On 10/13, Kuniyuki Iwashima wrote:
> This patch adds a new SOCK_OPS hook to validate arbitrary SYN Cookie.
>
> When the kernel receives ACK for SYN Cookie, the hook is invoked with
> bpf_sock_ops.op == BPF_SOCK_OPS_CHECK_SYNCOOKIE_CB if the listener has
> BPF_SOCK_OPS_SYNCOOKIE_CB_FLAG set by bpf_sock_ops_cb_flags_set().
>
> The BPF program can access the following information to validate ISN:
>
> bpf_sock_ops.sk : 4-tuple
> bpf_sock_ops.skb : TCP header
> bpf_sock_ops.args[0] : ISN
>
> The program must decode MSS and set it to bpf_sock_ops.replylong[0].
>
> By default, the kernel validates SYN Cookie before allocating reqsk, but
> the hook is invoked after allocating reqsk to keep the user interface
> consistent with BPF_SOCK_OPS_GEN_SYNCOOKIE_CB.
>
> Signed-off-by: Kuniyuki Iwashima <kuniyu@xxxxxxxxxx>
> ---
> include/net/tcp.h | 12 ++++++
> include/uapi/linux/bpf.h | 20 +++++++---
> net/ipv4/syncookies.c | 73 +++++++++++++++++++++++++++-------
> net/ipv6/syncookies.c | 44 +++++++++++++-------
> tools/include/uapi/linux/bpf.h | 20 +++++++---
> 5 files changed, 130 insertions(+), 39 deletions(-)
>
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index 676618c89bb7..90d95acdc34a 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -2158,6 +2158,18 @@ static inline __u32 cookie_init_sequence(const struct tcp_request_sock_ops *ops,
> __NET_INC_STATS(sock_net(sk), LINUX_MIB_SYNCOOKIESSENT);
> return ops->cookie_init_seq(skb, mss);
> }
> +
> +#ifdef CONFIG_CGROUP_BPF
> +int bpf_skops_cookie_check(struct sock *sk, struct request_sock *req,
> + struct sk_buff *skb);
> +#else
> +static inline int bpf_skops_cookie_check(struct sock *sk, struct request_sock *req,
> + struct sk_buff *skb)
> +{
> + return 0;
> +}
> +#endif
> +
> #else
> static inline __u32 cookie_init_sequence(const struct tcp_request_sock_ops *ops,
> const struct sock *sk, struct sk_buff *skb,
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index d3cc530613c0..e6f1507d7895 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -6738,13 +6738,16 @@ enum {
> * options first before the BPF program does.
> */
> BPF_SOCK_OPS_WRITE_HDR_OPT_CB_FLAG = (1<<6),
> - /* Call bpf when the kernel generates SYN Cookie (ISN) for SYN+ACK.
> + /* Call bpf when the kernel generates SYN Cookie (ISN) for SYN+ACK
> + * and validates ACK for SYN Cookie.
> *
> - * The bpf prog will be called to encode MSS into SYN Cookie with
> - * sock_ops->op == BPF_SOCK_OPS_GEN_SYNCOOKIE_CB.
> + * The bpf prog will be first called to encode MSS into SYN Cookie
> + * with sock_ops->op == BPF_SOCK_OPS_GEN_SYNCOOKIE_CB. Then, the
> + * bpf prog will be called to decode MSS from SYN Cookie with
> + * sock_ops->op == BPF_SOCK_OPS_CHECK_SYNCOOKIE_CB.
> *
> - * Please refer to the comment in BPF_SOCK_OPS_GEN_SYNCOOKIE_CB for
> - * input and output.
> + * Please refer to the comment in BPF_SOCK_OPS_GEN_SYNCOOKIE_CB and
> + * BPF_SOCK_OPS_CHECK_SYNCOOKIE_CB for input and output.
> */
> BPF_SOCK_OPS_SYNCOOKIE_CB_FLAG = (1<<7),
> /* Mask of all currently supported cb flags */
> @@ -6868,6 +6871,13 @@ enum {
> *
> * replylong[0]: ISN
> */
> + BPF_SOCK_OPS_CHECK_SYNCOOKIE_CB,/* Validate SYN Cookie and set
> + * MSS.
> + *
> + * args[0]: ISN
> + *
> + * replylong[0]: MSS
> + */
> };
>
> /* List of TCP states. There is a build check in net/ipv4/tcp.c to detect
> diff --git a/net/ipv4/syncookies.c b/net/ipv4/syncookies.c
> index 514f1a4abdee..b1dd415863ff 100644
> --- a/net/ipv4/syncookies.c
> +++ b/net/ipv4/syncookies.c
> @@ -317,6 +317,37 @@ struct request_sock *cookie_tcp_reqsk_alloc(const struct request_sock_ops *ops,
> }
> EXPORT_SYMBOL_GPL(cookie_tcp_reqsk_alloc);
>
> +#if IS_ENABLED(CONFIG_CGROUP_BPF) && IS_ENABLED(CONFIG_SYN_COOKIES)
> +int bpf_skops_cookie_check(struct sock *sk, struct request_sock *req, struct sk_buff *skb)
> +{
> + struct bpf_sock_ops_kern sock_ops;
> +
> + memset(&sock_ops, 0, offsetof(struct bpf_sock_ops_kern, temp));
> +
> + sock_ops.op = BPF_SOCK_OPS_CHECK_SYNCOOKIE_CB;
> + sock_ops.sk = req_to_sk(req);
> + sock_ops.args[0] = tcp_rsk(req)->snt_isn;
> +
> + bpf_skops_init_skb(&sock_ops, skb, tcp_hdrlen(skb));
> +
> + if (BPF_CGROUP_RUN_PROG_SOCK_OPS_SK(&sock_ops, sk))
> + goto err;
> +
> + if (!sock_ops.replylong[0])
> + goto err;
> +
> + __NET_INC_STATS(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);
I don't see LINUX_MIB_SYNCOOKIESSENT being incremented in the
previous patch, so maybe also don't touch the mib here? The bpf
program can do the counting if needed?
Or, alternatively, add LINUX_MIB_SYNCOOKIESSENT to
the BPF_SOCK_OPS_GEN_SYNCOOKIE_CB path?
