Merge tag 'driver-core-5.11-rc5' of git://git.kernel.org/pub/scm/linux/kernel/git...

[mirror_ubuntu-hirsute-kernel.git] / net / netfilter / nf_conntrack_pptp.c

// SPDX-License-Identifier: GPL-2.0-only
/*
 * Connection tracking support for PPTP (Point to Point Tunneling Protocol).
 * PPTP is a protocol for creating virtual private networks.
 * It is a specification defined by Microsoft and some vendors
 * working with Microsoft.  PPTP is built on top of a modified
 * version of the Internet Generic Routing Encapsulation Protocol.
 * GRE is defined in RFC 1701 and RFC 1702.  Documentation of
 * PPTP can be found in RFC 2637
 *
 * (C) 2000-2005 by Harald Welte <laforge@gnumonks.org>
 *
 * Development of this code funded by Astaro AG (http://www.astaro.com/)
 *
 * (C) 2006-2012 Patrick McHardy <kaber@trash.net>
 *
 * Limitations:
 *       - We blindly assume that control connections are always
 *         established in PNS->PAC direction.  This is a violation
 *         of RFC 2637
 *       - We can only support one single call within each session
 * TODO:
 *       - testing of incoming PPTP calls
 */

#include <linux/module.h>
#include <linux/skbuff.h>
#include <linux/in.h>
#include <linux/tcp.h>

#include <net/netfilter/nf_conntrack.h>
#include <net/netfilter/nf_conntrack_core.h>
#include <net/netfilter/nf_conntrack_helper.h>
#include <net/netfilter/nf_conntrack_zones.h>
#include <linux/netfilter/nf_conntrack_proto_gre.h>
#include <linux/netfilter/nf_conntrack_pptp.h>

#define NF_CT_PPTP_VERSION "3.1"

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Harald Welte <laforge@gnumonks.org>");
MODULE_DESCRIPTION("Netfilter connection tracking helper module for PPTP");
MODULE_ALIAS("ip_conntrack_pptp");
MODULE_ALIAS_NFCT_HELPER("pptp");

static DEFINE_SPINLOCK(nf_pptp_lock);

int
(*nf_nat_pptp_hook_outbound)(struct sk_buff *skb,
                             struct nf_conn *ct, enum ip_conntrack_info ctinfo,
                             unsigned int protoff, struct PptpControlHeader *ctlh,
                             union pptp_ctrl_union *pptpReq) __read_mostly;
EXPORT_SYMBOL_GPL(nf_nat_pptp_hook_outbound);

int
(*nf_nat_pptp_hook_inbound)(struct sk_buff *skb,
                            struct nf_conn *ct, enum ip_conntrack_info ctinfo,
                            unsigned int protoff, struct PptpControlHeader *ctlh,
                            union pptp_ctrl_union *pptpReq) __read_mostly;
EXPORT_SYMBOL_GPL(nf_nat_pptp_hook_inbound);

void
(*nf_nat_pptp_hook_exp_gre)(struct nf_conntrack_expect *expect_orig,
                            struct nf_conntrack_expect *expect_reply)
                            __read_mostly;
EXPORT_SYMBOL_GPL(nf_nat_pptp_hook_exp_gre);

void
(*nf_nat_pptp_hook_expectfn)(struct nf_conn *ct,
                             struct nf_conntrack_expect *exp) __read_mostly;
EXPORT_SYMBOL_GPL(nf_nat_pptp_hook_expectfn);

#if defined(DEBUG) || defined(CONFIG_DYNAMIC_DEBUG)
/* PptpControlMessageType names */
static const char *const pptp_msg_name_array[PPTP_MSG_MAX + 1] = {
        [0]                             = "UNKNOWN_MESSAGE",
        [PPTP_START_SESSION_REQUEST]    = "START_SESSION_REQUEST",
        [PPTP_START_SESSION_REPLY]      = "START_SESSION_REPLY",
        [PPTP_STOP_SESSION_REQUEST]     = "STOP_SESSION_REQUEST",
        [PPTP_STOP_SESSION_REPLY]       = "STOP_SESSION_REPLY",
        [PPTP_ECHO_REQUEST]             = "ECHO_REQUEST",
        [PPTP_ECHO_REPLY]               = "ECHO_REPLY",
        [PPTP_OUT_CALL_REQUEST]         = "OUT_CALL_REQUEST",
        [PPTP_OUT_CALL_REPLY]           = "OUT_CALL_REPLY",
        [PPTP_IN_CALL_REQUEST]          = "IN_CALL_REQUEST",
        [PPTP_IN_CALL_REPLY]            = "IN_CALL_REPLY",
        [PPTP_IN_CALL_CONNECT]          = "IN_CALL_CONNECT",
        [PPTP_CALL_CLEAR_REQUEST]       = "CALL_CLEAR_REQUEST",
        [PPTP_CALL_DISCONNECT_NOTIFY]   = "CALL_DISCONNECT_NOTIFY",
        [PPTP_WAN_ERROR_NOTIFY]         = "WAN_ERROR_NOTIFY",
        [PPTP_SET_LINK_INFO]            = "SET_LINK_INFO"
};

const char *pptp_msg_name(u_int16_t msg)
{
        if (msg > PPTP_MSG_MAX)
                return pptp_msg_name_array[0];

        return pptp_msg_name_array[msg];
}
EXPORT_SYMBOL(pptp_msg_name);
#endif

#define SECS *HZ
#define MINS * 60 SECS
#define HOURS * 60 MINS

#define PPTP_GRE_TIMEOUT                (10 MINS)
#define PPTP_GRE_STREAM_TIMEOUT         (5 HOURS)

static void pptp_expectfn(struct nf_conn *ct,
                         struct nf_conntrack_expect *exp)
{
        struct net *net = nf_ct_net(ct);
        typeof(nf_nat_pptp_hook_expectfn) nf_nat_pptp_expectfn;
        pr_debug("increasing timeouts\n");

        /* increase timeout of GRE data channel conntrack entry */
        ct->proto.gre.timeout        = PPTP_GRE_TIMEOUT;
        ct->proto.gre.stream_timeout = PPTP_GRE_STREAM_TIMEOUT;

        /* Can you see how rusty this code is, compared with the pre-2.6.11
         * one? That's what happened to my shiny newnat of 2002 ;( -HW */

        nf_nat_pptp_expectfn = rcu_dereference(nf_nat_pptp_hook_expectfn);
        if (nf_nat_pptp_expectfn && ct->master->status & IPS_NAT_MASK)
                nf_nat_pptp_expectfn(ct, exp);
        else {
                struct nf_conntrack_tuple inv_t;
                struct nf_conntrack_expect *exp_other;

                /* obviously this tuple inversion only works until you do NAT */
                nf_ct_invert_tuple(&inv_t, &exp->tuple);
                pr_debug("trying to unexpect other dir: ");
                nf_ct_dump_tuple(&inv_t);

                exp_other = nf_ct_expect_find_get(net, nf_ct_zone(ct), &inv_t);
                if (exp_other) {
                        /* delete other expectation.  */
                        pr_debug("found\n");
                        nf_ct_unexpect_related(exp_other);
                        nf_ct_expect_put(exp_other);
                } else {
                        pr_debug("not found\n");
                }
        }
}

static int destroy_sibling_or_exp(struct net *net, struct nf_conn *ct,
                                  const struct nf_conntrack_tuple *t)
{
        const struct nf_conntrack_tuple_hash *h;
        const struct nf_conntrack_zone *zone;
        struct nf_conntrack_expect *exp;
        struct nf_conn *sibling;

        pr_debug("trying to timeout ct or exp for tuple ");
        nf_ct_dump_tuple(t);

        zone = nf_ct_zone(ct);
        h = nf_conntrack_find_get(net, zone, t);
        if (h)  {
                sibling = nf_ct_tuplehash_to_ctrack(h);
                pr_debug("setting timeout of conntrack %p to 0\n", sibling);
                sibling->proto.gre.timeout        = 0;
                sibling->proto.gre.stream_timeout = 0;
                nf_ct_kill(sibling);
                nf_ct_put(sibling);
                return 1;
        } else {
                exp = nf_ct_expect_find_get(net, zone, t);
                if (exp) {
                        pr_debug("unexpect_related of expect %p\n", exp);
                        nf_ct_unexpect_related(exp);
                        nf_ct_expect_put(exp);
                        return 1;
                }
        }
        return 0;
}

/* timeout GRE data connections */
static void pptp_destroy_siblings(struct nf_conn *ct)
{
        struct net *net = nf_ct_net(ct);
        const struct nf_ct_pptp_master *ct_pptp_info = nfct_help_data(ct);
        struct nf_conntrack_tuple t;

        nf_ct_gre_keymap_destroy(ct);

        /* try original (pns->pac) tuple */
        memcpy(&t, &ct->tuplehash[IP_CT_DIR_ORIGINAL].tuple, sizeof(t));
        t.dst.protonum = IPPROTO_GRE;
        t.src.u.gre.key = ct_pptp_info->pns_call_id;
        t.dst.u.gre.key = ct_pptp_info->pac_call_id;
        if (!destroy_sibling_or_exp(net, ct, &t))
                pr_debug("failed to timeout original pns->pac ct/exp\n");

        /* try reply (pac->pns) tuple */
        memcpy(&t, &ct->tuplehash[IP_CT_DIR_REPLY].tuple, sizeof(t));
        t.dst.protonum = IPPROTO_GRE;
        t.src.u.gre.key = ct_pptp_info->pac_call_id;
        t.dst.u.gre.key = ct_pptp_info->pns_call_id;
        if (!destroy_sibling_or_exp(net, ct, &t))
                pr_debug("failed to timeout reply pac->pns ct/exp\n");
}

/* expect GRE connections (PNS->PAC and PAC->PNS direction) */
static int exp_gre(struct nf_conn *ct, __be16 callid, __be16 peer_callid)
{
        struct nf_conntrack_expect *exp_orig, *exp_reply;
        enum ip_conntrack_dir dir;
        int ret = 1;
        typeof(nf_nat_pptp_hook_exp_gre) nf_nat_pptp_exp_gre;

        exp_orig = nf_ct_expect_alloc(ct);
        if (exp_orig == NULL)
                goto out;

        exp_reply = nf_ct_expect_alloc(ct);
        if (exp_reply == NULL)
                goto out_put_orig;

        /* original direction, PNS->PAC */
        dir = IP_CT_DIR_ORIGINAL;
        nf_ct_expect_init(exp_orig, NF_CT_EXPECT_CLASS_DEFAULT,
                          nf_ct_l3num(ct),
                          &ct->tuplehash[dir].tuple.src.u3,
                          &ct->tuplehash[dir].tuple.dst.u3,
                          IPPROTO_GRE, &peer_callid, &callid);
        exp_orig->expectfn = pptp_expectfn;

        /* reply direction, PAC->PNS */
        dir = IP_CT_DIR_REPLY;
        nf_ct_expect_init(exp_reply, NF_CT_EXPECT_CLASS_DEFAULT,
                          nf_ct_l3num(ct),
                          &ct->tuplehash[dir].tuple.src.u3,
                          &ct->tuplehash[dir].tuple.dst.u3,
                          IPPROTO_GRE, &callid, &peer_callid);
        exp_reply->expectfn = pptp_expectfn;

        nf_nat_pptp_exp_gre = rcu_dereference(nf_nat_pptp_hook_exp_gre);
        if (nf_nat_pptp_exp_gre && ct->status & IPS_NAT_MASK)
                nf_nat_pptp_exp_gre(exp_orig, exp_reply);
        if (nf_ct_expect_related(exp_orig, 0) != 0)
                goto out_put_both;
        if (nf_ct_expect_related(exp_reply, 0) != 0)
                goto out_unexpect_orig;

        /* Add GRE keymap entries */
        if (nf_ct_gre_keymap_add(ct, IP_CT_DIR_ORIGINAL, &exp_orig->tuple) != 0)
                goto out_unexpect_both;
        if (nf_ct_gre_keymap_add(ct, IP_CT_DIR_REPLY, &exp_reply->tuple) != 0) {
                nf_ct_gre_keymap_destroy(ct);
                goto out_unexpect_both;
        }
        ret = 0;

out_put_both:
        nf_ct_expect_put(exp_reply);
out_put_orig:
        nf_ct_expect_put(exp_orig);
out:
        return ret;

out_unexpect_both:
        nf_ct_unexpect_related(exp_reply);
out_unexpect_orig:
        nf_ct_unexpect_related(exp_orig);
        goto out_put_both;
}

static int
pptp_inbound_pkt(struct sk_buff *skb, unsigned int protoff,
                 struct PptpControlHeader *ctlh,
                 union pptp_ctrl_union *pptpReq,
                 unsigned int reqlen,
                 struct nf_conn *ct,
                 enum ip_conntrack_info ctinfo)
{
        struct nf_ct_pptp_master *info = nfct_help_data(ct);
        u_int16_t msg;
        __be16 cid = 0, pcid = 0;
        typeof(nf_nat_pptp_hook_inbound) nf_nat_pptp_inbound;

        msg = ntohs(ctlh->messageType);
        pr_debug("inbound control message %s\n", pptp_msg_name(msg));

        switch (msg) {
        case PPTP_START_SESSION_REPLY:
                /* server confirms new control session */
                if (info->sstate < PPTP_SESSION_REQUESTED)
                        goto invalid;
                if (pptpReq->srep.resultCode == PPTP_START_OK)
                        info->sstate = PPTP_SESSION_CONFIRMED;
                else
                        info->sstate = PPTP_SESSION_ERROR;
                break;

        case PPTP_STOP_SESSION_REPLY:
                /* server confirms end of control session */
                if (info->sstate > PPTP_SESSION_STOPREQ)
                        goto invalid;
                if (pptpReq->strep.resultCode == PPTP_STOP_OK)
                        info->sstate = PPTP_SESSION_NONE;
                else
                        info->sstate = PPTP_SESSION_ERROR;
                break;

        case PPTP_OUT_CALL_REPLY:
                /* server accepted call, we now expect GRE frames */
                if (info->sstate != PPTP_SESSION_CONFIRMED)
                        goto invalid;
                if (info->cstate != PPTP_CALL_OUT_REQ &&
                    info->cstate != PPTP_CALL_OUT_CONF)
                        goto invalid;

                cid = pptpReq->ocack.callID;
                pcid = pptpReq->ocack.peersCallID;
                if (info->pns_call_id != pcid)
                        goto invalid;
                pr_debug("%s, CID=%X, PCID=%X\n", pptp_msg_name(msg),
                         ntohs(cid), ntohs(pcid));

                if (pptpReq->ocack.resultCode == PPTP_OUTCALL_CONNECT) {
                        info->cstate = PPTP_CALL_OUT_CONF;
                        info->pac_call_id = cid;
                        exp_gre(ct, cid, pcid);
                } else
                        info->cstate = PPTP_CALL_NONE;
                break;

        case PPTP_IN_CALL_REQUEST:
                /* server tells us about incoming call request */
                if (info->sstate != PPTP_SESSION_CONFIRMED)
                        goto invalid;

                cid = pptpReq->icreq.callID;
                pr_debug("%s, CID=%X\n", pptp_msg_name(msg), ntohs(cid));
                info->cstate = PPTP_CALL_IN_REQ;
                info->pac_call_id = cid;
                break;

        case PPTP_IN_CALL_CONNECT:
                /* server tells us about incoming call established */
                if (info->sstate != PPTP_SESSION_CONFIRMED)
                        goto invalid;
                if (info->cstate != PPTP_CALL_IN_REP &&
                    info->cstate != PPTP_CALL_IN_CONF)
                        goto invalid;

                pcid = pptpReq->iccon.peersCallID;
                cid = info->pac_call_id;

                if (info->pns_call_id != pcid)
                        goto invalid;

                pr_debug("%s, PCID=%X\n", pptp_msg_name(msg), ntohs(pcid));
                info->cstate = PPTP_CALL_IN_CONF;

                /* we expect a GRE connection from PAC to PNS */
                exp_gre(ct, cid, pcid);
                break;

        case PPTP_CALL_DISCONNECT_NOTIFY:
                /* server confirms disconnect */
                cid = pptpReq->disc.callID;
                pr_debug("%s, CID=%X\n", pptp_msg_name(msg), ntohs(cid));
                info->cstate = PPTP_CALL_NONE;

                /* untrack this call id, unexpect GRE packets */
                pptp_destroy_siblings(ct);
                break;

        case PPTP_WAN_ERROR_NOTIFY:
        case PPTP_SET_LINK_INFO:
        case PPTP_ECHO_REQUEST:
        case PPTP_ECHO_REPLY:
                /* I don't have to explain these ;) */
                break;

        default:
                goto invalid;
        }

        nf_nat_pptp_inbound = rcu_dereference(nf_nat_pptp_hook_inbound);
        if (nf_nat_pptp_inbound && ct->status & IPS_NAT_MASK)
                return nf_nat_pptp_inbound(skb, ct, ctinfo,
                                           protoff, ctlh, pptpReq);
        return NF_ACCEPT;

invalid:
        pr_debug("invalid %s: type=%d cid=%u pcid=%u "
                 "cstate=%d sstate=%d pns_cid=%u pac_cid=%u\n",
                 pptp_msg_name(msg),
                 msg, ntohs(cid), ntohs(pcid),  info->cstate, info->sstate,
                 ntohs(info->pns_call_id), ntohs(info->pac_call_id));
        return NF_ACCEPT;
}

static int
pptp_outbound_pkt(struct sk_buff *skb, unsigned int protoff,
                  struct PptpControlHeader *ctlh,
                  union pptp_ctrl_union *pptpReq,
                  unsigned int reqlen,
                  struct nf_conn *ct,
                  enum ip_conntrack_info ctinfo)
{
        struct nf_ct_pptp_master *info = nfct_help_data(ct);
        u_int16_t msg;
        __be16 cid = 0, pcid = 0;
        typeof(nf_nat_pptp_hook_outbound) nf_nat_pptp_outbound;

        msg = ntohs(ctlh->messageType);
        pr_debug("outbound control message %s\n", pptp_msg_name(msg));

        switch (msg) {
        case PPTP_START_SESSION_REQUEST:
                /* client requests for new control session */
                if (info->sstate != PPTP_SESSION_NONE)
                        goto invalid;
                info->sstate = PPTP_SESSION_REQUESTED;
                break;

        case PPTP_STOP_SESSION_REQUEST:
                /* client requests end of control session */
                info->sstate = PPTP_SESSION_STOPREQ;
                break;

        case PPTP_OUT_CALL_REQUEST:
                /* client initiating connection to server */
                if (info->sstate != PPTP_SESSION_CONFIRMED)
                        goto invalid;
                info->cstate = PPTP_CALL_OUT_REQ;
                /* track PNS call id */
                cid = pptpReq->ocreq.callID;
                pr_debug("%s, CID=%X\n", pptp_msg_name(msg), ntohs(cid));
                info->pns_call_id = cid;
                break;

        case PPTP_IN_CALL_REPLY:
                /* client answers incoming call */
                if (info->cstate != PPTP_CALL_IN_REQ &&
                    info->cstate != PPTP_CALL_IN_REP)
                        goto invalid;

                cid = pptpReq->icack.callID;
                pcid = pptpReq->icack.peersCallID;
                if (info->pac_call_id != pcid)
                        goto invalid;
                pr_debug("%s, CID=%X PCID=%X\n", pptp_msg_name(msg),
                         ntohs(cid), ntohs(pcid));

                if (pptpReq->icack.resultCode == PPTP_INCALL_ACCEPT) {
                        /* part two of the three-way handshake */
                        info->cstate = PPTP_CALL_IN_REP;
                        info->pns_call_id = cid;
                } else
                        info->cstate = PPTP_CALL_NONE;
                break;

        case PPTP_CALL_CLEAR_REQUEST:
                /* client requests hangup of call */
                if (info->sstate != PPTP_SESSION_CONFIRMED)
                        goto invalid;
                /* FUTURE: iterate over all calls and check if
                 * call ID is valid.  We don't do this without newnat,
                 * because we only know about last call */
                info->cstate = PPTP_CALL_CLEAR_REQ;
                break;

        case PPTP_SET_LINK_INFO:
        case PPTP_ECHO_REQUEST:
        case PPTP_ECHO_REPLY:
                /* I don't have to explain these ;) */
                break;

        default:
                goto invalid;
        }

        nf_nat_pptp_outbound = rcu_dereference(nf_nat_pptp_hook_outbound);
        if (nf_nat_pptp_outbound && ct->status & IPS_NAT_MASK)
                return nf_nat_pptp_outbound(skb, ct, ctinfo,
                                            protoff, ctlh, pptpReq);
        return NF_ACCEPT;

invalid:
        pr_debug("invalid %s: type=%d cid=%u pcid=%u "
                 "cstate=%d sstate=%d pns_cid=%u pac_cid=%u\n",
                 pptp_msg_name(msg),
                 msg, ntohs(cid), ntohs(pcid),  info->cstate, info->sstate,
                 ntohs(info->pns_call_id), ntohs(info->pac_call_id));
        return NF_ACCEPT;
}

static const unsigned int pptp_msg_size[] = {
        [PPTP_START_SESSION_REQUEST]  = sizeof(struct PptpStartSessionRequest),
        [PPTP_START_SESSION_REPLY]    = sizeof(struct PptpStartSessionReply),
        [PPTP_STOP_SESSION_REQUEST]   = sizeof(struct PptpStopSessionRequest),
        [PPTP_STOP_SESSION_REPLY]     = sizeof(struct PptpStopSessionReply),
        [PPTP_OUT_CALL_REQUEST]       = sizeof(struct PptpOutCallRequest),
        [PPTP_OUT_CALL_REPLY]         = sizeof(struct PptpOutCallReply),
        [PPTP_IN_CALL_REQUEST]        = sizeof(struct PptpInCallRequest),
        [PPTP_IN_CALL_REPLY]          = sizeof(struct PptpInCallReply),
        [PPTP_IN_CALL_CONNECT]        = sizeof(struct PptpInCallConnected),
        [PPTP_CALL_CLEAR_REQUEST]     = sizeof(struct PptpClearCallRequest),
        [PPTP_CALL_DISCONNECT_NOTIFY] = sizeof(struct PptpCallDisconnectNotify),
        [PPTP_WAN_ERROR_NOTIFY]       = sizeof(struct PptpWanErrorNotify),
        [PPTP_SET_LINK_INFO]          = sizeof(struct PptpSetLinkInfo),
};

/* track caller id inside control connection, call expect_related */
static int
conntrack_pptp_help(struct sk_buff *skb, unsigned int protoff,
                    struct nf_conn *ct, enum ip_conntrack_info ctinfo)

{
        int dir = CTINFO2DIR(ctinfo);
        const struct nf_ct_pptp_master *info = nfct_help_data(ct);
        const struct tcphdr *tcph;
        struct tcphdr _tcph;
        const struct pptp_pkt_hdr *pptph;
        struct pptp_pkt_hdr _pptph;
        struct PptpControlHeader _ctlh, *ctlh;
        union pptp_ctrl_union _pptpReq, *pptpReq;
        unsigned int tcplen = skb->len - protoff;
        unsigned int datalen, reqlen, nexthdr_off;
        int oldsstate, oldcstate;
        int ret;
        u_int16_t msg;

#if IS_ENABLED(CONFIG_NF_NAT)
        if (!nf_ct_is_confirmed(ct) && (ct->status & IPS_NAT_MASK)) {
                struct nf_conn_nat *nat = nf_ct_ext_find(ct, NF_CT_EXT_NAT);

                if (!nat && !nf_ct_ext_add(ct, NF_CT_EXT_NAT, GFP_ATOMIC))
                        return NF_DROP;
        }
#endif
        /* don't do any tracking before tcp handshake complete */
        if (ctinfo != IP_CT_ESTABLISHED && ctinfo != IP_CT_ESTABLISHED_REPLY)
                return NF_ACCEPT;

        nexthdr_off = protoff;
        tcph = skb_header_pointer(skb, nexthdr_off, sizeof(_tcph), &_tcph);
        BUG_ON(!tcph);
        nexthdr_off += tcph->doff * 4;
        datalen = tcplen - tcph->doff * 4;

        pptph = skb_header_pointer(skb, nexthdr_off, sizeof(_pptph), &_pptph);
        if (!pptph) {
                pr_debug("no full PPTP header, can't track\n");
                return NF_ACCEPT;
        }
        nexthdr_off += sizeof(_pptph);
        datalen -= sizeof(_pptph);

        /* if it's not a control message we can't do anything with it */
        if (ntohs(pptph->packetType) != PPTP_PACKET_CONTROL ||
            ntohl(pptph->magicCookie) != PPTP_MAGIC_COOKIE) {
                pr_debug("not a control packet\n");
                return NF_ACCEPT;
        }

        ctlh = skb_header_pointer(skb, nexthdr_off, sizeof(_ctlh), &_ctlh);
        if (!ctlh)
                return NF_ACCEPT;
        nexthdr_off += sizeof(_ctlh);
        datalen -= sizeof(_ctlh);

        reqlen = datalen;
        msg = ntohs(ctlh->messageType);
        if (msg > 0 && msg <= PPTP_MSG_MAX && reqlen < pptp_msg_size[msg])
                return NF_ACCEPT;
        if (reqlen > sizeof(*pptpReq))
                reqlen = sizeof(*pptpReq);

        pptpReq = skb_header_pointer(skb, nexthdr_off, reqlen, &_pptpReq);
        if (!pptpReq)
                return NF_ACCEPT;

        oldsstate = info->sstate;
        oldcstate = info->cstate;

        spin_lock_bh(&nf_pptp_lock);

        /* FIXME: We just blindly assume that the control connection is always
         * established from PNS->PAC.  However, RFC makes no guarantee */
        if (dir == IP_CT_DIR_ORIGINAL)
                /* client -> server (PNS -> PAC) */
                ret = pptp_outbound_pkt(skb, protoff, ctlh, pptpReq, reqlen, ct,
                                        ctinfo);
        else
                /* server -> client (PAC -> PNS) */
                ret = pptp_inbound_pkt(skb, protoff, ctlh, pptpReq, reqlen, ct,
                                       ctinfo);
        pr_debug("sstate: %d->%d, cstate: %d->%d\n",
                 oldsstate, info->sstate, oldcstate, info->cstate);
        spin_unlock_bh(&nf_pptp_lock);

        return ret;
}

static const struct nf_conntrack_expect_policy pptp_exp_policy = {
        .max_expected   = 2,
        .timeout        = 5 * 60,
};

/* control protocol helper */
static struct nf_conntrack_helper pptp __read_mostly = {
        .name                   = "pptp",
        .me                     = THIS_MODULE,
        .tuple.src.l3num        = AF_INET,
        .tuple.src.u.tcp.port   = cpu_to_be16(PPTP_CONTROL_PORT),
        .tuple.dst.protonum     = IPPROTO_TCP,
        .help                   = conntrack_pptp_help,
        .destroy                = pptp_destroy_siblings,
        .expect_policy          = &pptp_exp_policy,
};

static int __init nf_conntrack_pptp_init(void)
{
        NF_CT_HELPER_BUILD_BUG_ON(sizeof(struct nf_ct_pptp_master));

        return nf_conntrack_helper_register(&pptp);
}

static void __exit nf_conntrack_pptp_fini(void)
{
        nf_conntrack_helper_unregister(&pptp);
}

module_init(nf_conntrack_pptp_init);
module_exit(nf_conntrack_pptp_fini);