Date:   Fri, 18 May 2018 22:24:32 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Mathieu Xhonneux <m.xhonneux@...il.com>, netdev@...r.kernel.org
Cc:     dlebrun@...gle.com, alexei.starovoitov@...il.com
Subject: Re: [PATCH bpf-next v6 5/6] ipv6: sr: Add seg6local action End.BPF

On 05/17/2018 04:28 PM, Mathieu Xhonneux wrote:
> This patch adds the End.BPF action to the LWT seg6local infrastructure.
> This action works like any other seg6local End action, meaning that an IPv6
> header with SRH is needed, whose DA has to be equal to the SID of the
> action. It will also advance the SRH to the next segment, the BPF program
> does not have to take care of this.
> 
> Since the BPF program may not be a source of instability in the kernel, it
> is important to ensure that the integrity of the packet is maintained
> before yielding it back to the IPv6 layer. The hook hence keeps track if
> the SRH has been altered through the helpers, and re-validates its
> content if needed with seg6_validate_srh. The state kept for validation is
> stored in a per-CPU buffer. The BPF program is not allowed to directly
> write into the packet, and only some fields of the SRH can be altered
> through the helper bpf_lwt_seg6_store_bytes.
> 
> Performances profiling has shown that the SRH re-validation does not induce
> a significant overhead. If the altered SRH is deemed as invalid, the packet
> is dropped.
> 
> This validation is also done before executing any action through
> bpf_lwt_seg6_action, and will not be performed again if the SRH is not
> modified after calling the action.
> 
> The BPF program may return 3 types of return codes:
>     - BPF_OK: the End.BPF action will look up the next destination through
>              seg6_lookup_nexthop.
>     - BPF_REDIRECT: if an action has been executed through the
>           bpf_lwt_seg6_action helper, the BPF program should return this
>           value, as the skb's destination is already set and the default
>           lookup should not be performed.
>     - BPF_DROP : the packet will be dropped.
> 
> Signed-off-by: Mathieu Xhonneux <m.xhonneux@...il.com>
> Acked-by: David Lebrun <dlebrun@...gle.com>
[...]
>  static struct seg6_action_desc seg6_action_table[] = {
>  	{
>  		.action		= SEG6_LOCAL_ACTION_END,
> @@ -497,7 +568,13 @@ static struct seg6_action_desc seg6_action_table[] = {
>  		.attrs		= (1 << SEG6_LOCAL_SRH),
>  		.input		= input_action_end_b6_encap,
>  		.static_headroom	= sizeof(struct ipv6hdr),
> -	}
> +	},
> +	{
> +		.action		= SEG6_LOCAL_ACTION_END_BPF,
> +		.attrs		= (1 << SEG6_LOCAL_BPF),
> +		.input		= input_action_end_bpf,
> +	},
> +
>  };
>  
>  static struct seg6_action_desc *__get_action_desc(int action)
> @@ -542,6 +619,7 @@ static const struct nla_policy seg6_local_policy[SEG6_LOCAL_MAX + 1] = {
>  				    .len = sizeof(struct in6_addr) },
>  	[SEG6_LOCAL_IIF]	= { .type = NLA_U32 },
>  	[SEG6_LOCAL_OIF]	= { .type = NLA_U32 },
> +	[SEG6_LOCAL_BPF]	= { .type = NLA_NESTED },
>  };
>  
>  static int parse_nla_srh(struct nlattr **attrs, struct seg6_local_lwt *slwt)
> @@ -719,6 +797,71 @@ static int cmp_nla_oif(struct seg6_local_lwt *a, struct seg6_local_lwt *b)
>  	return 0;
>  }
>  
> +#define MAX_PROG_NAME 256
> +static const struct nla_policy bpf_prog_policy[LWT_BPF_PROG_MAX + 1] = {
> +	[LWT_BPF_PROG_FD]   = { .type = NLA_U32, },

>From UAPI point of view, I wouldn't name it LWT_BPF_PROG_FD but rather something like
LWT_BPF_PROG for example. That way, the setup can contain the fd number, but on the
dump you can put the prog->aux->id in there so that prog lookup can be done again.

> +	[LWT_BPF_PROG_NAME] = { .type = NLA_NUL_STRING,
> +				.len = MAX_PROG_NAME },
> +};
> +
> +static int parse_nla_bpf(struct nlattr **attrs, struct seg6_local_lwt *slwt)
> +{
> +	struct nlattr *tb[LWT_BPF_PROG_MAX + 1];
> +	struct bpf_prog *p;
> +	int ret;
> +	u32 fd;
> +
> +	ret = nla_parse_nested(tb, LWT_BPF_PROG_MAX, attrs[SEG6_LOCAL_BPF],
> +			       bpf_prog_policy, NULL);
> +	if (ret < 0)
> +		return ret;
> +
> +	if (!tb[LWT_BPF_PROG_FD] || !tb[LWT_BPF_PROG_NAME])
> +		return -EINVAL;
> +
> +	slwt->bpf.name = nla_memdup(tb[LWT_BPF_PROG_NAME], GFP_KERNEL);
> +	if (!slwt->bpf.name)
> +		return -ENOMEM;
> +
> +	fd = nla_get_u32(tb[LWT_BPF_PROG_FD]);
> +	p = bpf_prog_get_type(fd, BPF_PROG_TYPE_LWT_SEG6LOCAL);
> +	if (IS_ERR(p))
> +		return PTR_ERR(p);

Here in the above error path is definitely a bug in that you don't free the
prior allocated slwt->bpf.name from nla_memdup().

Also when you destroy the struct seg6_local_lwt object, what I'm not getting
is where you drop the prog reference again and free slwt->bpf.name there?

> +
> +	slwt->bpf.prog = p;
> +
> +	return 0;
> +}
> +
> +static int put_nla_bpf(struct sk_buff *skb, struct seg6_local_lwt *slwt)
> +{
> +	struct nlattr *nest;
> +
> +	if (!slwt->bpf.prog)
> +		return 0;
> +
> +	nest = nla_nest_start(skb, SEG6_LOCAL_BPF);
> +	if (!nest)
> +		return -EMSGSIZE;
> +
> +	if (slwt->bpf.name &&
> +	    nla_put_string(skb, LWT_BPF_PROG_NAME, slwt->bpf.name))
> +		return -EMSGSIZE;
> +
> +	return nla_nest_end(skb, nest);
> +}
> +
> +static int cmp_nla_bpf(struct seg6_local_lwt *a, struct seg6_local_lwt *b)
> +{
> +	if (!a->bpf.name && !b->bpf.name)
> +		return 0;
> +
> +	if (!a->bpf.name || !b->bpf.name)
> +		return 1;
> +
> +	return strcmp(a->bpf.name, b->bpf.name);
> +}
> +
>  struct seg6_action_param {
>  	int (*parse)(struct nlattr **attrs, struct seg6_local_lwt *slwt);
>  	int (*put)(struct sk_buff *skb, struct seg6_local_lwt *slwt);
> @@ -749,6 +892,11 @@ static struct seg6_action_param seg6_action_params[SEG6_LOCAL_MAX + 1] = {
>  	[SEG6_LOCAL_OIF]	= { .parse = parse_nla_oif,
>  				    .put = put_nla_oif,
>  				    .cmp = cmp_nla_oif },
> +
> +	[SEG6_LOCAL_BPF]	= { .parse = parse_nla_bpf,
> +				    .put = put_nla_bpf,
> +				    .cmp = cmp_nla_bpf },
> +
>  };
>  
>  static int parse_nla_action(struct nlattr **attrs, struct seg6_local_lwt *slwt)
> @@ -797,7 +945,6 @@ static int seg6_local_build_state(struct nlattr *nla, unsigned int family,
>  
>  	err = nla_parse_nested(tb, SEG6_LOCAL_MAX, nla, seg6_local_policy,
>  			       extack);
> -
>  	if (err < 0)
>  		return err;
>  
> @@ -886,6 +1033,11 @@ static int seg6_local_get_encap_size(struct lwtunnel_state *lwt)
>  	if (attrs & (1 << SEG6_LOCAL_OIF))
>  		nlsize += nla_total_size(4);
>  
> +	if (attrs & (1 << SEG6_LOCAL_BPF))
> +		nlsize += nla_total_size(sizeof(struct nlattr)) +
> +		       nla_total_size(MAX_PROG_NAME) +
> +		       nla_total_size(4);
> +
>  	return nlsize;
>  }
>  
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 3dbe217bf23e..a29fed1dfce2 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -1456,6 +1456,7 @@ static bool bpf_prog_type__needs_kver(enum bpf_prog_type type)
>  	case BPF_PROG_TYPE_LWT_IN:
>  	case BPF_PROG_TYPE_LWT_OUT:
>  	case BPF_PROG_TYPE_LWT_XMIT:
> +	case BPF_PROG_TYPE_LWT_SEG6LOCAL:
>  	case BPF_PROG_TYPE_SOCK_OPS:
>  	case BPF_PROG_TYPE_SK_SKB:
>  	case BPF_PROG_TYPE_CGROUP_DEVICE:
> 

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