Commit 6578171a authored by Daniel Borkmann's avatar Daniel Borkmann Committed by David S. Miller
Browse files

bpf: add bpf_skb_change_proto helper 



This patch adds a minimal helper for doing the groundwork of changing
the skb->protocol in a controlled way. Currently supported is v4 to
v6 and vice versa transitions, which allows f.e. for a minimal, static
nat64 implementation where applications in containers that still
require IPv4 can be transparently operated in an IPv6-only environment.
For example, host facing veth of the container can transparently do
the transitions in a programmatic way with the help of clsact qdisc
and cls_bpf.

Idea is to separate concerns for keeping complexity of the helper
lower, which means that the programs utilize bpf_skb_change_proto(),
bpf_skb_store_bytes() and bpf_lX_csum_replace() to get the job done,
instead of doing everything in a single helper (and thus partially
duplicating helper functionality). Also, bpf_skb_change_proto()
shouldn't need to deal with raw packet data as this is done by other
helpers.

bpf_skb_proto_6_to_4() and bpf_skb_proto_4_to_6() unclone the skb to
operate on a private one, push or pop additionally required header
space and migrate the gso/gro meta data from the shared info. We do
mark the gso type as dodgy so that headers are checked and segs
recalculated by the gso/gro engine. The gso_size target is adapted
as well. The flags argument added is currently reserved and can be
used for future extensions.

Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
Acked-by: default avatarAlexei Starovoitov <ast@kernel.org>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 80b48c44

include/uapi/linux/bpf.h

+14 −0
Original line number Diff line number Diff line
@@ -313,6 +313,20 @@ enum bpf_func_id { 
	 */

	BPF_FUNC_skb_get_tunnel_opt,

	BPF_FUNC_skb_set_tunnel_opt,



	/**

	 * bpf_skb_change_proto(skb, proto, flags)

	 * Change protocol of the skb. Currently supported is

	 * v4 -> v6, v6 -> v4 transitions. The helper will also

	 * resize the skb. eBPF program is expected to fill the

	 * new headers via skb_store_bytes and lX_csum_replace.

	 * @skb: pointer to skb

	 * @proto: new skb->protocol type

	 * @flags: reserved

	 * Return: 0 on success or negative error

	 */

	BPF_FUNC_skb_change_proto,



	__BPF_FUNC_MAX_ID,

};

net/core/filter.c

+200 −0
Original line number Diff line number Diff line
@@ -1783,6 +1783,202 @@ const struct bpf_func_proto bpf_skb_vlan_pop_proto = { 
};

EXPORT_SYMBOL_GPL(bpf_skb_vlan_pop_proto);



static int bpf_skb_generic_push(struct sk_buff *skb, u32 off, u32 len)

{

	/* Caller already did skb_cow() with len as headroom,

	 * so no need to do it here.

	 */

	skb_push(skb, len);

	memmove(skb->data, skb->data + len, off);

	memset(skb->data + off, 0, len);



	/* No skb_postpush_rcsum(skb, skb->data + off, len)

	 * needed here as it does not change the skb->csum

	 * result for checksum complete when summing over

	 * zeroed blocks.

	 */

	return 0;

}



static int bpf_skb_generic_pop(struct sk_buff *skb, u32 off, u32 len)

{

	/* skb_ensure_writable() is not needed here, as we're

	 * already working on an uncloned skb.

	 */

	if (unlikely(!pskb_may_pull(skb, off + len)))

		return -ENOMEM;



	skb_postpull_rcsum(skb, skb->data + off, len);

	memmove(skb->data + len, skb->data, off);

	__skb_pull(skb, len);



	return 0;

}



static int bpf_skb_net_hdr_push(struct sk_buff *skb, u32 off, u32 len)

{

	bool trans_same = skb->transport_header == skb->network_header;

	int ret;



	/* There's no need for __skb_push()/__skb_pull() pair to

	 * get to the start of the mac header as we're guaranteed

	 * to always start from here under eBPF.

	 */

	ret = bpf_skb_generic_push(skb, off, len);

	if (likely(!ret)) {

		skb->mac_header -= len;

		skb->network_header -= len;

		if (trans_same)

			skb->transport_header = skb->network_header;

	}



	return ret;

}



static int bpf_skb_net_hdr_pop(struct sk_buff *skb, u32 off, u32 len)

{

	bool trans_same = skb->transport_header == skb->network_header;

	int ret;



	/* Same here, __skb_push()/__skb_pull() pair not needed. */

	ret = bpf_skb_generic_pop(skb, off, len);

	if (likely(!ret)) {

		skb->mac_header += len;

		skb->network_header += len;

		if (trans_same)

			skb->transport_header = skb->network_header;

	}



	return ret;

}



static int bpf_skb_proto_4_to_6(struct sk_buff *skb)

{

	const u32 len_diff = sizeof(struct ipv6hdr) - sizeof(struct iphdr);

	u32 off = skb->network_header - skb->mac_header;

	int ret;



	ret = skb_cow(skb, len_diff);

	if (unlikely(ret < 0))

		return ret;



	ret = bpf_skb_net_hdr_push(skb, off, len_diff);

	if (unlikely(ret < 0))

		return ret;



	if (skb_is_gso(skb)) {

		/* SKB_GSO_UDP stays as is. SKB_GSO_TCPV4 needs to

		 * be changed into SKB_GSO_TCPV6.

		 */

		if (skb_shinfo(skb)->gso_type & SKB_GSO_TCPV4) {

			skb_shinfo(skb)->gso_type &= ~SKB_GSO_TCPV4;

			skb_shinfo(skb)->gso_type |=  SKB_GSO_TCPV6;

		}



		/* Due to IPv6 header, MSS needs to be downgraded. */

		skb_shinfo(skb)->gso_size -= len_diff;

		/* Header must be checked, and gso_segs recomputed. */

		skb_shinfo(skb)->gso_type |= SKB_GSO_DODGY;

		skb_shinfo(skb)->gso_segs = 0;

	}



	skb->protocol = htons(ETH_P_IPV6);

	skb_clear_hash(skb);



	return 0;

}



static int bpf_skb_proto_6_to_4(struct sk_buff *skb)

{

	const u32 len_diff = sizeof(struct ipv6hdr) - sizeof(struct iphdr);

	u32 off = skb->network_header - skb->mac_header;

	int ret;



	ret = skb_unclone(skb, GFP_ATOMIC);

	if (unlikely(ret < 0))

		return ret;



	ret = bpf_skb_net_hdr_pop(skb, off, len_diff);

	if (unlikely(ret < 0))

		return ret;



	if (skb_is_gso(skb)) {

		/* SKB_GSO_UDP stays as is. SKB_GSO_TCPV6 needs to

		 * be changed into SKB_GSO_TCPV4.

		 */

		if (skb_shinfo(skb)->gso_type & SKB_GSO_TCPV6) {

			skb_shinfo(skb)->gso_type &= ~SKB_GSO_TCPV6;

			skb_shinfo(skb)->gso_type |=  SKB_GSO_TCPV4;

		}



		/* Due to IPv4 header, MSS can be upgraded. */

		skb_shinfo(skb)->gso_size += len_diff;

		/* Header must be checked, and gso_segs recomputed. */

		skb_shinfo(skb)->gso_type |= SKB_GSO_DODGY;

		skb_shinfo(skb)->gso_segs = 0;

	}



	skb->protocol = htons(ETH_P_IP);

	skb_clear_hash(skb);



	return 0;

}



static int bpf_skb_proto_xlat(struct sk_buff *skb, __be16 to_proto)

{

	__be16 from_proto = skb->protocol;



	if (from_proto == htons(ETH_P_IP) &&

	      to_proto == htons(ETH_P_IPV6))

		return bpf_skb_proto_4_to_6(skb);



	if (from_proto == htons(ETH_P_IPV6) &&

	      to_proto == htons(ETH_P_IP))

		return bpf_skb_proto_6_to_4(skb);



	return -ENOTSUPP;

}



static u64 bpf_skb_change_proto(u64 r1, u64 r2, u64 flags, u64 r4, u64 r5)

{

	struct sk_buff *skb = (struct sk_buff *) (long) r1;

	__be16 proto = (__force __be16) r2;

	int ret;



	if (unlikely(flags))

		return -EINVAL;



	/* General idea is that this helper does the basic groundwork

	 * needed for changing the protocol, and eBPF program fills the

	 * rest through bpf_skb_store_bytes(), bpf_lX_csum_replace()

	 * and other helpers, rather than passing a raw buffer here.

	 *

	 * The rationale is to keep this minimal and without a need to

	 * deal with raw packet data. F.e. even if we would pass buffers

	 * here, the program still needs to call the bpf_lX_csum_replace()

	 * helpers anyway. Plus, this way we keep also separation of

	 * concerns, since f.e. bpf_skb_store_bytes() should only take

	 * care of stores.

	 *

	 * Currently, additional options and extension header space are

	 * not supported, but flags register is reserved so we can adapt

	 * that. For offloads, we mark packet as dodgy, so that headers

	 * need to be verified first.

	 */

	ret = bpf_skb_proto_xlat(skb, proto);

	bpf_compute_data_end(skb);

	return ret;

}



static const struct bpf_func_proto bpf_skb_change_proto_proto = {

	.func		= bpf_skb_change_proto,

	.gpl_only	= false,

	.ret_type	= RET_INTEGER,

	.arg1_type	= ARG_PTR_TO_CTX,

	.arg2_type	= ARG_ANYTHING,

	.arg3_type	= ARG_ANYTHING,

};



bool bpf_helper_changes_skb_data(void *func)

{

	if (func == bpf_skb_vlan_push)
@@ -1791,6 +1987,8 @@ bool bpf_helper_changes_skb_data(void *func) 
		return true;

	if (func == bpf_skb_store_bytes)

		return true;

	if (func == bpf_skb_change_proto)

		return true;

	if (func == bpf_l3_csum_replace)

		return true;

	if (func == bpf_l4_csum_replace)
@@ -2078,6 +2276,8 @@ tc_cls_act_func_proto(enum bpf_func_id func_id) 
		return &bpf_skb_vlan_push_proto;

	case BPF_FUNC_skb_vlan_pop:

		return &bpf_skb_vlan_pop_proto;

	case BPF_FUNC_skb_change_proto:

		return &bpf_skb_change_proto_proto;

	case BPF_FUNC_skb_get_tunnel_key:

		return &bpf_skb_get_tunnel_key_proto;

	case BPF_FUNC_skb_set_tunnel_key:

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