netfilter.c

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/* IPv4 specific functions of netfilter core */
#include <linux/kernel.h>
#include <linux/netfilter.h>
#include <linux/netfilter_ipv4.h>
#include <linux/ip.h>
#include <linux/skbuff.h>
#include <linux/gfp.h>
#include <linux/export.h>
#include <net/route.h>
#include <net/xfrm.h>
#include <net/ip.h>
#include <net/netfilter/nf_queue.h>

/* route_me_harder function, used by iptable_nat, iptable_mangle + ip_queue */
int ip_route_me_harder(struct sk_buff *skb, unsigned int addr_type)
{
    struct net *net = dev_net(skb_dst(skb)->dev);
    const struct iphdr *iph = ip_hdr(skb);
    struct rtable *rt;
    struct flowi4 fl4 = {};
    __be32 saddr = iph->saddr;
    __u8 flags = skb->sk ? inet_sk_flowi_flags(skb->sk) : 0;
    unsigned int hh_len;

    if (addr_type == RTN_UNSPEC)
        addr_type = inet_addr_type(net, saddr);
    if (addr_type == RTN_LOCAL || addr_type == RTN_UNICAST)
        flags |= FLOWI_FLAG_ANYSRC;
    else
        saddr = 0;

    /* some non-standard hacks like ipt_REJECT.c:send_reset() can cause
     * packets with foreign saddr to appear on the NF_INET_LOCAL_OUT hook.
     */
    fl4.daddr = iph->daddr;
    fl4.saddr = saddr;
    fl4.flowi4_tos = RT_TOS(iph->tos);
    fl4.flowi4_oif = skb->sk ? skb->sk->sk_bound_dev_if : 0;
    fl4.flowi4_mark = skb->mark;
    fl4.flowi4_flags = flags;
    rt = ip_route_output_key(net, &fl4);
    if (IS_ERR(rt))
        return -1;

    /* Drop old route. */
    skb_dst_drop(skb);
    skb_dst_set(skb, &rt->dst);

    if (skb_dst(skb)->error)
        return -1;

#ifdef CONFIG_XFRM
    if (!(IPCB(skb)->flags & IPSKB_XFRM_TRANSFORMED) &&
        xfrm_decode_session(skb, flowi4_to_flowi(&fl4), AF_INET) == 0) {
        struct dst_entry *dst = skb_dst(skb);
        skb_dst_set(skb, NULL);
        dst = xfrm_lookup(net, dst, flowi4_to_flowi(&fl4), skb->sk, 0);
        if (IS_ERR(dst))
            return -1;
        skb_dst_set(skb, dst);
    }
#endif

    /* Change in oif may mean change in hh_len. */
    hh_len = skb_dst(skb)->dev->hard_header_len;
    if (skb_headroom(skb) < hh_len &&
        pskb_expand_head(skb, HH_DATA_ALIGN(hh_len - skb_headroom(skb)),
                0, GFP_ATOMIC))
        return -1;

    return 0;
}
EXPORT_SYMBOL(ip_route_me_harder);

/*
 * Extra routing may needed on local out, as the QUEUE target never
 * returns control to the table.
 */

struct ip_rt_info {
    __be32 daddr;
    __be32 saddr;
    u_int8_t tos;
    u_int32_t mark;
};

static void nf_ip_saveroute(const struct sk_buff *skb,
                struct nf_queue_entry *entry)
{
    struct ip_rt_info *rt_info = nf_queue_entry_reroute(entry);

    if (entry->hook == NF_INET_LOCAL_OUT) {
        const struct iphdr *iph = ip_hdr(skb);

        rt_info->tos = iph->tos;
        rt_info->daddr = iph->daddr;
        rt_info->saddr = iph->saddr;
        rt_info->mark = skb->mark;
    }
}

static int nf_ip_reroute(struct sk_buff *skb,
             const struct nf_queue_entry *entry)
{
    const struct ip_rt_info *rt_info = nf_queue_entry_reroute(entry);

    if (entry->hook == NF_INET_LOCAL_OUT) {
        const struct iphdr *iph = ip_hdr(skb);

        if (!(iph->tos == rt_info->tos &&
              skb->mark == rt_info->mark &&
              iph->daddr == rt_info->daddr &&
              iph->saddr == rt_info->saddr))
            return ip_route_me_harder(skb, RTN_UNSPEC);
    }
    return 0;
}

__sum16 nf_ip_checksum(struct sk_buff *skb, unsigned int hook,
                unsigned int dataoff, u_int8_t protocol)
{
    const struct iphdr *iph = ip_hdr(skb);
    __sum16 csum = 0;

    switch (skb->ip_summed) {
    case CHECKSUM_COMPLETE:
        if (hook != NF_INET_PRE_ROUTING && hook != NF_INET_LOCAL_IN)
            break;
        if ((protocol == 0 && !csum_fold(skb->csum)) ||
            !csum_tcpudp_magic(iph->saddr, iph->daddr,
                       skb->len - dataoff, protocol,
                       skb->csum)) {
            skb->ip_summed = CHECKSUM_UNNECESSARY;
            break;
        }
        /* fall through */
    case CHECKSUM_NONE:
        if (protocol == 0)
            skb->csum = 0;
        else
            skb->csum = csum_tcpudp_nofold(iph->saddr, iph->daddr,
                               skb->len - dataoff,
                               protocol, 0);
        csum = __skb_checksum_complete(skb);
    }
    return csum;
}
EXPORT_SYMBOL(nf_ip_checksum);

static __sum16 nf_ip_checksum_partial(struct sk_buff *skb, unsigned int hook,
                      unsigned int dataoff, unsigned int len,
                      u_int8_t protocol)
{
    const struct iphdr *iph = ip_hdr(skb);
    __sum16 csum = 0;

    switch (skb->ip_summed) {
    case CHECKSUM_COMPLETE:
        if (len == skb->len - dataoff)
            return nf_ip_checksum(skb, hook, dataoff, protocol);
        /* fall through */
    case CHECKSUM_NONE:
        skb->csum = csum_tcpudp_nofold(iph->saddr, iph->daddr, protocol,
                           skb->len - dataoff, 0);
        skb->ip_summed = CHECKSUM_NONE;
        return __skb_checksum_complete_head(skb, dataoff + len);
    }
    return csum;
}

static int nf_ip_route(struct net *net, struct dst_entry **dst,
               struct flowi *fl, bool strict __always_unused)
{
    struct rtable *rt = ip_route_output_key(net, &fl->u.ip4);
    if (IS_ERR(rt))
        return PTR_ERR(rt);
    *dst = &rt->dst;
    return 0;
}

static const struct nf_afinfo nf_ip_afinfo = {
    .family         = AF_INET,
    .checksum       = nf_ip_checksum,
    .checksum_partial   = nf_ip_checksum_partial,
    .route          = nf_ip_route,
    .saveroute      = nf_ip_saveroute,
    .reroute        = nf_ip_reroute,
    .route_key_size     = sizeof(struct ip_rt_info),
};

static int __init ipv4_netfilter_init(void)
{
    return nf_register_afinfo(&nf_ip_afinfo);
}

static void __exit ipv4_netfilter_fini(void)
{
    nf_unregister_afinfo(&nf_ip_afinfo);
}

module_init(ipv4_netfilter_init);
module_exit(ipv4_netfilter_fini);