On Fri, Feb 22, 2019 at 09:50:55AM +0000, Lorenz Bauer wrote:
> Using bpf_sk_lookup_tcp it's possible to ascertain whether a packet belongs
> to a known connection. However, there is one corner case: no sockets are
> created if SYN cookies are active. This means that the final ACK in the
> 3WHS is misclassified.
>
> Using the helper, we can look up the listening socket via bpf_sk_lookup_tcp
> and then check whether a packet is a valid SYN cookie ACK.
>
> Signed-off-by: Lorenz Bauer <lmb@xxxxxxxxxxxxxx>
> ---
> include/uapi/linux/bpf.h | 18 ++++++++++-
> net/core/filter.c | 68 ++++++++++++++++++++++++++++++++++++++++
> 2 files changed, 85 insertions(+), 1 deletion(-)
>
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index bcdd2474eee7..bc2af87e9621 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -2359,6 +2359,21 @@ union bpf_attr {
> * Return
> * A **struct bpf_tcp_sock** pointer on success, or NULL in
> * case of failure.
> + *
> + * int bpf_sk_check_syncookie(struct bpf_sock *sk, void *iph, u32 iph_len, struct tcphdr *th, u32 th_len)
> + * Description
> + * Check whether iph and th contain a valid SYN cookie ACK for
> + * the listening socket in sk.
> + *
> + * iph points to the start of the IPv4 or IPv6 header, while
> + * iph_len contains sizeof(struct iphdr) or sizeof(struct ip6hdr).
> + *
> + * th points to the start of the TCP header, while th_len contains
> + * sizeof(struct tcphdr).
> + *
> + * Return
> + * 0 if iph and th are a valid SYN cookie ACK, or a negative error
> + * otherwise.
> */
> #define __BPF_FUNC_MAPPER(FN) \
> FN(unspec), \
> @@ -2457,7 +2472,8 @@ union bpf_attr {
> FN(spin_lock), \
> FN(spin_unlock), \
> FN(sk_fullsock), \
> - FN(tcp_sock),
> + FN(tcp_sock), \
> + FN(sk_check_syncookie),
>
> /* integer value in 'imm' field of BPF_CALL instruction selects which helper
> * function eBPF program intends to call
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 85749f6ec789..9e68897cc7ed 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -5426,6 +5426,70 @@ static const struct bpf_func_proto bpf_tcp_sock_proto = {
> .arg1_type = ARG_PTR_TO_SOCK_COMMON,
> };
>
> +BPF_CALL_5(bpf_sk_check_syncookie, struct sock *, sk, void *, iph, u32, iph_len,
s/bpf_sk_check_syncookie/bpf_tcp_check_syncookie/
> + struct tcphdr *, th, u32, th_len)
> +{
> +#if IS_ENABLED(CONFIG_SYN_COOKIES)
nit. "#ifdef CONFIG_SYN_COOKIES" such that it is clear it is a bool kconfig.
> + u32 cookie;
> + int ret;
> +
> + if (unlikely(th_len < sizeof(*th)))
> + return -EINVAL;
> +
> + /* sk_listener() allows TCP_NEW_SYN_RECV, which makes no sense here. */
> + if (sk->sk_protocol != IPPROTO_TCP || sk->sk_state != TCP_LISTEN)
>From the test program in patch 3, the "sk" here is obtained from
bpf_sk_lookup_tcp() which does a sk_to_full_sk() before returning.
AFAICT, meaning bpf_sk_lookup_tcp() will return the listening sk
even if there is a request_sock. Does it make sense to check
syncookie if there is already a request_sock?
> + return -EINVAL;
> +
> + if (!sock_net(sk)->ipv4.sysctl_tcp_syncookies)
Should tcp_synq_no_recent_overflow(tp) be checked also?
> + return -EINVAL;
> +
> + if (!th->ack || th->rst)
How about th->syn?
> + return -ENOENT;
> +
> + cookie = ntohl(th->ack_seq) - 1;
> +
> + switch (sk->sk_family) {
> + case AF_INET:
> + if (unlikely(iph_len < sizeof(struct iphdr)))
> + return -EINVAL;
> +
> + ret = __cookie_v4_check((struct iphdr *)iph, th, cookie);
> + break;
> +
> +#if IS_ENABLED(CONFIG_IPV6)
> + case AF_INET6:
> + if (unlikely(iph_len < sizeof(struct ipv6hdr)))
> + return -EINVAL;
> +
> + ret = __cookie_v6_check((struct ipv6hdr *)iph, th, cookie);
> + break;
> +#endif /* CONFIG_IPV6 */
> +
> + default:
> + return -EPROTONOSUPPORT;
> + }
> +
> + if (ret > 0)
> + return 0;
> +
> + return -ENOENT;
> +#else
> + return -ENOTSUP;
> +#endif
> +}
> +
> +static const struct bpf_func_proto bpf_sk_check_syncookie_proto = {
> + .func = bpf_sk_check_syncookie,
> + .gpl_only = true,
> + .pkt_access = true,
> + .ret_type = RET_INTEGER,
> + .arg1_type = ARG_PTR_TO_SOCKET,
I think it should be ARG_PTR_TO_TCP_SOCK
> + .arg2_type = ARG_PTR_TO_MEM,
> + .arg3_type = ARG_CONST_SIZE,
> + .arg4_type = ARG_PTR_TO_MEM,
> + .arg5_type = ARG_CONST_SIZE,
> +};
> +
> #endif /* CONFIG_INET */
