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Commit 4f74fede authored by David Ahern's avatar David Ahern Committed by Daniel Borkmann
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bpf: Add mtu checking to FIB forwarding helper 



Add check that egress MTU can handle packet to be forwarded. If
the MTU is less than the packet length, return 0 meaning the
packet is expected to continue up the stack for help - eg.,
fragmenting the packet or sending an ICMP.

The XDP path needs to leverage the FIB entry for an MTU on the
route spec or an exception entry for a given destination. The
skb path lets is_skb_forwardable decide if the packet can be
sent.

Signed-off-by: default avatarDavid Ahern <dsahern@gmail.com>
Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
parent 901731b8

net/core/filter.c

+35 −7
Original line number Diff line number Diff line
@@ -4089,7 +4089,7 @@ static int bpf_fib_set_fwd_params(struct bpf_fib_lookup *params, 


#if IS_ENABLED(CONFIG_INET)

static int bpf_ipv4_fib_lookup(struct net *net, struct bpf_fib_lookup *params,

			       u32 flags)

			       u32 flags, bool check_mtu)

{

	struct in_device *in_dev;

	struct neighbour *neigh;
@@ -4098,6 +4098,7 @@ static int bpf_ipv4_fib_lookup(struct net *net, struct bpf_fib_lookup *params, 
	struct fib_nh *nh;

	struct flowi4 fl4;

	int err;

	u32 mtu;



	dev = dev_get_by_index_rcu(net, params->ifindex);

	if (unlikely(!dev))
@@ -4149,6 +4150,12 @@ static int bpf_ipv4_fib_lookup(struct net *net, struct bpf_fib_lookup *params, 
	if (res.fi->fib_nhs > 1)

		fib_select_path(net, &res, &fl4, NULL);



	if (check_mtu) {

		mtu = ip_mtu_from_fib_result(&res, params->ipv4_dst);

		if (params->tot_len > mtu)

			return 0;

	}



	nh = &res.fi->fib_nh[res.nh_sel];



	/* do not handle lwt encaps right now */
@@ -4177,7 +4184,7 @@ static int bpf_ipv4_fib_lookup(struct net *net, struct bpf_fib_lookup *params, 


#if IS_ENABLED(CONFIG_IPV6)

static int bpf_ipv6_fib_lookup(struct net *net, struct bpf_fib_lookup *params,

			       u32 flags)

			       u32 flags, bool check_mtu)

{

	struct in6_addr *src = (struct in6_addr *) params->ipv6_src;

	struct in6_addr *dst = (struct in6_addr *) params->ipv6_dst;
@@ -4188,6 +4195,7 @@ static int bpf_ipv6_fib_lookup(struct net *net, struct bpf_fib_lookup *params, 
	struct flowi6 fl6;

	int strict = 0;

	int oif;

	u32 mtu;



	/* link local addresses are never forwarded */

	if (rt6_need_strict(dst) || rt6_need_strict(src))
@@ -4250,6 +4258,12 @@ static int bpf_ipv6_fib_lookup(struct net *net, struct bpf_fib_lookup *params, 
						       fl6.flowi6_oif, NULL,

						       strict);



	if (check_mtu) {

		mtu = ipv6_stub->ip6_mtu_from_fib6(f6i, dst, src);

		if (params->tot_len > mtu)

			return 0;

	}



	if (f6i->fib6_nh.nh_lwtstate)

		return 0;
@@ -4282,12 +4296,12 @@ BPF_CALL_4(bpf_xdp_fib_lookup, struct xdp_buff *, ctx, 
#if IS_ENABLED(CONFIG_INET)

	case AF_INET:

		return bpf_ipv4_fib_lookup(dev_net(ctx->rxq->dev), params,

					   flags);

					   flags, true);

#endif

#if IS_ENABLED(CONFIG_IPV6)

	case AF_INET6:

		return bpf_ipv6_fib_lookup(dev_net(ctx->rxq->dev), params,

					   flags);

					   flags, true);

#endif

	}

	return 0;
@@ -4306,20 +4320,34 @@ static const struct bpf_func_proto bpf_xdp_fib_lookup_proto = { 
BPF_CALL_4(bpf_skb_fib_lookup, struct sk_buff *, skb,

	   struct bpf_fib_lookup *, params, int, plen, u32, flags)

{

	struct net *net = dev_net(skb->dev);

	int index = 0;



	if (plen < sizeof(*params))

		return -EINVAL;



	switch (params->family) {

#if IS_ENABLED(CONFIG_INET)

	case AF_INET:

		return bpf_ipv4_fib_lookup(dev_net(skb->dev), params, flags);

		index = bpf_ipv4_fib_lookup(net, params, flags, false);

		break;

#endif

#if IS_ENABLED(CONFIG_IPV6)

	case AF_INET6:

		return bpf_ipv6_fib_lookup(dev_net(skb->dev), params, flags);

		index = bpf_ipv6_fib_lookup(net, params, flags, false);

		break;

#endif

	}

	return -ENOTSUPP;



	if (index > 0) {

		struct net_device *dev;



		dev = dev_get_by_index_rcu(net, index);

		if (!is_skb_forwardable(dev, skb))

			index = 0;

	}



	return index;

}



static const struct bpf_func_proto bpf_skb_fib_lookup_proto = {