xt_osf.c

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/*
 * Copyright (c) 2003+ Evgeniy Polyakov <[email protected]>
 *
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 */
#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
#include <linux/module.h>
#include <linux/kernel.h>

#include <linux/if.h>
#include <linux/inetdevice.h>
#include <linux/ip.h>
#include <linux/list.h>
#include <linux/rculist.h>
#include <linux/skbuff.h>
#include <linux/slab.h>
#include <linux/tcp.h>

#include <net/ip.h>
#include <net/tcp.h>

#include <linux/netfilter/nfnetlink.h>
#include <linux/netfilter/x_tables.h>
#include <net/netfilter/nf_log.h>
#include <linux/netfilter/xt_osf.h>

struct xt_osf_finger {
    struct rcu_head         rcu_head;
    struct list_head        finger_entry;
    struct xt_osf_user_finger   finger;
};

enum osf_fmatch_states {
    /* Packet does not match the fingerprint */
    FMATCH_WRONG = 0,
    /* Packet matches the fingerprint */
    FMATCH_OK,
    /* Options do not match the fingerprint, but header does */
    FMATCH_OPT_WRONG,
};

/*
 * Indexed by dont-fragment bit.
 * It is the only constant value in the fingerprint.
 */
static struct list_head xt_osf_fingers[2];

static const struct nla_policy xt_osf_policy[OSF_ATTR_MAX + 1] = {
    [OSF_ATTR_FINGER]   = { .len = sizeof(struct xt_osf_user_finger) },
};

static int xt_osf_add_callback(struct sock *ctnl, struct sk_buff *skb,
                   const struct nlmsghdr *nlh,
                   const struct nlattr * const osf_attrs[])
{
    struct xt_osf_user_finger *f;
    struct xt_osf_finger *kf = NULL, *sf;
    int err = 0;

    if (!osf_attrs[OSF_ATTR_FINGER])
        return -EINVAL;

    if (!(nlh->nlmsg_flags & NLM_F_CREATE))
        return -EINVAL;

    f = nla_data(osf_attrs[OSF_ATTR_FINGER]);

    kf = kmalloc(sizeof(struct xt_osf_finger), GFP_KERNEL);
    if (!kf)
        return -ENOMEM;

    memcpy(&kf->finger, f, sizeof(struct xt_osf_user_finger));

    list_for_each_entry(sf, &xt_osf_fingers[!!f->df], finger_entry) {
        if (memcmp(&sf->finger, f, sizeof(struct xt_osf_user_finger)))
            continue;

        kfree(kf);
        kf = NULL;

        if (nlh->nlmsg_flags & NLM_F_EXCL)
            err = -EEXIST;
        break;
    }

    /*
     * We are protected by nfnl mutex.
     */
    if (kf)
        list_add_tail_rcu(&kf->finger_entry, &xt_osf_fingers[!!f->df]);

    return err;
}

static int xt_osf_remove_callback(struct sock *ctnl, struct sk_buff *skb,
                  const struct nlmsghdr *nlh,
                  const struct nlattr * const osf_attrs[])
{
    struct xt_osf_user_finger *f;
    struct xt_osf_finger *sf;
    int err = -ENOENT;

    if (!osf_attrs[OSF_ATTR_FINGER])
        return -EINVAL;

    f = nla_data(osf_attrs[OSF_ATTR_FINGER]);

    list_for_each_entry(sf, &xt_osf_fingers[!!f->df], finger_entry) {
        if (memcmp(&sf->finger, f, sizeof(struct xt_osf_user_finger)))
            continue;

        /*
         * We are protected by nfnl mutex.
         */
        list_del_rcu(&sf->finger_entry);
        kfree_rcu(sf, rcu_head);

        err = 0;
        break;
    }

    return err;
}

static const struct nfnl_callback xt_osf_nfnetlink_callbacks[OSF_MSG_MAX] = {
    [OSF_MSG_ADD]   = {
        .call       = xt_osf_add_callback,
        .attr_count = OSF_ATTR_MAX,
        .policy     = xt_osf_policy,
    },
    [OSF_MSG_REMOVE]    = {
        .call       = xt_osf_remove_callback,
        .attr_count = OSF_ATTR_MAX,
        .policy     = xt_osf_policy,
    },
};

static const struct nfnetlink_subsystem xt_osf_nfnetlink = {
    .name           = "osf",
    .subsys_id      = NFNL_SUBSYS_OSF,
    .cb_count       = OSF_MSG_MAX,
    .cb         = xt_osf_nfnetlink_callbacks,
};

static inline int xt_osf_ttl(const struct sk_buff *skb, const struct xt_osf_info *info,
                unsigned char f_ttl)
{
    const struct iphdr *ip = ip_hdr(skb);

    if (info->flags & XT_OSF_TTL) {
        if (info->ttl == XT_OSF_TTL_TRUE)
            return ip->ttl == f_ttl;
        if (info->ttl == XT_OSF_TTL_NOCHECK)
            return 1;
        else if (ip->ttl <= f_ttl)
            return 1;
        else {
            struct in_device *in_dev = __in_dev_get_rcu(skb->dev);
            int ret = 0;

            for_ifa(in_dev) {
                if (inet_ifa_match(ip->saddr, ifa)) {
                    ret = (ip->ttl == f_ttl);
                    break;
                }
            }
            endfor_ifa(in_dev);

            return ret;
        }
    }

    return ip->ttl == f_ttl;
}

static bool
xt_osf_match_packet(const struct sk_buff *skb, struct xt_action_param *p)
{
    const struct xt_osf_info *info = p->matchinfo;
    const struct iphdr *ip = ip_hdr(skb);
    const struct tcphdr *tcp;
    struct tcphdr _tcph;
    int fmatch = FMATCH_WRONG, fcount = 0;
    unsigned int optsize = 0, check_WSS = 0;
    u16 window, totlen, mss = 0;
    bool df;
    const unsigned char *optp = NULL, *_optp = NULL;
    unsigned char opts[MAX_IPOPTLEN];
    const struct xt_osf_finger *kf;
    const struct xt_osf_user_finger *f;

    if (!info)
        return false;

    tcp = skb_header_pointer(skb, ip_hdrlen(skb), sizeof(struct tcphdr), &_tcph);
    if (!tcp)
        return false;

    if (!tcp->syn)
        return false;

    totlen = ntohs(ip->tot_len);
    df = ntohs(ip->frag_off) & IP_DF;
    window = ntohs(tcp->window);

    if (tcp->doff * 4 > sizeof(struct tcphdr)) {
        optsize = tcp->doff * 4 - sizeof(struct tcphdr);

        _optp = optp = skb_header_pointer(skb, ip_hdrlen(skb) +
                sizeof(struct tcphdr), optsize, opts);
    }

    rcu_read_lock();
    list_for_each_entry_rcu(kf, &xt_osf_fingers[df], finger_entry) {
        f = &kf->finger;

        if (!(info->flags & XT_OSF_LOG) && strcmp(info->genre, f->genre))
            continue;

        optp = _optp;
        fmatch = FMATCH_WRONG;

        if (totlen == f->ss && xt_osf_ttl(skb, info, f->ttl)) {
            int foptsize, optnum;

            /*
             * Should not happen if userspace parser was written correctly.
             */
            if (f->wss.wc >= OSF_WSS_MAX)
                continue;

            /* Check options */

            foptsize = 0;
            for (optnum = 0; optnum < f->opt_num; ++optnum)
                foptsize += f->opt[optnum].length;

            if (foptsize > MAX_IPOPTLEN ||
                optsize > MAX_IPOPTLEN ||
                optsize != foptsize)
                continue;

            check_WSS = f->wss.wc;

            for (optnum = 0; optnum < f->opt_num; ++optnum) {
                if (f->opt[optnum].kind == (*optp)) {
                    __u32 len = f->opt[optnum].length;
                    const __u8 *optend = optp + len;
                    int loop_cont = 0;

                    fmatch = FMATCH_OK;

                    switch (*optp) {
                    case OSFOPT_MSS:
                        mss = optp[3];
                        mss <<= 8;
                        mss |= optp[2];

                        mss = ntohs((__force __be16)mss);
                        break;
                    case OSFOPT_TS:
                        loop_cont = 1;
                        break;
                    }

                    optp = optend;
                } else
                    fmatch = FMATCH_OPT_WRONG;

                if (fmatch != FMATCH_OK)
                    break;
            }

            if (fmatch != FMATCH_OPT_WRONG) {
                fmatch = FMATCH_WRONG;

                switch (check_WSS) {
                case OSF_WSS_PLAIN:
                    if (f->wss.val == 0 || window == f->wss.val)
                        fmatch = FMATCH_OK;
                    break;
                case OSF_WSS_MSS:
                    /*
                     * Some smart modems decrease mangle MSS to 
                     * SMART_MSS_2, so we check standard, decreased
                     * and the one provided in the fingerprint MSS
                     * values.
                     */
#define SMART_MSS_1 1460
#define SMART_MSS_2 1448
                    if (window == f->wss.val * mss ||
                        window == f->wss.val * SMART_MSS_1 ||
                        window == f->wss.val * SMART_MSS_2)
                        fmatch = FMATCH_OK;
                    break;
                case OSF_WSS_MTU:
                    if (window == f->wss.val * (mss + 40) ||
                        window == f->wss.val * (SMART_MSS_1 + 40) ||
                        window == f->wss.val * (SMART_MSS_2 + 40))
                        fmatch = FMATCH_OK;
                    break;
                case OSF_WSS_MODULO:
                    if ((window % f->wss.val) == 0)
                        fmatch = FMATCH_OK;
                    break;
                }
            }

            if (fmatch != FMATCH_OK)
                continue;

            fcount++;

            if (info->flags & XT_OSF_LOG)
                nf_log_packet(p->family, p->hooknum, skb,
                    p->in, p->out, NULL,
                    "%s [%s:%s] : %pI4:%d -> %pI4:%d hops=%d\n",
                    f->genre, f->version, f->subtype,
                    &ip->saddr, ntohs(tcp->source),
                    &ip->daddr, ntohs(tcp->dest),
                    f->ttl - ip->ttl);

            if ((info->flags & XT_OSF_LOG) &&
                info->loglevel == XT_OSF_LOGLEVEL_FIRST)
                break;
        }
    }
    rcu_read_unlock();

    if (!fcount && (info->flags & XT_OSF_LOG))
        nf_log_packet(p->family, p->hooknum, skb, p->in, p->out, NULL,
            "Remote OS is not known: %pI4:%u -> %pI4:%u\n",
                &ip->saddr, ntohs(tcp->source),
                &ip->daddr, ntohs(tcp->dest));

    if (fcount)
        fmatch = FMATCH_OK;

    return fmatch == FMATCH_OK;
}

static struct xt_match xt_osf_match = {
    .name       = "osf",
    .revision   = 0,
    .family     = NFPROTO_IPV4,
    .proto      = IPPROTO_TCP,
    .hooks          = (1 << NF_INET_LOCAL_IN) |
                (1 << NF_INET_PRE_ROUTING) |
                (1 << NF_INET_FORWARD),
    .match      = xt_osf_match_packet,
    .matchsize  = sizeof(struct xt_osf_info),
    .me     = THIS_MODULE,
};

static int __init xt_osf_init(void)
{
    int err = -EINVAL;
    int i;

    for (i=0; i<ARRAY_SIZE(xt_osf_fingers); ++i)
        INIT_LIST_HEAD(&xt_osf_fingers[i]);

    err = nfnetlink_subsys_register(&xt_osf_nfnetlink);
    if (err < 0) {
        pr_err("Failed to register OSF nsfnetlink helper (%d)\n", err);
        goto err_out_exit;
    }

    err = xt_register_match(&xt_osf_match);
    if (err) {
        pr_err("Failed to register OS fingerprint "
               "matching module (%d)\n", err);
        goto err_out_remove;
    }

    return 0;

err_out_remove:
    nfnetlink_subsys_unregister(&xt_osf_nfnetlink);
err_out_exit:
    return err;
}

static void __exit xt_osf_fini(void)
{
    struct xt_osf_finger *f;
    int i;

    nfnetlink_subsys_unregister(&xt_osf_nfnetlink);
    xt_unregister_match(&xt_osf_match);

    rcu_read_lock();
    for (i=0; i<ARRAY_SIZE(xt_osf_fingers); ++i) {

        list_for_each_entry_rcu(f, &xt_osf_fingers[i], finger_entry) {
            list_del_rcu(&f->finger_entry);
            kfree_rcu(f, rcu_head);
        }
    }
    rcu_read_unlock();

    rcu_barrier();
}

module_init(xt_osf_init);
module_exit(xt_osf_fini);

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Evgeniy Polyakov <[email protected]>");
MODULE_DESCRIPTION("Passive OS fingerprint matching.");
MODULE_ALIAS_NFNL_SUBSYS(NFNL_SUBSYS_OSF);