nf_nat_helper.c

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/* nf_nat_helper.c - generic support functions for NAT helpers
 *
 * (C) 2000-2002 Harald Welte <[email protected]>
 * (C) 2003-2006 Netfilter Core Team <[email protected]>
 *
 * 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
 * published by the Free Software Foundation.
 */
#include <linux/module.h>
#include <linux/gfp.h>
#include <linux/types.h>
#include <linux/skbuff.h>
#include <linux/tcp.h>
#include <linux/udp.h>
#include <net/tcp.h>

#include <net/netfilter/nf_conntrack.h>
#include <net/netfilter/nf_conntrack_helper.h>
#include <net/netfilter/nf_conntrack_ecache.h>
#include <net/netfilter/nf_conntrack_expect.h>
#include <net/netfilter/nf_nat.h>
#include <net/netfilter/nf_nat_l3proto.h>
#include <net/netfilter/nf_nat_l4proto.h>
#include <net/netfilter/nf_nat_core.h>
#include <net/netfilter/nf_nat_helper.h>

#define DUMP_OFFSET(x) \
    pr_debug("offset_before=%d, offset_after=%d, correction_pos=%u\n", \
         x->offset_before, x->offset_after, x->correction_pos);

static DEFINE_SPINLOCK(nf_nat_seqofs_lock);

/* Setup TCP sequence correction given this change at this sequence */
static inline void
adjust_tcp_sequence(u32 seq,
            int sizediff,
            struct nf_conn *ct,
            enum ip_conntrack_info ctinfo)
{
    enum ip_conntrack_dir dir = CTINFO2DIR(ctinfo);
    struct nf_conn_nat *nat = nfct_nat(ct);
    struct nf_nat_seq *this_way = &nat->seq[dir];

    pr_debug("adjust_tcp_sequence: seq = %u, sizediff = %d\n",
         seq, sizediff);

    pr_debug("adjust_tcp_sequence: Seq_offset before: ");
    DUMP_OFFSET(this_way);

    spin_lock_bh(&nf_nat_seqofs_lock);

    /* SYN adjust. If it's uninitialized, or this is after last
     * correction, record it: we don't handle more than one
     * adjustment in the window, but do deal with common case of a
     * retransmit */
    if (this_way->offset_before == this_way->offset_after ||
        before(this_way->correction_pos, seq)) {
        this_way->correction_pos = seq;
        this_way->offset_before = this_way->offset_after;
        this_way->offset_after += sizediff;
    }
    spin_unlock_bh(&nf_nat_seqofs_lock);

    pr_debug("adjust_tcp_sequence: Seq_offset after: ");
    DUMP_OFFSET(this_way);
}

/* Get the offset value, for conntrack */
s16 nf_nat_get_offset(const struct nf_conn *ct,
              enum ip_conntrack_dir dir,
              u32 seq)
{
    struct nf_conn_nat *nat = nfct_nat(ct);
    struct nf_nat_seq *this_way;
    s16 offset;

    if (!nat)
        return 0;

    this_way = &nat->seq[dir];
    spin_lock_bh(&nf_nat_seqofs_lock);
    offset = after(seq, this_way->correction_pos)
         ? this_way->offset_after : this_way->offset_before;
    spin_unlock_bh(&nf_nat_seqofs_lock);

    return offset;
}

/* Frobs data inside this packet, which is linear. */
static void mangle_contents(struct sk_buff *skb,
                unsigned int dataoff,
                unsigned int match_offset,
                unsigned int match_len,
                const char *rep_buffer,
                unsigned int rep_len)
{
    unsigned char *data;

    BUG_ON(skb_is_nonlinear(skb));
    data = skb_network_header(skb) + dataoff;

    /* move post-replacement */
    memmove(data + match_offset + rep_len,
        data + match_offset + match_len,
        skb->tail - (skb->network_header + dataoff +
                 match_offset + match_len));

    /* insert data from buffer */
    memcpy(data + match_offset, rep_buffer, rep_len);

    /* update skb info */
    if (rep_len > match_len) {
        pr_debug("nf_nat_mangle_packet: Extending packet by "
             "%u from %u bytes\n", rep_len - match_len, skb->len);
        skb_put(skb, rep_len - match_len);
    } else {
        pr_debug("nf_nat_mangle_packet: Shrinking packet from "
             "%u from %u bytes\n", match_len - rep_len, skb->len);
        __skb_trim(skb, skb->len + rep_len - match_len);
    }

    if (nf_ct_l3num((struct nf_conn *)skb->nfct) == NFPROTO_IPV4) {
        /* fix IP hdr checksum information */
        ip_hdr(skb)->tot_len = htons(skb->len);
        ip_send_check(ip_hdr(skb));
    } else
        ipv6_hdr(skb)->payload_len =
            htons(skb->len - sizeof(struct ipv6hdr));
}

/* Unusual, but possible case. */
static int enlarge_skb(struct sk_buff *skb, unsigned int extra)
{
    if (skb->len + extra > 65535)
        return 0;

    if (pskb_expand_head(skb, 0, extra - skb_tailroom(skb), GFP_ATOMIC))
        return 0;

    return 1;
}

void nf_nat_set_seq_adjust(struct nf_conn *ct, enum ip_conntrack_info ctinfo,
               __be32 seq, s16 off)
{
    if (!off)
        return;
    set_bit(IPS_SEQ_ADJUST_BIT, &ct->status);
    adjust_tcp_sequence(ntohl(seq), off, ct, ctinfo);
    nf_conntrack_event_cache(IPCT_NATSEQADJ, ct);
}
EXPORT_SYMBOL_GPL(nf_nat_set_seq_adjust);

void nf_nat_tcp_seq_adjust(struct sk_buff *skb, struct nf_conn *ct,
               u32 ctinfo, int off)
{
    const struct tcphdr *th;

    if (nf_ct_protonum(ct) != IPPROTO_TCP)
        return;

    th = (struct tcphdr *)(skb_network_header(skb)+ ip_hdrlen(skb));
    nf_nat_set_seq_adjust(ct, ctinfo, th->seq, off);
}
EXPORT_SYMBOL_GPL(nf_nat_tcp_seq_adjust);

/* Generic function for mangling variable-length address changes inside
 * NATed TCP connections (like the PORT XXX,XXX,XXX,XXX,XXX,XXX
 * command in FTP).
 *
 * Takes care about all the nasty sequence number changes, checksumming,
 * skb enlargement, ...
 *
 * */
int __nf_nat_mangle_tcp_packet(struct sk_buff *skb,
                   struct nf_conn *ct,
                   enum ip_conntrack_info ctinfo,
                   unsigned int protoff,
                   unsigned int match_offset,
                   unsigned int match_len,
                   const char *rep_buffer,
                   unsigned int rep_len, bool adjust)
{
    const struct nf_nat_l3proto *l3proto;
    struct tcphdr *tcph;
    int oldlen, datalen;

    if (!skb_make_writable(skb, skb->len))
        return 0;

    if (rep_len > match_len &&
        rep_len - match_len > skb_tailroom(skb) &&
        !enlarge_skb(skb, rep_len - match_len))
        return 0;

    SKB_LINEAR_ASSERT(skb);

    tcph = (void *)skb->data + protoff;

    oldlen = skb->len - protoff;
    mangle_contents(skb, protoff + tcph->doff*4,
            match_offset, match_len, rep_buffer, rep_len);

    datalen = skb->len - protoff;

    l3proto = __nf_nat_l3proto_find(nf_ct_l3num(ct));
    l3proto->csum_recalc(skb, IPPROTO_TCP, tcph, &tcph->check,
                 datalen, oldlen);

    if (adjust && rep_len != match_len)
        nf_nat_set_seq_adjust(ct, ctinfo, tcph->seq,
                      (int)rep_len - (int)match_len);

    return 1;
}
EXPORT_SYMBOL(__nf_nat_mangle_tcp_packet);

/* Generic function for mangling variable-length address changes inside
 * NATed UDP connections (like the CONNECT DATA XXXXX MESG XXXXX INDEX XXXXX
 * command in the Amanda protocol)
 *
 * Takes care about all the nasty sequence number changes, checksumming,
 * skb enlargement, ...
 *
 * XXX - This function could be merged with nf_nat_mangle_tcp_packet which
 *       should be fairly easy to do.
 */
int
nf_nat_mangle_udp_packet(struct sk_buff *skb,
             struct nf_conn *ct,
             enum ip_conntrack_info ctinfo,
             unsigned int protoff,
             unsigned int match_offset,
             unsigned int match_len,
             const char *rep_buffer,
             unsigned int rep_len)
{
    const struct nf_nat_l3proto *l3proto;
    struct udphdr *udph;
    int datalen, oldlen;

    if (!skb_make_writable(skb, skb->len))
        return 0;

    if (rep_len > match_len &&
        rep_len - match_len > skb_tailroom(skb) &&
        !enlarge_skb(skb, rep_len - match_len))
        return 0;

    udph = (void *)skb->data + protoff;

    oldlen = skb->len - protoff;
    mangle_contents(skb, protoff + sizeof(*udph),
            match_offset, match_len, rep_buffer, rep_len);

    /* update the length of the UDP packet */
    datalen = skb->len - protoff;
    udph->len = htons(datalen);

    /* fix udp checksum if udp checksum was previously calculated */
    if (!udph->check && skb->ip_summed != CHECKSUM_PARTIAL)
        return 1;

    l3proto = __nf_nat_l3proto_find(nf_ct_l3num(ct));
    l3proto->csum_recalc(skb, IPPROTO_UDP, udph, &udph->check,
                 datalen, oldlen);

    return 1;
}
EXPORT_SYMBOL(nf_nat_mangle_udp_packet);

/* Adjust one found SACK option including checksum correction */
static void
sack_adjust(struct sk_buff *skb,
        struct tcphdr *tcph,
        unsigned int sackoff,
        unsigned int sackend,
        struct nf_nat_seq *natseq)
{
    while (sackoff < sackend) {
        struct tcp_sack_block_wire *sack;
        __be32 new_start_seq, new_end_seq;

        sack = (void *)skb->data + sackoff;
        if (after(ntohl(sack->start_seq) - natseq->offset_before,
              natseq->correction_pos))
            new_start_seq = htonl(ntohl(sack->start_seq)
                    - natseq->offset_after);
        else
            new_start_seq = htonl(ntohl(sack->start_seq)
                    - natseq->offset_before);

        if (after(ntohl(sack->end_seq) - natseq->offset_before,
              natseq->correction_pos))
            new_end_seq = htonl(ntohl(sack->end_seq)
                      - natseq->offset_after);
        else
            new_end_seq = htonl(ntohl(sack->end_seq)
                      - natseq->offset_before);

        pr_debug("sack_adjust: start_seq: %d->%d, end_seq: %d->%d\n",
             ntohl(sack->start_seq), new_start_seq,
             ntohl(sack->end_seq), new_end_seq);

        inet_proto_csum_replace4(&tcph->check, skb,
                     sack->start_seq, new_start_seq, 0);
        inet_proto_csum_replace4(&tcph->check, skb,
                     sack->end_seq, new_end_seq, 0);
        sack->start_seq = new_start_seq;
        sack->end_seq = new_end_seq;
        sackoff += sizeof(*sack);
    }
}

/* TCP SACK sequence number adjustment */
static inline unsigned int
nf_nat_sack_adjust(struct sk_buff *skb,
           unsigned int protoff,
           struct tcphdr *tcph,
           struct nf_conn *ct,
           enum ip_conntrack_info ctinfo)
{
    unsigned int dir, optoff, optend;
    struct nf_conn_nat *nat = nfct_nat(ct);

    optoff = protoff + sizeof(struct tcphdr);
    optend = protoff + tcph->doff * 4;

    if (!skb_make_writable(skb, optend))
        return 0;

    dir = CTINFO2DIR(ctinfo);

    while (optoff < optend) {
        /* Usually: option, length. */
        unsigned char *op = skb->data + optoff;

        switch (op[0]) {
        case TCPOPT_EOL:
            return 1;
        case TCPOPT_NOP:
            optoff++;
            continue;
        default:
            /* no partial options */
            if (optoff + 1 == optend ||
                optoff + op[1] > optend ||
                op[1] < 2)
                return 0;
            if (op[0] == TCPOPT_SACK &&
                op[1] >= 2+TCPOLEN_SACK_PERBLOCK &&
                ((op[1] - 2) % TCPOLEN_SACK_PERBLOCK) == 0)
                sack_adjust(skb, tcph, optoff+2,
                        optoff+op[1], &nat->seq[!dir]);
            optoff += op[1];
        }
    }
    return 1;
}

/* TCP sequence number adjustment.  Returns 1 on success, 0 on failure */
int
nf_nat_seq_adjust(struct sk_buff *skb,
          struct nf_conn *ct,
          enum ip_conntrack_info ctinfo,
          unsigned int protoff)
{
    struct tcphdr *tcph;
    int dir;
    __be32 newseq, newack;
    s16 seqoff, ackoff;
    struct nf_conn_nat *nat = nfct_nat(ct);
    struct nf_nat_seq *this_way, *other_way;

    dir = CTINFO2DIR(ctinfo);

    this_way = &nat->seq[dir];
    other_way = &nat->seq[!dir];

    if (!skb_make_writable(skb, protoff + sizeof(*tcph)))
        return 0;

    tcph = (void *)skb->data + protoff;
    if (after(ntohl(tcph->seq), this_way->correction_pos))
        seqoff = this_way->offset_after;
    else
        seqoff = this_way->offset_before;

    if (after(ntohl(tcph->ack_seq) - other_way->offset_before,
          other_way->correction_pos))
        ackoff = other_way->offset_after;
    else
        ackoff = other_way->offset_before;

    newseq = htonl(ntohl(tcph->seq) + seqoff);
    newack = htonl(ntohl(tcph->ack_seq) - ackoff);

    inet_proto_csum_replace4(&tcph->check, skb, tcph->seq, newseq, 0);
    inet_proto_csum_replace4(&tcph->check, skb, tcph->ack_seq, newack, 0);

    pr_debug("Adjusting sequence number from %u->%u, ack from %u->%u\n",
         ntohl(tcph->seq), ntohl(newseq), ntohl(tcph->ack_seq),
         ntohl(newack));

    tcph->seq = newseq;
    tcph->ack_seq = newack;

    return nf_nat_sack_adjust(skb, protoff, tcph, ct, ctinfo);
}

/* Setup NAT on this expected conntrack so it follows master. */
/* If we fail to get a free NAT slot, we'll get dropped on confirm */
void nf_nat_follow_master(struct nf_conn *ct,
              struct nf_conntrack_expect *exp)
{
    struct nf_nat_range range;

    /* This must be a fresh one. */
    BUG_ON(ct->status & IPS_NAT_DONE_MASK);

    /* Change src to where master sends to */
    range.flags = NF_NAT_RANGE_MAP_IPS;
    range.min_addr = range.max_addr
        = ct->master->tuplehash[!exp->dir].tuple.dst.u3;
    nf_nat_setup_info(ct, &range, NF_NAT_MANIP_SRC);

    /* For DST manip, map port here to where it's expected. */
    range.flags = (NF_NAT_RANGE_MAP_IPS | NF_NAT_RANGE_PROTO_SPECIFIED);
    range.min_proto = range.max_proto = exp->saved_proto;
    range.min_addr = range.max_addr
        = ct->master->tuplehash[!exp->dir].tuple.src.u3;
    nf_nat_setup_info(ct, &range, NF_NAT_MANIP_DST);
}
EXPORT_SYMBOL(nf_nat_follow_master);