Re: [PATCH bpf-next v10 1/5] bpf: add bpf_icmp_send kfunc

[Stanislav Fomichev <sdf.kernel@gmail.com>]

On 06/25, Mahe Tardy wrote:
> 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.
> 
> For normal cgroup_skb paths, the skb dst route should already be set.
> However, bpf_prog_test_run_skb can create synthetic IPv4 skbs without an
> attached route. In that case, icmp_send returns early, and the kfunc
> would otherwise report success despite no ICMP reply being sent. The
> check also rejects metadata dsts, which are not valid struct rtable
> instances. For IPv6, reject metadata dsts only: icmpv6_send can reach
> icmp6_dev, where skb_rt6_info treats any non-NULL skb dst as a struct
> rt6_info, which is not valid for metadata_dst.
> 
> Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
> Reviewed-by: Jordan Rife <jordan@jrife.io>
> Signed-off-by: Mahe Tardy <mahe.tardy@gmail.com>
> ---
>  net/core/filter.c | 95 +++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 95 insertions(+)
> 
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 2e96b4b847ce..0a0191586b44 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 +12549,88 @@ __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: No usable route/dst for the ICMP reply
> + *         -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 expects skb_dst to be a real rtable. */
> +		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;

[..]

> +		/* icmpv6_send may treat skb_dst as rt6_info. */
> +		if (skb_metadata_dst(skb))
> +			return -ENETUNREACH;

A bit confused about this. Which part of icmpv6_send treats skb_dst as rt6_info?
(I see the original sashiko report about dst, but icmp6 seems to be not
requiring it)