On Tue, Mar 19, 2019 at 10:20:59AM +0000, Lorenz Bauer wrote:
> Using bpf_skc_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_skc_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 | 72 ++++++++++++++++++++++++++++++++++++++++
> 2 files changed, 89 insertions(+), 1 deletion(-)
>
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 8e4f8276942a..587d7a3295bf 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -2391,6 +2391,21 @@ union bpf_attr {
> * Pointer to **struct bpf_sock**, or **NULL** in case of failure.
> * For sockets with reuseport option, the **struct bpf_sock**
> * result is from **reuse->socks**\ [] using the hash of the tuple.
> + *
> + * int bpf_tcp_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), \
> @@ -2492,7 +2507,8 @@ union bpf_attr {
> FN(tcp_sock), \
> FN(skb_ecn_set_ce), \
> FN(get_listener_sock), \
> - FN(skc_lookup_tcp),
> + FN(skc_lookup_tcp), \
> + FN(tcp_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 f5210773cfd8..45a46fbe4ee0 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -5546,6 +5546,74 @@ static const struct bpf_func_proto bpf_skb_ecn_set_ce_proto = {
> .ret_type = RET_INTEGER,
> .arg1_type = ARG_PTR_TO_CTX,
> };
> +
> +BPF_CALL_5(bpf_tcp_check_syncookie, struct sock *, sk, void *, iph, u32, iph_len,
> + struct tcphdr *, th, u32, th_len)
what was the reason to pass ip and tcp header pointers separately?
Why not to pass 'void* iph' with len long enough to cover both?
the helper can compute tcp header and can take into account variable length ipv4 hdr
while checking that resulting ptr is within 'iph + len' validated by the verifier.
Last patch example is buggy in that sense, since it's doing:
tcph = data + sizeof(struct ethhdr) + sizeof(struct iphdr);
If we drop two args then there will be room for 'flags'.
It can be used for example to indicate whether LINUX_MIB_SYNCOOKIESFAILED
should be incremented or not.
May be it shold be unconditionally?
> +{
> +#ifdef CONFIG_SYN_COOKIES
> + 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)
> + return -EINVAL;
> +
> + if (!sock_net(sk)->ipv4.sysctl_tcp_syncookies)
> + return -EINVAL;
> +
> + if (!th->ack || th->rst || th->syn)
> + return -ENOENT;
> +
> + if (tcp_synq_no_recent_overflow(sk))
> + 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_BUILTIN(CONFIG_IPV6)
> + case AF_INET6:
> + if (unlikely(iph_len < sizeof(struct ipv6hdr)))
> + return -EINVAL;
> +
> + ret = __cookie_v6_check((struct ipv6hdr *)iph, th, cookie);
I guess this is fast path and you don't want indirect call via ipv6_bpf_stub ?
