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netfilter: nft_payload: layer 4 checksum adjustment for pseudoheader fields

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This patch adds a new flag that signals the kernel to update layer 4
checksum if the packet field belongs to the layer 4 pseudoheader. This
implicitly provides stateless NAT 1:1 that is useful under very specific
usecases.

Since rules mangling layer 3 fields that are part of the pseudoheader
may potentially convey any layer 4 packet, we have to deal with the
layer 4 checksum adjustment using protocol specific code.

This patch adds support for TCP, UDP and ICMPv6, since they include the
pseudoheader in the layer 4 checksum calculation. ICMP doesn't, so we
can skip it.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
This commit is contained in:
Pablo Neira Ayuso 2016-11-24 12:04:55 +01:00
parent e0ffdbc78d
commit 1814096980
3 changed files with 109 additions and 5 deletions
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1
include/net/netfilter/nf_tables_core.h
View File

@ -45,6 +45,7 @@ struct nft_payload_set {
enum nft_registers sreg:8;
u8 csum_type;
u8 csum_offset;
u8 csum_flags;
};
extern const struct nft_expr_ops nft_payload_fast_ops;

6
include/uapi/linux/netfilter/nf_tables.h
View File

@ -659,6 +659,10 @@ enum nft_payload_csum_types {
NFT_PAYLOAD_CSUM_INET,
};
enum nft_payload_csum_flags {
NFT_PAYLOAD_L4CSUM_PSEUDOHDR = (1 << 0),
};
/**
* enum nft_payload_attributes - nf_tables payload expression netlink attributes
*
@ -669,6 +673,7 @@ enum nft_payload_csum_types {
* @NFTA_PAYLOAD_SREG: source register to load data from (NLA_U32: nft_registers)
* @NFTA_PAYLOAD_CSUM_TYPE: checksum type (NLA_U32)
* @NFTA_PAYLOAD_CSUM_OFFSET: checksum offset relative to base (NLA_U32)
* @NFTA_PAYLOAD_CSUM_FLAGS: checksum flags (NLA_U32)
*/
enum nft_payload_attributes {
NFTA_PAYLOAD_UNSPEC,
@ -679,6 +684,7 @@ enum nft_payload_attributes {
NFTA_PAYLOAD_SREG,
NFTA_PAYLOAD_CSUM_TYPE,
NFTA_PAYLOAD_CSUM_OFFSET,
NFTA_PAYLOAD_CSUM_FLAGS,
__NFTA_PAYLOAD_MAX
};
#define NFTA_PAYLOAD_MAX (__NFTA_PAYLOAD_MAX - 1)

107
net/netfilter/nft_payload.c
View File

@ -1,5 +1,6 @@
/*
* Copyright (c) 2008-2009 Patrick McHardy <kaber@trash.net>
* Copyright (c) 2016 Pablo Neira Ayuso <pablo@netfilter.org>
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License version 2 as
@ -17,6 +18,10 @@
#include <linux/netfilter/nf_tables.h>
#include <net/netfilter/nf_tables_core.h>
#include <net/netfilter/nf_tables.h>
/* For layer 4 checksum field offset. */
#include <linux/tcp.h>
#include <linux/udp.h>
#include <linux/icmpv6.h>
/* add vlan header into the user buffer for if tag was removed by offloads */
static bool
@ -164,6 +169,87 @@ const struct nft_expr_ops nft_payload_fast_ops = {
.dump = nft_payload_dump,
};
static inline void nft_csum_replace(__sum16 *sum, __wsum fsum, __wsum tsum)
{
*sum = csum_fold(csum_add(csum_sub(~csum_unfold(*sum), fsum), tsum));
if (*sum == 0)
*sum = CSUM_MANGLED_0;
}
static bool nft_payload_udp_checksum(struct sk_buff *skb, unsigned int thoff)
{
struct udphdr *uh, _uh;
uh = skb_header_pointer(skb, thoff, sizeof(_uh), &_uh);
if (!uh)
return false;
return uh->check;
}
static int nft_payload_l4csum_offset(const struct nft_pktinfo *pkt,
struct sk_buff *skb,
unsigned int *l4csum_offset)
{
switch (pkt->tprot) {
case IPPROTO_TCP:
*l4csum_offset = offsetof(struct tcphdr, check);
break;
case IPPROTO_UDP:
if (!nft_payload_udp_checksum(skb, pkt->xt.thoff))
return -1;
/* Fall through. */
case IPPROTO_UDPLITE:
*l4csum_offset = offsetof(struct udphdr, check);
break;
case IPPROTO_ICMPV6:
*l4csum_offset = offsetof(struct icmp6hdr, icmp6_cksum);
break;
default:
return -1;
}
*l4csum_offset += pkt->xt.thoff;
return 0;
}
static int nft_payload_l4csum_update(const struct nft_pktinfo *pkt,
struct sk_buff *skb,
__wsum fsum, __wsum tsum)
{
int l4csum_offset;
__sum16 sum;
/* If we cannot determine layer 4 checksum offset or this packet doesn't
* require layer 4 checksum recalculation, skip this packet.
*/
if (nft_payload_l4csum_offset(pkt, skb, &l4csum_offset) < 0)
return 0;
if (skb_copy_bits(skb, l4csum_offset, &sum, sizeof(sum)) < 0)
return -1;
/* Checksum mangling for an arbitrary amount of bytes, based on
* inet_proto_csum_replace*() functions.
*/
if (skb->ip_summed != CHECKSUM_PARTIAL) {
nft_csum_replace(&sum, fsum, tsum);
if (skb->ip_summed == CHECKSUM_COMPLETE) {
skb->csum = ~csum_add(csum_sub(~(skb->csum), fsum),
tsum);
}
} else {
sum = ~csum_fold(csum_add(csum_sub(csum_unfold(sum), fsum),
tsum));
}
if (!skb_make_writable(skb, l4csum_offset + sizeof(sum)) ||
skb_store_bits(skb, l4csum_offset, &sum, sizeof(sum)) < 0)
return -1;
return 0;
}
static void nft_payload_set_eval(const struct nft_expr *expr,
struct nft_regs *regs,
const struct nft_pktinfo *pkt)
@ -204,14 +290,15 @@ static void nft_payload_set_eval(const struct nft_expr *expr,
fsum = skb_checksum(skb, offset, priv->len, 0);
tsum = csum_partial(src, priv->len, 0);
sum = csum_fold(csum_add(csum_sub(~csum_unfold(sum), fsum),
tsum));
if (sum == 0)
sum = CSUM_MANGLED_0;
nft_csum_replace(&sum, fsum, tsum);
if (!skb_make_writable(skb, csum_offset + sizeof(sum)) ||
skb_store_bits(skb, csum_offset, &sum, sizeof(sum)) < 0)
goto err;
if (priv->csum_flags &&
nft_payload_l4csum_update(pkt, skb, fsum, tsum) < 0)
goto err;
}
if (!skb_make_writable(skb, max(offset + priv->len, 0)) ||
@ -240,6 +327,15 @@ static int nft_payload_set_init(const struct nft_ctx *ctx,
if (tb[NFTA_PAYLOAD_CSUM_OFFSET])
priv->csum_offset =
ntohl(nla_get_be32(tb[NFTA_PAYLOAD_CSUM_OFFSET]));
if (tb[NFTA_PAYLOAD_CSUM_FLAGS]) {
u32 flags;
flags = ntohl(nla_get_be32(tb[NFTA_PAYLOAD_CSUM_FLAGS]));
if (flags & ~NFT_PAYLOAD_L4CSUM_PSEUDOHDR)
return -EINVAL;
priv->csum_flags = flags;
}
switch (priv->csum_type) {
case NFT_PAYLOAD_CSUM_NONE:
@ -262,7 +358,8 @@ static int nft_payload_set_dump(struct sk_buff *skb, const struct nft_expr *expr
nla_put_be32(skb, NFTA_PAYLOAD_LEN, htonl(priv->len)) ||
nla_put_be32(skb, NFTA_PAYLOAD_CSUM_TYPE, htonl(priv->csum_type)) ||
nla_put_be32(skb, NFTA_PAYLOAD_CSUM_OFFSET,
htonl(priv->csum_offset)))
htonl(priv->csum_offset)) ||
nla_put_be32(skb, NFTA_PAYLOAD_CSUM_FLAGS, htonl(priv->csum_flags)))
goto nla_put_failure;
return 0;