xt_tcpudp.c

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#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
#include <linux/types.h>
#include <linux/module.h>
#include <net/ip.h>
#include <linux/ipv6.h>
#include <net/ipv6.h>
#include <net/tcp.h>
#include <net/udp.h>
#include <linux/netfilter/x_tables.h>
#include <linux/netfilter/xt_tcpudp.h>
#include <linux/netfilter_ipv4/ip_tables.h>
#include <linux/netfilter_ipv6/ip6_tables.h>

MODULE_DESCRIPTION("Xtables: TCP, UDP and UDP-Lite match");
MODULE_LICENSE("GPL");
MODULE_ALIAS("xt_tcp");
MODULE_ALIAS("xt_udp");
MODULE_ALIAS("ipt_udp");
MODULE_ALIAS("ipt_tcp");
MODULE_ALIAS("ip6t_udp");
MODULE_ALIAS("ip6t_tcp");

/* Returns 1 if the port is matched by the range, 0 otherwise */
static inline bool
port_match(u_int16_t min, u_int16_t max, u_int16_t port, bool invert)
{
    return (port >= min && port <= max) ^ invert;
}

static bool
tcp_find_option(u_int8_t option,
        const struct sk_buff *skb,
        unsigned int protoff,
        unsigned int optlen,
        bool invert,
        bool *hotdrop)
{
    /* tcp.doff is only 4 bits, ie. max 15 * 4 bytes */
    const u_int8_t *op;
    u_int8_t _opt[60 - sizeof(struct tcphdr)];
    unsigned int i;

    pr_debug("finding option\n");

    if (!optlen)
        return invert;

    /* If we don't have the whole header, drop packet. */
    op = skb_header_pointer(skb, protoff + sizeof(struct tcphdr),
                optlen, _opt);
    if (op == NULL) {
        *hotdrop = true;
        return false;
    }

    for (i = 0; i < optlen; ) {
        if (op[i] == option) return !invert;
        if (op[i] < 2) i++;
        else i += op[i+1]?:1;
    }

    return invert;
}

static bool tcp_mt(const struct sk_buff *skb, struct xt_action_param *par)
{
    const struct tcphdr *th;
    struct tcphdr _tcph;
    const struct xt_tcp *tcpinfo = par->matchinfo;

    if (par->fragoff != 0) {
        /* To quote Alan:

           Don't allow a fragment of TCP 8 bytes in. Nobody normal
           causes this. Its a cracker trying to break in by doing a
           flag overwrite to pass the direction checks.
        */
        if (par->fragoff == 1) {
            pr_debug("Dropping evil TCP offset=1 frag.\n");
            par->hotdrop = true;
        }
        /* Must not be a fragment. */
        return false;
    }

#define FWINVTCP(bool, invflg) ((bool) ^ !!(tcpinfo->invflags & (invflg)))

    th = skb_header_pointer(skb, par->thoff, sizeof(_tcph), &_tcph);
    if (th == NULL) {
        /* We've been asked to examine this packet, and we
           can't.  Hence, no choice but to drop. */
        pr_debug("Dropping evil TCP offset=0 tinygram.\n");
        par->hotdrop = true;
        return false;
    }

    if (!port_match(tcpinfo->spts[0], tcpinfo->spts[1],
            ntohs(th->source),
            !!(tcpinfo->invflags & XT_TCP_INV_SRCPT)))
        return false;
    if (!port_match(tcpinfo->dpts[0], tcpinfo->dpts[1],
            ntohs(th->dest),
            !!(tcpinfo->invflags & XT_TCP_INV_DSTPT)))
        return false;
    if (!FWINVTCP((((unsigned char *)th)[13] & tcpinfo->flg_mask)
              == tcpinfo->flg_cmp,
              XT_TCP_INV_FLAGS))
        return false;
    if (tcpinfo->option) {
        if (th->doff * 4 < sizeof(_tcph)) {
            par->hotdrop = true;
            return false;
        }
        if (!tcp_find_option(tcpinfo->option, skb, par->thoff,
                     th->doff*4 - sizeof(_tcph),
                     tcpinfo->invflags & XT_TCP_INV_OPTION,
                     &par->hotdrop))
            return false;
    }
    return true;
}

static int tcp_mt_check(const struct xt_mtchk_param *par)
{
    const struct xt_tcp *tcpinfo = par->matchinfo;

    /* Must specify no unknown invflags */
    return (tcpinfo->invflags & ~XT_TCP_INV_MASK) ? -EINVAL : 0;
}

static bool udp_mt(const struct sk_buff *skb, struct xt_action_param *par)
{
    const struct udphdr *uh;
    struct udphdr _udph;
    const struct xt_udp *udpinfo = par->matchinfo;

    /* Must not be a fragment. */
    if (par->fragoff != 0)
        return false;

    uh = skb_header_pointer(skb, par->thoff, sizeof(_udph), &_udph);
    if (uh == NULL) {
        /* We've been asked to examine this packet, and we
           can't.  Hence, no choice but to drop. */
        pr_debug("Dropping evil UDP tinygram.\n");
        par->hotdrop = true;
        return false;
    }

    return port_match(udpinfo->spts[0], udpinfo->spts[1],
              ntohs(uh->source),
              !!(udpinfo->invflags & XT_UDP_INV_SRCPT))
        && port_match(udpinfo->dpts[0], udpinfo->dpts[1],
                  ntohs(uh->dest),
                  !!(udpinfo->invflags & XT_UDP_INV_DSTPT));
}

static int udp_mt_check(const struct xt_mtchk_param *par)
{
    const struct xt_udp *udpinfo = par->matchinfo;

    /* Must specify no unknown invflags */
    return (udpinfo->invflags & ~XT_UDP_INV_MASK) ? -EINVAL : 0;
}

static struct xt_match tcpudp_mt_reg[] __read_mostly = {
    {
        .name       = "tcp",
        .family     = NFPROTO_IPV4,
        .checkentry = tcp_mt_check,
        .match      = tcp_mt,
        .matchsize  = sizeof(struct xt_tcp),
        .proto      = IPPROTO_TCP,
        .me     = THIS_MODULE,
    },
    {
        .name       = "tcp",
        .family     = NFPROTO_IPV6,
        .checkentry = tcp_mt_check,
        .match      = tcp_mt,
        .matchsize  = sizeof(struct xt_tcp),
        .proto      = IPPROTO_TCP,
        .me     = THIS_MODULE,
    },
    {
        .name       = "udp",
        .family     = NFPROTO_IPV4,
        .checkentry = udp_mt_check,
        .match      = udp_mt,
        .matchsize  = sizeof(struct xt_udp),
        .proto      = IPPROTO_UDP,
        .me     = THIS_MODULE,
    },
    {
        .name       = "udp",
        .family     = NFPROTO_IPV6,
        .checkentry = udp_mt_check,
        .match      = udp_mt,
        .matchsize  = sizeof(struct xt_udp),
        .proto      = IPPROTO_UDP,
        .me     = THIS_MODULE,
    },
    {
        .name       = "udplite",
        .family     = NFPROTO_IPV4,
        .checkentry = udp_mt_check,
        .match      = udp_mt,
        .matchsize  = sizeof(struct xt_udp),
        .proto      = IPPROTO_UDPLITE,
        .me     = THIS_MODULE,
    },
    {
        .name       = "udplite",
        .family     = NFPROTO_IPV6,
        .checkentry = udp_mt_check,
        .match      = udp_mt,
        .matchsize  = sizeof(struct xt_udp),
        .proto      = IPPROTO_UDPLITE,
        .me     = THIS_MODULE,
    },
};

static int __init tcpudp_mt_init(void)
{
    return xt_register_matches(tcpudp_mt_reg, ARRAY_SIZE(tcpudp_mt_reg));
}

static void __exit tcpudp_mt_exit(void)
{
    xt_unregister_matches(tcpudp_mt_reg, ARRAY_SIZE(tcpudp_mt_reg));
}

module_init(tcpudp_mt_init);
module_exit(tcpudp_mt_exit);