[PATCH bpf-next v9 1/5] bpf: add bpf_icmp_send kfunc

[Mahe Tardy <mahe.tardy@gmail.com>]

This is needed in the context of Tetragon to provide improved feedback
(in contrast to just dropping packets) to east-west traffic when blocked
by policies using cgroup_skb programs.

This reuses concepts from netfilter reject target codepath with the
differences that:
* Packets are cloned since the BPF user can still let the packet pass
  (SK_PASS from the cgroup_skb progs for example) and the current skb
  need to stay untouched (cgroup_skb hooks only allow read-only skb
  payload).
* We protect against recursion since the kfunc, by generating an ICMP
  error message, could retrigger the BPF prog that invoked it.

Only ICMP_DEST_UNREACH and ICMPV6_DEST_UNREACH are currently supported.
The interface accepts a type parameter to facilitate future extension to
other ICMP control message types.

Signed-off-by: Mahe Tardy <mahe.tardy@gmail.com>
---
 net/core/filter.c | 91 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 91 insertions(+)

diff --git a/net/core/filter.c b/net/core/filter.c
index 2e96b4b847ce..f3aa494ed105 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -84,6 +84,9 @@
 #include <linux/un.h>
 #include <net/xdp_sock_drv.h>
 #include <net/inet_dscp.h>
+#include <linux/icmpv6.h>
+#include <net/icmp.h>
+#include <net/ip6_route.h>

 #include "dev.h"

@@ -12546,6 +12549,84 @@ __bpf_kfunc int bpf_xdp_pull_data(struct xdp_md *x, u32 len)
 	return 0;
 }

+/**
+ * bpf_icmp_send - Send an ICMP control message
+ * @skb_ctx: Packet that triggered the control message
+ * @type: ICMP type (only ICMP_DEST_UNREACH/ICMPV6_DEST_UNREACH supported)
+ * @code: ICMP code (0-15 except ICMP_FRAG_NEEDED for IPv4, 0-6 for IPv6)
+ *
+ * Sends an ICMP control message in response to the packet. The original packet
+ * is cloned before sending the ICMP message, so the BPF program can still let
+ * the packet pass if desired.
+ *
+ * Currently only ICMP_DEST_UNREACH (IPv4) and ICMPV6_DEST_UNREACH (IPv6) are
+ * supported.
+ *
+ * Return: 0 on success (send attempt), negative error code on failure:
+ *         -EBUSY: Recursion detected
+ *         -EPROTONOSUPPORT: Non-IP protocol
+ *         -EOPNOTSUPP: Unsupported ICMP type
+ *         -EINVAL: Invalid code parameter
+ *         -ENETUNREACH: Unusable IPv4 route/dst attached to the skb
+ *         -ENOMEM: Memory allocation failed
+ */
+__bpf_kfunc int bpf_icmp_send(struct __sk_buff *skb_ctx, int type, int code)
+{
+	struct sk_buff *skb = (struct sk_buff *)skb_ctx;
+	struct sk_buff *nskb;
+	struct sock *sk;
+
+	sk = skb_to_full_sk(skb);
+	if (sk && sk->sk_kern_sock &&
+	    (sk->sk_protocol == IPPROTO_ICMP || sk->sk_protocol == IPPROTO_ICMPV6))
+		return -EBUSY;
+
+	switch (skb->protocol) {
+#if IS_ENABLED(CONFIG_INET)
+	case htons(ETH_P_IP): {
+		if (type != ICMP_DEST_UNREACH)
+			return -EOPNOTSUPP;
+		if (code < 0 || code > NR_ICMP_UNREACH ||
+		    code == ICMP_FRAG_NEEDED) /* needs a valid next-hop MTU */
+			return -EINVAL;
+
+		/* icmp_send requires a rtable; test-run synthetic skbs lack one. */
+		if (!skb_valid_dst(skb))
+			return -ENETUNREACH;
+
+		nskb = skb_clone(skb, GFP_ATOMIC);
+		if (!nskb)
+			return -ENOMEM;
+
+		memset(IPCB(nskb), 0, sizeof(*IPCB(nskb)));
+		icmp_send(nskb, type, code, 0);
+		consume_skb(nskb);
+		break;
+	}
+#endif
+#if IS_ENABLED(CONFIG_IPV6)
+	case htons(ETH_P_IPV6):
+		if (type != ICMPV6_DEST_UNREACH)
+			return -EOPNOTSUPP;
+		if (code < 0 || code > ICMPV6_REJECT_ROUTE)
+			return -EINVAL;
+
+		nskb = skb_clone(skb, GFP_ATOMIC);
+		if (!nskb)
+			return -ENOMEM;
+
+		memset(IP6CB(nskb), 0, sizeof(*IP6CB(nskb)));
+		icmpv6_send(nskb, type, code, 0);
+		consume_skb(nskb);
+		break;
+#endif
+	default:
+		return -EPROTONOSUPPORT;
+	}
+
+	return 0;
+}
+
 __bpf_kfunc_end_defs();

 int bpf_dynptr_from_skb_rdonly(struct __sk_buff *skb, u64 flags,
@@ -12588,6 +12669,10 @@ BTF_KFUNCS_START(bpf_kfunc_check_set_sock_ops)
 BTF_ID_FLAGS(func, bpf_sock_ops_enable_tx_tstamp)
 BTF_KFUNCS_END(bpf_kfunc_check_set_sock_ops)

+BTF_KFUNCS_START(bpf_kfunc_check_set_icmp_send)
+BTF_ID_FLAGS(func, bpf_icmp_send)
+BTF_KFUNCS_END(bpf_kfunc_check_set_icmp_send)
+
 static const struct btf_kfunc_id_set bpf_kfunc_set_skb = {
 	.owner = THIS_MODULE,
 	.set = &bpf_kfunc_check_set_skb,
@@ -12618,6 +12703,11 @@ static const struct btf_kfunc_id_set bpf_kfunc_set_sock_ops = {
 	.set = &bpf_kfunc_check_set_sock_ops,
 };

+static const struct btf_kfunc_id_set bpf_kfunc_set_icmp_send = {
+	.owner = THIS_MODULE,
+	.set = &bpf_kfunc_check_set_icmp_send,
+};
+
 static int __init bpf_kfunc_init(void)
 {
 	int ret;
@@ -12639,6 +12729,7 @@ static int __init bpf_kfunc_init(void)
 	ret = ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_CGROUP_SOCK_ADDR,
 					       &bpf_kfunc_set_sock_addr);
 	ret = ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_SCHED_CLS, &bpf_kfunc_set_tcp_reqsk);
+	ret = ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_CGROUP_SKB, &bpf_kfunc_set_icmp_send);
 	return ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_SOCK_OPS, &bpf_kfunc_set_sock_ops);
 }
 late_initcall(bpf_kfunc_init);
--
2.34.1