bpf: add bpf_tcp_gen_syncookie helper

This helper function allows BPF programs to try to generate SYN
cookies, given a reference to a listener socket. The function works
from XDP and with an skb context since bpf_skc_lookup_tcp can lookup a
socket in both cases.

Signed-off-by: Petar Penkov <ppenkov@google.com>
Suggested-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Lorenz Bauer <lmb@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 73 insertions, 0 deletions

diff --git a/net/core/filter.c b/net/core/filter.c
index 1eee70f80fba..5a2707918629 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -5855,6 +5855,75 @@ static const struct bpf_func_proto bpf_tcp_check_syncookie_proto = {
 	.arg5_type	= ARG_CONST_SIZE,
 };
 
+BPF_CALL_5(bpf_tcp_gen_syncookie, struct sock *, sk, void *, iph, u32, iph_len,
+	   struct tcphdr *, th, u32, th_len)
+{
+#ifdef CONFIG_SYN_COOKIES
+	u32 cookie;
+	u16 mss;
+
+	if (unlikely(th_len < sizeof(*th) || th_len != th->doff * 4))
+		return -EINVAL;
+
+	if (sk->sk_protocol != IPPROTO_TCP || sk->sk_state != TCP_LISTEN)
+		return -EINVAL;
+
+	if (!sock_net(sk)->ipv4.sysctl_tcp_syncookies)
+		return -ENOENT;
+
+	if (!th->syn || th->ack || th->fin || th->rst)
+		return -EINVAL;
+
+	if (unlikely(iph_len < sizeof(struct iphdr)))
+		return -EINVAL;
+
+	/* Both struct iphdr and struct ipv6hdr have the version field at the
+	 * same offset so we can cast to the shorter header (struct iphdr).
+	 */
+	switch (((struct iphdr *)iph)->version) {
+	case 4:
+		if (sk->sk_family == AF_INET6 && sk->sk_ipv6only)
+			return -EINVAL;
+
+		mss = tcp_v4_get_syncookie(sk, iph, th, &cookie);
+		break;
+
+#if IS_BUILTIN(CONFIG_IPV6)
+	case 6:
+		if (unlikely(iph_len < sizeof(struct ipv6hdr)))
+			return -EINVAL;
+
+		if (sk->sk_family != AF_INET6)
+			return -EINVAL;
+
+		mss = tcp_v6_get_syncookie(sk, iph, th, &cookie);
+		break;
+#endif /* CONFIG_IPV6 */
+
+	default:
+		return -EPROTONOSUPPORT;
+	}
+	if (mss <= 0)
+		return -ENOENT;
+
+	return cookie | ((u64)mss << 32);
+#else
+	return -EOPNOTSUPP;
+#endif /* CONFIG_SYN_COOKIES */
+}
+
+static const struct bpf_func_proto bpf_tcp_gen_syncookie_proto = {
+	.func		= bpf_tcp_gen_syncookie,
+	.gpl_only	= true, /* __cookie_v*_init_sequence() is GPL */
+	.pkt_access	= true,
+	.ret_type	= RET_INTEGER,
+	.arg1_type	= ARG_PTR_TO_SOCK_COMMON,
+	.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 */
 
 bool bpf_helper_changes_pkt_data(void *func)
@@ -6144,6 +6213,8 @@ tc_cls_act_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 		return &bpf_tcp_check_syncookie_proto;
 	case BPF_FUNC_skb_ecn_set_ce:
 		return &bpf_skb_ecn_set_ce_proto;
+	case BPF_FUNC_tcp_gen_syncookie:
+		return &bpf_tcp_gen_syncookie_proto;
 #endif
 	default:
 		return bpf_base_func_proto(func_id);
@@ -6183,6 +6254,8 @@ xdp_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 		return &bpf_xdp_skc_lookup_tcp_proto;
 	case BPF_FUNC_tcp_check_syncookie:
 		return &bpf_tcp_check_syncookie_proto;
+	case BPF_FUNC_tcp_gen_syncookie:
+		return &bpf_tcp_gen_syncookie_proto;
 #endif
 	default:
 		return bpf_base_func_proto(func_id);