[bpf-next,v3,4/8] bpf: add helper to check for a valid SYN cookie

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index fab05317f5e7..3c04410137d9 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -2448,6 +2448,21 @@  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),			\
@@ -2549,7 +2564,8 @@  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 b6d83ba97621..d2511fe46db3 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -5553,6 +5553,74 @@  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)
+{
+#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);
+		break;
+#endif /* CONFIG_IPV6 */
+
+	default:
+		return -EPROTONOSUPPORT;
+	}
+
+	if (ret > 0)
+		return 0;
+
+	return -ENOENT;
+#else
+	return -ENOTSUPP;
+#endif
+}
+
+static const struct bpf_func_proto bpf_tcp_check_syncookie_proto = {
+	.func		= bpf_tcp_check_syncookie,
+	.gpl_only	= true,
+	.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)
@@ -5815,6 +5883,8 @@  tc_cls_act_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 		return &bpf_get_listener_sock_proto;
 	case BPF_FUNC_skc_lookup_tcp:
 		return &bpf_skc_lookup_tcp_proto;
+	case BPF_FUNC_tcp_check_syncookie:
+		return &bpf_tcp_check_syncookie_proto;
 #endif
 	default:
 		return bpf_base_func_proto(func_id);
@@ -5852,6 +5922,8 @@  xdp_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 		return &bpf_sk_release_proto;
 	case BPF_FUNC_skc_lookup_tcp:
 		return &bpf_xdp_skc_lookup_tcp_proto;
+	case BPF_FUNC_tcp_check_syncookie:
+		return &bpf_tcp_check_syncookie_proto;
 #endif
 	default:
 		return bpf_base_func_proto(func_id);