lwt_bpf.c

// SPDX-License-Identifier: GPL-2.0-only
/* Copyright (c) 2016 Thomas Graf <tgraf@tgraf.ch>
 */

#include <linux/filter.h>
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/skbuff.h>
#include <linux/types.h>
#include <linux/bpf.h>
#include <net/lwtunnel.h>
#include <net/gre.h>
#include <net/ip6_route.h>
#include <net/ipv6_stubs.h>

struct bpf_lwt_prog {
	struct bpf_prog *prog;
	char *name;
};

struct bpf_lwt {
	struct bpf_lwt_prog in;
	struct bpf_lwt_prog out;
	struct bpf_lwt_prog xmit;
	int family;
};

#define MAX_PROG_NAME 256

static inline struct bpf_lwt *bpf_lwt_lwtunnel(struct lwtunnel_state *lwt)
{
	return (struct bpf_lwt *)lwt->data;
}

#define NO_REDIRECT false
#define CAN_REDIRECT true

static int run_lwt_bpf(struct sk_buff *skb, struct bpf_lwt_prog *lwt,
		       struct dst_entry *dst, bool can_redirect)
{
	int ret;

	/* Migration disable and BH disable are needed to protect per-cpu
	 * redirect_info between BPF prog and skb_do_redirect().
	 */
	migrate_disable();
	local_bh_disable();
	bpf_compute_data_pointers(skb);
	ret = bpf_prog_run_save_cb(lwt->prog, skb);

	switch (ret) {
	case BPF_OK:
	case BPF_LWT_REROUTE:
		break;

	case BPF_REDIRECT:
		if (unlikely(!can_redirect)) {
			pr_warn_once("Illegal redirect return code in prog %s\n",
				     lwt->name ? : "<unknown>");
			ret = BPF_OK;
		} else {
			skb_reset_mac_header(skb);
			skb_do_redirect(skb);
			ret = BPF_REDIRECT;
		}
		break;

	case BPF_DROP:
		kfree_skb(skb);
		ret = -EPERM;
		break;

	default:
		pr_warn_once("bpf-lwt: Illegal return value %u, expect packet loss\n", ret);
		kfree_skb(skb);
		ret = -EINVAL;
		break;
	}

	local_bh_enable();
	migrate_enable();

	return ret;
}

static int bpf_lwt_input_reroute(struct sk_buff *skb)
{
	int err = -EINVAL;

	if (skb->protocol == htons(ETH_P_IP)) {
		struct net_device *dev = skb_dst(skb)->dev;
		struct iphdr *iph = ip_hdr(skb);

		dev_hold(dev);
		skb_dst_drop(skb);
		err = ip_route_input_noref(skb, iph->daddr, iph->saddr,
					   iph->tos, dev);
		dev_put(dev);
	} else if (skb->protocol == htons(ETH_P_IPV6)) {
		skb_dst_drop(skb);
		err = ipv6_stub->ipv6_route_input(skb);
	} else {
		err = -EAFNOSUPPORT;
	}

	if (err)
		goto err;
	return dst_input(skb);

err:
	kfree_skb(skb);
	return err;
}

static int bpf_input(struct sk_buff *skb)
{
	struct dst_entry *dst = skb_dst(skb);
	struct bpf_lwt *bpf;
	int ret;

	bpf = bpf_lwt_lwtunnel(dst->lwtstate);
	if (bpf->in.prog) {
		ret = run_lwt_bpf(skb, &bpf->in, dst, NO_REDIRECT);
		if (ret < 0)
			return ret;
		if (ret == BPF_LWT_REROUTE)
			return bpf_lwt_input_reroute(skb);
	}

	if (unlikely(!dst->lwtstate->orig_input)) {
		kfree_skb(skb);
		return -EINVAL;
	}

	return dst->lwtstate->orig_input(skb);
}

static int bpf_output(struct net *net, struct sock *sk, struct sk_buff *skb)
{
	struct dst_entry *dst = skb_dst(skb);
	struct bpf_lwt *bpf;
	int ret;

	bpf = bpf_lwt_lwtunnel(dst->lwtstate);
	if (bpf->out.prog) {
		ret = run_lwt_bpf(skb, &bpf->out, dst, NO_REDIRECT);
		if (ret < 0)
			return ret;
	}

	if (unlikely(!dst->lwtstate->orig_output)) {
		pr_warn_once("orig_output not set on dst for prog %s\n",
			     bpf->out.name);
		kfree_skb(skb);
		return -EINVAL;
	}

	return dst->lwtstate->orig_output(net, sk, skb);
}

static int xmit_check_hhlen(struct sk_buff *skb, int hh_len)
{
	if (skb_headroom(skb) < hh_len) {
		int nhead = HH_DATA_ALIGN(hh_len - skb_headroom(skb));

		if (pskb_expand_head(skb, nhead, 0, GFP_ATOMIC))
			return -ENOMEM;
	}

	return 0;
}

static int bpf_lwt_xmit_reroute(struct sk_buff *skb)
{
	struct net_device *l3mdev = l3mdev_master_dev_rcu(skb_dst(skb)->dev);
	int oif = l3mdev ? l3mdev->ifindex : 0;
	struct dst_entry *dst = NULL;
	int err = -EAFNOSUPPORT;
	struct sock *sk;
	struct net *net;
	bool ipv4;

	if (skb->protocol == htons(ETH_P_IP))
		ipv4 = true;
	else if (skb->protocol == htons(ETH_P_IPV6))
		ipv4 = false;
	else
		goto err;

	sk = sk_to_full_sk(skb->sk);
	if (sk) {
		if (sk->sk_bound_dev_if)
			oif = sk->sk_bound_dev_if;
		net = sock_net(sk);
	} else {
		net = dev_net(skb_dst(skb)->dev);
	}

	if (ipv4) {
		struct iphdr *iph = ip_hdr(skb);
		struct flowi4 fl4 = {};
		struct rtable *rt;

		fl4.flowi4_oif = oif;
		fl4.flowi4_mark = skb->mark;
		fl4.flowi4_uid = sock_net_uid(net, sk);
		fl4.flowi4_tos = RT_TOS(iph->tos);
		fl4.flowi4_flags = FLOWI_FLAG_ANYSRC;
		fl4.flowi4_proto = iph->protocol;
		fl4.daddr = iph->daddr;
		fl4.saddr = iph->saddr;

		rt = ip_route_output_key(net, &fl4);
		if (IS_ERR(rt)) {
			err = PTR_ERR(rt);
			goto err;
		}
		dst = &rt->dst;
	} else {
		struct ipv6hdr *iph6 = ipv6_hdr(skb);
		struct flowi6 fl6 = {};

		fl6.flowi6_oif = oif;
		fl6.flowi6_mark = skb->mark;
		fl6.flowi6_uid = sock_net_uid(net, sk);
		fl6.flowlabel = ip6_flowinfo(iph6);
		fl6.flowi6_proto = iph6->nexthdr;
		fl6.daddr = iph6->daddr;
		fl6.saddr = iph6->saddr;

		dst = ipv6_stub->ipv6_dst_lookup_flow(net, skb->sk, &fl6, NULL);
		if (IS_ERR(dst)) {
			err = PTR_ERR(dst);
			goto err;
		}
	}
	if (unlikely(dst->error)) {
		err = dst->error;
		dst_release(dst);
		goto err;
	}

	/* Although skb header was reserved in bpf_lwt_push_ip_encap(), it
	 * was done for the previous dst, so we are doing it here again, in
	 * case the new dst needs much more space. The call below is a noop
	 * if there is enough header space in skb.
	 */
	err = skb_cow_head(skb, LL_RESERVED_SPACE(dst->dev));
	if (unlikely(err))
		goto err;

	skb_dst_drop(skb);
	skb_dst_set(skb, dst);

	err = dst_output(dev_net(skb_dst(skb)->dev), skb->sk, skb);
	if (unlikely(err))
		return net_xmit_errno(err);

	/* ip[6]_finish_output2 understand LWTUNNEL_XMIT_DONE */
	return LWTUNNEL_XMIT_DONE;

err:
	kfree_skb(skb);
	return err;
}

static int bpf_xmit(struct sk_buff *skb)
{
	struct dst_entry *dst = skb_dst(skb);
	struct bpf_lwt *bpf;

	bpf = bpf_lwt_lwtunnel(dst->lwtstate);
	if (bpf->xmit.prog) {
		int hh_len = dst->dev->hard_header_len;
		__be16 proto = skb->protocol;
		int ret;

		ret = run_lwt_bpf(skb, &bpf->xmit, dst, CAN_REDIRECT);
		switch (ret) {
		case BPF_OK:
			/* If the header changed, e.g. via bpf_lwt_push_encap,
			 * BPF_LWT_REROUTE below should have been used if the
			 * protocol was also changed.
			 */
			if (skb->protocol != proto) {
				kfree_skb(skb);
				return -EINVAL;
			}
			/* If the header was expanded, headroom might be too
			 * small for L2 header to come, expand as needed.
			 */
			ret = xmit_check_hhlen(skb, hh_len);
			if (unlikely(ret))
				return ret;

			return LWTUNNEL_XMIT_CONTINUE;
		case BPF_REDIRECT:
			return LWTUNNEL_XMIT_DONE;
		case BPF_LWT_REROUTE:
			return bpf_lwt_xmit_reroute(skb);
		default:
			return ret;
		}
	}

	return LWTUNNEL_XMIT_CONTINUE;
}

static void bpf_lwt_prog_destroy(struct bpf_lwt_prog *prog)
{
	if (prog->prog)
		bpf_prog_put(prog->prog);

	kfree(prog->name);
}

static void bpf_destroy_state(struct lwtunnel_state *lwt)
{
	struct bpf_lwt *bpf = bpf_lwt_lwtunnel(lwt);

	bpf_lwt_prog_destroy(&bpf->in);
	bpf_lwt_prog_destroy(&bpf->out);
	bpf_lwt_prog_destroy(&bpf->xmit);
}

static const struct nla_policy bpf_prog_policy[LWT_BPF_PROG_MAX + 1] = {
	[LWT_BPF_PROG_FD]   = { .type = NLA_U32, },
	[LWT_BPF_PROG_NAME] = { .type = NLA_NUL_STRING,
				.len = MAX_PROG_NAME },
};

static int bpf_parse_prog(struct nlattr *attr, struct bpf_lwt_prog *prog,
			  enum bpf_prog_type type)
{
	struct nlattr *tb[LWT_BPF_PROG_MAX + 1];
	struct bpf_prog *p;
	int ret;
	u32 fd;

	ret = nla_parse_nested_deprecated(tb, LWT_BPF_PROG_MAX, attr,
					  bpf_prog_policy, NULL);
	if (ret < 0)
		return ret;

	if (!tb[LWT_BPF_PROG_FD] || !tb[LWT_BPF_PROG_NAME])
		return -EINVAL;

	prog->name = nla_memdup(tb[LWT_BPF_PROG_NAME], GFP_ATOMIC);
	if (!prog->name)
		return -ENOMEM;

	fd = nla_get_u32(tb[LWT_BPF_PROG_FD]);
	p = bpf_prog_get_type(fd, type);
	if (IS_ERR(p))
		return PTR_ERR(p);

	prog->prog = p;

	return 0;
}

static const struct nla_policy bpf_nl_policy[LWT_BPF_MAX + 1] = {
	[LWT_BPF_IN]		= { .type = NLA_NESTED, },
	[LWT_BPF_OUT]		= { .type = NLA_NESTED, },
	[LWT_BPF_XMIT]		= { .type = NLA_NESTED, },
	[LWT_BPF_XMIT_HEADROOM]	= { .type = NLA_U32 },
};

static int bpf_build_state(struct net *net, struct nlattr *nla,
			   unsigned int family, const void *cfg,
			   struct lwtunnel_state **ts,
			   struct netlink_ext_ack *extack)
{
	struct nlattr *tb[LWT_BPF_MAX + 1];
	struct lwtunnel_state *newts;
	struct bpf_lwt *bpf;
	int ret;

	if (family != AF_INET && family != AF_INET6)
		return -EAFNOSUPPORT;

	ret = nla_parse_nested_deprecated(tb, LWT_BPF_MAX, nla, bpf_nl_policy,
					  extack);
	if (ret < 0)
		return ret;

	if (!tb[LWT_BPF_IN] && !tb[LWT_BPF_OUT] && !tb[LWT_BPF_XMIT])
		return -EINVAL;

	newts = lwtunnel_state_alloc(sizeof(*bpf));
	if (!newts)
		return -ENOMEM;

	newts->type = LWTUNNEL_ENCAP_BPF;
	bpf = bpf_lwt_lwtunnel(newts);

	if (tb[LWT_BPF_IN]) {
		newts->flags |= LWTUNNEL_STATE_INPUT_REDIRECT;
		ret = bpf_parse_prog(tb[LWT_BPF_IN], &bpf->in,
				     BPF_PROG_TYPE_LWT_IN);
		if (ret  < 0)
			goto errout;
	}

	if (tb[LWT_BPF_OUT]) {
		newts->flags |= LWTUNNEL_STATE_OUTPUT_REDIRECT;
		ret = bpf_parse_prog(tb[LWT_BPF_OUT], &bpf->out,
				     BPF_PROG_TYPE_LWT_OUT);
		if (ret < 0)
			goto errout;
	}

	if (tb[LWT_BPF_XMIT]) {
		newts->flags |= LWTUNNEL_STATE_XMIT_REDIRECT;
		ret = bpf_parse_prog(tb[LWT_BPF_XMIT], &bpf->xmit,
				     BPF_PROG_TYPE_LWT_XMIT);
		if (ret < 0)
			goto errout;
	}

	if (tb[LWT_BPF_XMIT_HEADROOM]) {
		u32 headroom = nla_get_u32(tb[LWT_BPF_XMIT_HEADROOM]);

		if (headroom > LWT_BPF_MAX_HEADROOM) {
			ret = -ERANGE;
			goto errout;
		}

		newts->headroom = headroom;
	}

	bpf->family = family;
	*ts = newts;

	return 0;

errout:
	bpf_destroy_state(newts);
	kfree(newts);
	return ret;
}

static int bpf_fill_lwt_prog(struct sk_buff *skb, int attr,
			     struct bpf_lwt_prog *prog)
{
	struct nlattr *nest;

	if (!prog->prog)
		return 0;

	nest = nla_nest_start_noflag(skb, attr);
	if (!nest)
		return -EMSGSIZE;

	if (prog->name &&
	    nla_put_string(skb, LWT_BPF_PROG_NAME, prog->name))
		return -EMSGSIZE;

	return nla_nest_end(skb, nest);
}

static int bpf_fill_encap_info(struct sk_buff *skb, struct lwtunnel_state *lwt)
{
	struct bpf_lwt *bpf = bpf_lwt_lwtunnel(lwt);

	if (bpf_fill_lwt_prog(skb, LWT_BPF_IN, &bpf->in) < 0 ||
	    bpf_fill_lwt_prog(skb, LWT_BPF_OUT, &bpf->out) < 0 ||
	    bpf_fill_lwt_prog(skb, LWT_BPF_XMIT, &bpf->xmit) < 0)
		return -EMSGSIZE;

	return 0;
}

static int bpf_encap_nlsize(struct lwtunnel_state *lwtstate)
{
	int nest_len = nla_total_size(sizeof(struct nlattr)) +
		       nla_total_size(MAX_PROG_NAME) + /* LWT_BPF_PROG_NAME */
		       0;

	return nest_len + /* LWT_BPF_IN */
	       nest_len + /* LWT_BPF_OUT */
	       nest_len + /* LWT_BPF_XMIT */
	       0;
}

static int bpf_lwt_prog_cmp(struct bpf_lwt_prog *a, struct bpf_lwt_prog *b)
{
	/* FIXME:
	 * The LWT state is currently rebuilt for delete requests which
	 * results in a new bpf_prog instance. Comparing names for now.
	 */
	if (!a->name && !b->name)
		return 0;

	if (!a->name || !b->name)
		return 1;

	return strcmp(a->name, b->name);
}

static int bpf_encap_cmp(struct lwtunnel_state *a, struct lwtunnel_state *b)
{
	struct bpf_lwt *a_bpf = bpf_lwt_lwtunnel(a);
	struct bpf_lwt *b_bpf = bpf_lwt_lwtunnel(b);

	return bpf_lwt_prog_cmp(&a_bpf->in, &b_bpf->in) ||
	       bpf_lwt_prog_cmp(&a_bpf->out, &b_bpf->out) ||
	       bpf_lwt_prog_cmp(&a_bpf->xmit, &b_bpf->xmit);
}

static const struct lwtunnel_encap_ops bpf_encap_ops = {
	.build_state	= bpf_build_state,
	.destroy_state	= bpf_destroy_state,
	.input		= bpf_input,
	.output		= bpf_output,
	.xmit		= bpf_xmit,
	.fill_encap	= bpf_fill_encap_info,
	.get_encap_size = bpf_encap_nlsize,
	.cmp_encap	= bpf_encap_cmp,
	.owner		= THIS_MODULE,
};

static int handle_gso_type(struct sk_buff *skb, unsigned int gso_type,
			   int encap_len)
{
	struct skb_shared_info *shinfo = skb_shinfo(skb);

	gso_type |= SKB_GSO_DODGY;
	shinfo->gso_type |= gso_type;
	skb_decrease_gso_size(shinfo, encap_len);
	shinfo->gso_segs = 0;
	return 0;
}

static int handle_gso_encap(struct sk_buff *skb, bool ipv4, int encap_len)
{
	int next_hdr_offset;
	void *next_hdr;
	__u8 protocol;

	/* SCTP and UDP_L4 gso need more nuanced handling than what
	 * handle_gso_type() does above: skb_decrease_gso_size() is not enough.
	 * So at the moment only TCP GSO packets are let through.
	 */
	if (!(skb_shinfo(skb)->gso_type & (SKB_GSO_TCPV4 | SKB_GSO_TCPV6)))
		return -ENOTSUPP;

	if (ipv4) {
		protocol = ip_hdr(skb)->protocol;
		next_hdr_offset = sizeof(struct iphdr);
		next_hdr = skb_network_header(skb) + next_hdr_offset;
	} else {
		protocol = ipv6_hdr(skb)->nexthdr;
		next_hdr_offset = sizeof(struct ipv6hdr);
		next_hdr = skb_network_header(skb) + next_hdr_offset;
	}

	switch (protocol) {
	case IPPROTO_GRE:
		next_hdr_offset += sizeof(struct gre_base_hdr);
		if (next_hdr_offset > encap_len)
			return -EINVAL;

		if (((struct gre_base_hdr *)next_hdr)->flags & GRE_CSUM)
			return handle_gso_type(skb, SKB_GSO_GRE_CSUM,
					       encap_len);
		return handle_gso_type(skb, SKB_GSO_GRE, encap_len);

	case IPPROTO_UDP:
		next_hdr_offset += sizeof(struct udphdr);
		if (next_hdr_offset > encap_len)
			return -EINVAL;

		if (((struct udphdr *)next_hdr)->check)
			return handle_gso_type(skb, SKB_GSO_UDP_TUNNEL_CSUM,
					       encap_len);
		return handle_gso_type(skb, SKB_GSO_UDP_TUNNEL, encap_len);

	case IPPROTO_IP:
	case IPPROTO_IPV6:
		if (ipv4)
			return handle_gso_type(skb, SKB_GSO_IPXIP4, encap_len);
		else
			return handle_gso_type(skb, SKB_GSO_IPXIP6, encap_len);

	default:
		return -EPROTONOSUPPORT;
	}
}

int bpf_lwt_push_ip_encap(struct sk_buff *skb, void *hdr, u32 len, bool ingress)
{
	struct iphdr *iph;
	bool ipv4;
	int err;

	if (unlikely(len < sizeof(struct iphdr) || len > LWT_BPF_MAX_HEADROOM))
		return -EINVAL;

	/* validate protocol and length */
	iph = (struct iphdr *)hdr;
	if (iph->version == 4) {
		ipv4 = true;
		if (unlikely(len < iph->ihl * 4))
			return -EINVAL;
	} else if (iph->version == 6) {
		ipv4 = false;
		if (unlikely(len < sizeof(struct ipv6hdr)))
			return -EINVAL;
	} else {
		return -EINVAL;
	}

	if (ingress)
		err = skb_cow_head(skb, len + skb->mac_len);
	else
		err = skb_cow_head(skb,
				   len + LL_RESERVED_SPACE(skb_dst(skb)->dev));
	if (unlikely(err))
		return err;

	/* push the encap headers and fix pointers */
	skb_reset_inner_headers(skb);
	skb_reset_inner_mac_header(skb);  /* mac header is not yet set */
	skb_set_inner_protocol(skb, skb->protocol);
	skb->encapsulation = 1;
	skb_push(skb, len);
	if (ingress)
		skb_postpush_rcsum(skb, iph, len);
	skb_reset_network_header(skb);
	memcpy(skb_network_header(skb), hdr, len);
	bpf_compute_data_pointers(skb);
	skb_clear_hash(skb);

	if (ipv4) {
		skb->protocol = htons(ETH_P_IP);
		iph = ip_hdr(skb);

		if (!iph->check)
			iph->check = ip_fast_csum((unsigned char *)iph,
						  iph->ihl);
	} else {
		skb->protocol = htons(ETH_P_IPV6);
	}

	if (skb_is_gso(skb))
		return handle_gso_encap(skb, ipv4, len);

	return 0;
}

static int __init bpf_lwt_init(void)
{
	return lwtunnel_encap_add_ops(&bpf_encap_ops, LWTUNNEL_ENCAP_BPF);
}

subsys_initcall(bpf_lwt_init)