct bpf_tcp_req_attrs". There is an 
existing example in __bpf_nf_ct_lookup() when checking the 'struct bpf_ct_opts 
*opts'.