l2tp_core.c

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/*
 * L2TP core.
 *
 * Copyright (c) 2008,2009,2010 Katalix Systems Ltd
 *
 * This file contains some code of the original L2TPv2 pppol2tp
 * driver, which has the following copyright:
 *
 * Authors: Martijn van Oosterhout <[email protected]>
 *      James Chapman ([email protected])
 * Contributors:
 *      Michal Ostrowski <[email protected]>
 *      Arnaldo Carvalho de Melo <[email protected]>
 *      David S. Miller ([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.
 */

#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#include <linux/module.h>
#include <linux/string.h>
#include <linux/list.h>
#include <linux/rculist.h>
#include <linux/uaccess.h>

#include <linux/kernel.h>
#include <linux/spinlock.h>
#include <linux/kthread.h>
#include <linux/sched.h>
#include <linux/slab.h>
#include <linux/errno.h>
#include <linux/jiffies.h>

#include <linux/netdevice.h>
#include <linux/net.h>
#include <linux/inetdevice.h>
#include <linux/skbuff.h>
#include <linux/init.h>
#include <linux/in.h>
#include <linux/ip.h>
#include <linux/udp.h>
#include <linux/l2tp.h>
#include <linux/hash.h>
#include <linux/sort.h>
#include <linux/file.h>
#include <linux/nsproxy.h>
#include <net/net_namespace.h>
#include <net/netns/generic.h>
#include <net/dst.h>
#include <net/ip.h>
#include <net/udp.h>
#include <net/inet_common.h>
#include <net/xfrm.h>
#include <net/protocol.h>
#include <net/inet6_connection_sock.h>
#include <net/inet_ecn.h>
#include <net/ip6_route.h>
#include <net/ip6_checksum.h>

#include <asm/byteorder.h>
#include <linux/atomic.h>

#include "l2tp_core.h"

#define L2TP_DRV_VERSION    "V2.0"

/* L2TP header constants */
#define L2TP_HDRFLAG_T     0x8000
#define L2TP_HDRFLAG_L     0x4000
#define L2TP_HDRFLAG_S     0x0800
#define L2TP_HDRFLAG_O     0x0200
#define L2TP_HDRFLAG_P     0x0100

#define L2TP_HDR_VER_MASK  0x000F
#define L2TP_HDR_VER_2     0x0002
#define L2TP_HDR_VER_3     0x0003

/* L2TPv3 default L2-specific sublayer */
#define L2TP_SLFLAG_S      0x40000000
#define L2TP_SL_SEQ_MASK   0x00ffffff

#define L2TP_HDR_SIZE_SEQ       10
#define L2TP_HDR_SIZE_NOSEQ     6

/* Default trace flags */
#define L2TP_DEFAULT_DEBUG_FLAGS    0

/* Private data stored for received packets in the skb.
 */
struct l2tp_skb_cb {
    u32         ns;
    u16         has_seq;
    u16         length;
    unsigned long       expires;
};

#define L2TP_SKB_CB(skb)    ((struct l2tp_skb_cb *) &skb->cb[sizeof(struct inet_skb_parm)])

static atomic_t l2tp_tunnel_count;
static atomic_t l2tp_session_count;

/* per-net private data for this module */
static unsigned int l2tp_net_id;
struct l2tp_net {
    struct list_head l2tp_tunnel_list;
    spinlock_t l2tp_tunnel_list_lock;
    struct hlist_head l2tp_session_hlist[L2TP_HASH_SIZE_2];
    spinlock_t l2tp_session_hlist_lock;
};

static void l2tp_session_set_header_len(struct l2tp_session *session, int version);
static void l2tp_tunnel_free(struct l2tp_tunnel *tunnel);
static void l2tp_tunnel_closeall(struct l2tp_tunnel *tunnel);

static inline struct l2tp_net *l2tp_pernet(struct net *net)
{
    BUG_ON(!net);

    return net_generic(net, l2tp_net_id);
}


/* Tunnel reference counts. Incremented per session that is added to
 * the tunnel.
 */
static inline void l2tp_tunnel_inc_refcount_1(struct l2tp_tunnel *tunnel)
{
    atomic_inc(&tunnel->ref_count);
}

static inline void l2tp_tunnel_dec_refcount_1(struct l2tp_tunnel *tunnel)
{
    if (atomic_dec_and_test(&tunnel->ref_count))
        l2tp_tunnel_free(tunnel);
}
#ifdef L2TP_REFCNT_DEBUG
#define l2tp_tunnel_inc_refcount(_t)                    \
do {                                    \
    pr_debug("l2tp_tunnel_inc_refcount: %s:%d %s: cnt=%d\n",    \
         __func__, __LINE__, (_t)->name,            \
         atomic_read(&_t->ref_count));              \
    l2tp_tunnel_inc_refcount_1(_t);                 \
} while (0)
#define l2tp_tunnel_dec_refcount(_t)
do {                                    \
    pr_debug("l2tp_tunnel_dec_refcount: %s:%d %s: cnt=%d\n",    \
         __func__, __LINE__, (_t)->name,            \
         atomic_read(&_t->ref_count));              \
    l2tp_tunnel_dec_refcount_1(_t);                 \
} while (0)
#else
#define l2tp_tunnel_inc_refcount(t) l2tp_tunnel_inc_refcount_1(t)
#define l2tp_tunnel_dec_refcount(t) l2tp_tunnel_dec_refcount_1(t)
#endif

/* Session hash global list for L2TPv3.
 * The session_id SHOULD be random according to RFC3931, but several
 * L2TP implementations use incrementing session_ids.  So we do a real
 * hash on the session_id, rather than a simple bitmask.
 */
static inline struct hlist_head *
l2tp_session_id_hash_2(struct l2tp_net *pn, u32 session_id)
{
    return &pn->l2tp_session_hlist[hash_32(session_id, L2TP_HASH_BITS_2)];

}

/* Lookup a session by id in the global session list
 */
static struct l2tp_session *l2tp_session_find_2(struct net *net, u32 session_id)
{
    struct l2tp_net *pn = l2tp_pernet(net);
    struct hlist_head *session_list =
        l2tp_session_id_hash_2(pn, session_id);
    struct l2tp_session *session;
    struct hlist_node *walk;

    rcu_read_lock_bh();
    hlist_for_each_entry_rcu(session, walk, session_list, global_hlist) {
        if (session->session_id == session_id) {
            rcu_read_unlock_bh();
            return session;
        }
    }
    rcu_read_unlock_bh();

    return NULL;
}

/* Session hash list.
 * The session_id SHOULD be random according to RFC2661, but several
 * L2TP implementations (Cisco and Microsoft) use incrementing
 * session_ids.  So we do a real hash on the session_id, rather than a
 * simple bitmask.
 */
static inline struct hlist_head *
l2tp_session_id_hash(struct l2tp_tunnel *tunnel, u32 session_id)
{
    return &tunnel->session_hlist[hash_32(session_id, L2TP_HASH_BITS)];
}

/* Lookup a session by id
 */
struct l2tp_session *l2tp_session_find(struct net *net, struct l2tp_tunnel *tunnel, u32 session_id)
{
    struct hlist_head *session_list;
    struct l2tp_session *session;
    struct hlist_node *walk;

    /* In L2TPv3, session_ids are unique over all tunnels and we
     * sometimes need to look them up before we know the
     * tunnel.
     */
    if (tunnel == NULL)
        return l2tp_session_find_2(net, session_id);

    session_list = l2tp_session_id_hash(tunnel, session_id);
    read_lock_bh(&tunnel->hlist_lock);
    hlist_for_each_entry(session, walk, session_list, hlist) {
        if (session->session_id == session_id) {
            read_unlock_bh(&tunnel->hlist_lock);
            return session;
        }
    }
    read_unlock_bh(&tunnel->hlist_lock);

    return NULL;
}
EXPORT_SYMBOL_GPL(l2tp_session_find);

struct l2tp_session *l2tp_session_find_nth(struct l2tp_tunnel *tunnel, int nth)
{
    int hash;
    struct hlist_node *walk;
    struct l2tp_session *session;
    int count = 0;

    read_lock_bh(&tunnel->hlist_lock);
    for (hash = 0; hash < L2TP_HASH_SIZE; hash++) {
        hlist_for_each_entry(session, walk, &tunnel->session_hlist[hash], hlist) {
            if (++count > nth) {
                read_unlock_bh(&tunnel->hlist_lock);
                return session;
            }
        }
    }

    read_unlock_bh(&tunnel->hlist_lock);

    return NULL;
}
EXPORT_SYMBOL_GPL(l2tp_session_find_nth);

/* Lookup a session by interface name.
 * This is very inefficient but is only used by management interfaces.
 */
struct l2tp_session *l2tp_session_find_by_ifname(struct net *net, char *ifname)
{
    struct l2tp_net *pn = l2tp_pernet(net);
    int hash;
    struct hlist_node *walk;
    struct l2tp_session *session;

    rcu_read_lock_bh();
    for (hash = 0; hash < L2TP_HASH_SIZE_2; hash++) {
        hlist_for_each_entry_rcu(session, walk, &pn->l2tp_session_hlist[hash], global_hlist) {
            if (!strcmp(session->ifname, ifname)) {
                rcu_read_unlock_bh();
                return session;
            }
        }
    }

    rcu_read_unlock_bh();

    return NULL;
}
EXPORT_SYMBOL_GPL(l2tp_session_find_by_ifname);

/* Lookup a tunnel by id
 */
struct l2tp_tunnel *l2tp_tunnel_find(struct net *net, u32 tunnel_id)
{
    struct l2tp_tunnel *tunnel;
    struct l2tp_net *pn = l2tp_pernet(net);

    rcu_read_lock_bh();
    list_for_each_entry_rcu(tunnel, &pn->l2tp_tunnel_list, list) {
        if (tunnel->tunnel_id == tunnel_id) {
            rcu_read_unlock_bh();
            return tunnel;
        }
    }
    rcu_read_unlock_bh();

    return NULL;
}
EXPORT_SYMBOL_GPL(l2tp_tunnel_find);

struct l2tp_tunnel *l2tp_tunnel_find_nth(struct net *net, int nth)
{
    struct l2tp_net *pn = l2tp_pernet(net);
    struct l2tp_tunnel *tunnel;
    int count = 0;

    rcu_read_lock_bh();
    list_for_each_entry_rcu(tunnel, &pn->l2tp_tunnel_list, list) {
        if (++count > nth) {
            rcu_read_unlock_bh();
            return tunnel;
        }
    }

    rcu_read_unlock_bh();

    return NULL;
}
EXPORT_SYMBOL_GPL(l2tp_tunnel_find_nth);

/*****************************************************************************
 * Receive data handling
 *****************************************************************************/

/* Queue a skb in order. We come here only if the skb has an L2TP sequence
 * number.
 */
static void l2tp_recv_queue_skb(struct l2tp_session *session, struct sk_buff *skb)
{
    struct sk_buff *skbp;
    struct sk_buff *tmp;
    u32 ns = L2TP_SKB_CB(skb)->ns;
    struct l2tp_stats *sstats;

    spin_lock_bh(&session->reorder_q.lock);
    sstats = &session->stats;
    skb_queue_walk_safe(&session->reorder_q, skbp, tmp) {
        if (L2TP_SKB_CB(skbp)->ns > ns) {
            __skb_queue_before(&session->reorder_q, skbp, skb);
            l2tp_dbg(session, L2TP_MSG_SEQ,
                 "%s: pkt %hu, inserted before %hu, reorder_q len=%d\n",
                 session->name, ns, L2TP_SKB_CB(skbp)->ns,
                 skb_queue_len(&session->reorder_q));
            u64_stats_update_begin(&sstats->syncp);
            sstats->rx_oos_packets++;
            u64_stats_update_end(&sstats->syncp);
            goto out;
        }
    }

    __skb_queue_tail(&session->reorder_q, skb);

out:
    spin_unlock_bh(&session->reorder_q.lock);
}

/* Dequeue a single skb.
 */
static void l2tp_recv_dequeue_skb(struct l2tp_session *session, struct sk_buff *skb)
{
    struct l2tp_tunnel *tunnel = session->tunnel;
    int length = L2TP_SKB_CB(skb)->length;
    struct l2tp_stats *tstats, *sstats;

    /* We're about to requeue the skb, so return resources
     * to its current owner (a socket receive buffer).
     */
    skb_orphan(skb);

    tstats = &tunnel->stats;
    u64_stats_update_begin(&tstats->syncp);
    sstats = &session->stats;
    u64_stats_update_begin(&sstats->syncp);
    tstats->rx_packets++;
    tstats->rx_bytes += length;
    sstats->rx_packets++;
    sstats->rx_bytes += length;
    u64_stats_update_end(&tstats->syncp);
    u64_stats_update_end(&sstats->syncp);

    if (L2TP_SKB_CB(skb)->has_seq) {
        /* Bump our Nr */
        session->nr++;
        if (tunnel->version == L2TP_HDR_VER_2)
            session->nr &= 0xffff;
        else
            session->nr &= 0xffffff;

        l2tp_dbg(session, L2TP_MSG_SEQ, "%s: updated nr to %hu\n",
             session->name, session->nr);
    }

    /* call private receive handler */
    if (session->recv_skb != NULL)
        (*session->recv_skb)(session, skb, L2TP_SKB_CB(skb)->length);
    else
        kfree_skb(skb);

    if (session->deref)
        (*session->deref)(session);
}

/* Dequeue skbs from the session's reorder_q, subject to packet order.
 * Skbs that have been in the queue for too long are simply discarded.
 */
static void l2tp_recv_dequeue(struct l2tp_session *session)
{
    struct sk_buff *skb;
    struct sk_buff *tmp;
    struct l2tp_stats *sstats;

    /* If the pkt at the head of the queue has the nr that we
     * expect to send up next, dequeue it and any other
     * in-sequence packets behind it.
     */
start:
    spin_lock_bh(&session->reorder_q.lock);
    sstats = &session->stats;
    skb_queue_walk_safe(&session->reorder_q, skb, tmp) {
        if (time_after(jiffies, L2TP_SKB_CB(skb)->expires)) {
            u64_stats_update_begin(&sstats->syncp);
            sstats->rx_seq_discards++;
            sstats->rx_errors++;
            u64_stats_update_end(&sstats->syncp);
            l2tp_dbg(session, L2TP_MSG_SEQ,
                 "%s: oos pkt %u len %d discarded (too old), waiting for %u, reorder_q_len=%d\n",
                 session->name, L2TP_SKB_CB(skb)->ns,
                 L2TP_SKB_CB(skb)->length, session->nr,
                 skb_queue_len(&session->reorder_q));
            session->reorder_skip = 1;
            __skb_unlink(skb, &session->reorder_q);
            kfree_skb(skb);
            if (session->deref)
                (*session->deref)(session);
            continue;
        }

        if (L2TP_SKB_CB(skb)->has_seq) {
            if (session->reorder_skip) {
                l2tp_dbg(session, L2TP_MSG_SEQ,
                     "%s: advancing nr to next pkt: %u -> %u",
                     session->name, session->nr,
                     L2TP_SKB_CB(skb)->ns);
                session->reorder_skip = 0;
                session->nr = L2TP_SKB_CB(skb)->ns;
            }
            if (L2TP_SKB_CB(skb)->ns != session->nr) {
                l2tp_dbg(session, L2TP_MSG_SEQ,
                     "%s: holding oos pkt %u len %d, waiting for %u, reorder_q_len=%d\n",
                     session->name, L2TP_SKB_CB(skb)->ns,
                     L2TP_SKB_CB(skb)->length, session->nr,
                     skb_queue_len(&session->reorder_q));
                goto out;
            }
        }
        __skb_unlink(skb, &session->reorder_q);

        /* Process the skb. We release the queue lock while we
         * do so to let other contexts process the queue.
         */
        spin_unlock_bh(&session->reorder_q.lock);
        l2tp_recv_dequeue_skb(session, skb);
        goto start;
    }

out:
    spin_unlock_bh(&session->reorder_q.lock);
}

static inline int l2tp_verify_udp_checksum(struct sock *sk,
                       struct sk_buff *skb)
{
    struct udphdr *uh = udp_hdr(skb);
    u16 ulen = ntohs(uh->len);
    __wsum psum;

    if (sk->sk_no_check || skb_csum_unnecessary(skb))
        return 0;

#if IS_ENABLED(CONFIG_IPV6)
    if (sk->sk_family == PF_INET6) {
        if (!uh->check) {
            LIMIT_NETDEBUG(KERN_INFO "L2TP: IPv6: checksum is 0\n");
            return 1;
        }
        if ((skb->ip_summed == CHECKSUM_COMPLETE) &&
            !csum_ipv6_magic(&ipv6_hdr(skb)->saddr,
                     &ipv6_hdr(skb)->daddr, ulen,
                     IPPROTO_UDP, skb->csum)) {
            skb->ip_summed = CHECKSUM_UNNECESSARY;
            return 0;
        }
        skb->csum = ~csum_unfold(csum_ipv6_magic(&ipv6_hdr(skb)->saddr,
                             &ipv6_hdr(skb)->daddr,
                             skb->len, IPPROTO_UDP,
                             0));
    } else
#endif
    {
        struct inet_sock *inet;
        if (!uh->check)
            return 0;
        inet = inet_sk(sk);
        psum = csum_tcpudp_nofold(inet->inet_saddr, inet->inet_daddr,
                      ulen, IPPROTO_UDP, 0);

        if ((skb->ip_summed == CHECKSUM_COMPLETE) &&
            !csum_fold(csum_add(psum, skb->csum)))
            return 0;
        skb->csum = psum;
    }

    return __skb_checksum_complete(skb);
}

/* Do receive processing of L2TP data frames. We handle both L2TPv2
 * and L2TPv3 data frames here.
 *
 * L2TPv2 Data Message Header
 *
 *  0                   1                   2                   3
 *  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 * |T|L|x|x|S|x|O|P|x|x|x|x|  Ver  |          Length (opt)         |
 * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 * |           Tunnel ID           |           Session ID          |
 * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 * |             Ns (opt)          |             Nr (opt)          |
 * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 * |      Offset Size (opt)        |    Offset pad... (opt)
 * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 *
 * Data frames are marked by T=0. All other fields are the same as
 * those in L2TP control frames.
 *
 * L2TPv3 Data Message Header
 *
 * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 * |                      L2TP Session Header                      |
 * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 * |                      L2-Specific Sublayer                     |
 * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 * |                        Tunnel Payload                      ...
 * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 *
 * L2TPv3 Session Header Over IP
 *
 *  0                   1                   2                   3
 *  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 * |                           Session ID                          |
 * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 * |               Cookie (optional, maximum 64 bits)...
 * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 *                                                                 |
 * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 *
 * L2TPv3 L2-Specific Sublayer Format
 *
 *  0                   1                   2                   3
 *  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 * |x|S|x|x|x|x|x|x|              Sequence Number                  |
 * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 *
 * Cookie value, sublayer format and offset (pad) are negotiated with
 * the peer when the session is set up. Unlike L2TPv2, we do not need
 * to parse the packet header to determine if optional fields are
 * present.
 *
 * Caller must already have parsed the frame and determined that it is
 * a data (not control) frame before coming here. Fields up to the
 * session-id have already been parsed and ptr points to the data
 * after the session-id.
 */
void l2tp_recv_common(struct l2tp_session *session, struct sk_buff *skb,
              unsigned char *ptr, unsigned char *optr, u16 hdrflags,
              int length, int (*payload_hook)(struct sk_buff *skb))
{
    struct l2tp_tunnel *tunnel = session->tunnel;
    int offset;
    u32 ns, nr;
    struct l2tp_stats *sstats = &session->stats;

    /* The ref count is increased since we now hold a pointer to
     * the session. Take care to decrement the refcnt when exiting
     * this function from now on...
     */
    l2tp_session_inc_refcount(session);
    if (session->ref)
        (*session->ref)(session);

    /* Parse and check optional cookie */
    if (session->peer_cookie_len > 0) {
        if (memcmp(ptr, &session->peer_cookie[0], session->peer_cookie_len)) {
            l2tp_info(tunnel, L2TP_MSG_DATA,
                  "%s: cookie mismatch (%u/%u). Discarding.\n",
                  tunnel->name, tunnel->tunnel_id,
                  session->session_id);
            u64_stats_update_begin(&sstats->syncp);
            sstats->rx_cookie_discards++;
            u64_stats_update_end(&sstats->syncp);
            goto discard;
        }
        ptr += session->peer_cookie_len;
    }

    /* Handle the optional sequence numbers. Sequence numbers are
     * in different places for L2TPv2 and L2TPv3.
     *
     * If we are the LAC, enable/disable sequence numbers under
     * the control of the LNS.  If no sequence numbers present but
     * we were expecting them, discard frame.
     */
    ns = nr = 0;
    L2TP_SKB_CB(skb)->has_seq = 0;
    if (tunnel->version == L2TP_HDR_VER_2) {
        if (hdrflags & L2TP_HDRFLAG_S) {
            ns = ntohs(*(__be16 *) ptr);
            ptr += 2;
            nr = ntohs(*(__be16 *) ptr);
            ptr += 2;

            /* Store L2TP info in the skb */
            L2TP_SKB_CB(skb)->ns = ns;
            L2TP_SKB_CB(skb)->has_seq = 1;

            l2tp_dbg(session, L2TP_MSG_SEQ,
                 "%s: recv data ns=%u, nr=%u, session nr=%u\n",
                 session->name, ns, nr, session->nr);
        }
    } else if (session->l2specific_type == L2TP_L2SPECTYPE_DEFAULT) {
        u32 l2h = ntohl(*(__be32 *) ptr);

        if (l2h & 0x40000000) {
            ns = l2h & 0x00ffffff;

            /* Store L2TP info in the skb */
            L2TP_SKB_CB(skb)->ns = ns;
            L2TP_SKB_CB(skb)->has_seq = 1;

            l2tp_dbg(session, L2TP_MSG_SEQ,
                 "%s: recv data ns=%u, session nr=%u\n",
                 session->name, ns, session->nr);
        }
    }

    /* Advance past L2-specific header, if present */
    ptr += session->l2specific_len;

    if (L2TP_SKB_CB(skb)->has_seq) {
        /* Received a packet with sequence numbers. If we're the LNS,
         * check if we sre sending sequence numbers and if not,
         * configure it so.
         */
        if ((!session->lns_mode) && (!session->send_seq)) {
            l2tp_info(session, L2TP_MSG_SEQ,
                  "%s: requested to enable seq numbers by LNS\n",
                  session->name);
            session->send_seq = -1;
            l2tp_session_set_header_len(session, tunnel->version);
        }
    } else {
        /* No sequence numbers.
         * If user has configured mandatory sequence numbers, discard.
         */
        if (session->recv_seq) {
            l2tp_warn(session, L2TP_MSG_SEQ,
                  "%s: recv data has no seq numbers when required. Discarding.\n",
                  session->name);
            u64_stats_update_begin(&sstats->syncp);
            sstats->rx_seq_discards++;
            u64_stats_update_end(&sstats->syncp);
            goto discard;
        }

        /* If we're the LAC and we're sending sequence numbers, the
         * LNS has requested that we no longer send sequence numbers.
         * If we're the LNS and we're sending sequence numbers, the
         * LAC is broken. Discard the frame.
         */
        if ((!session->lns_mode) && (session->send_seq)) {
            l2tp_info(session, L2TP_MSG_SEQ,
                  "%s: requested to disable seq numbers by LNS\n",
                  session->name);
            session->send_seq = 0;
            l2tp_session_set_header_len(session, tunnel->version);
        } else if (session->send_seq) {
            l2tp_warn(session, L2TP_MSG_SEQ,
                  "%s: recv data has no seq numbers when required. Discarding.\n",
                  session->name);
            u64_stats_update_begin(&sstats->syncp);
            sstats->rx_seq_discards++;
            u64_stats_update_end(&sstats->syncp);
            goto discard;
        }
    }

    /* Session data offset is handled differently for L2TPv2 and
     * L2TPv3. For L2TPv2, there is an optional 16-bit value in
     * the header. For L2TPv3, the offset is negotiated using AVPs
     * in the session setup control protocol.
     */
    if (tunnel->version == L2TP_HDR_VER_2) {
        /* If offset bit set, skip it. */
        if (hdrflags & L2TP_HDRFLAG_O) {
            offset = ntohs(*(__be16 *)ptr);
            ptr += 2 + offset;
        }
    } else
        ptr += session->offset;

    offset = ptr - optr;
    if (!pskb_may_pull(skb, offset))
        goto discard;

    __skb_pull(skb, offset);

    /* If caller wants to process the payload before we queue the
     * packet, do so now.
     */
    if (payload_hook)
        if ((*payload_hook)(skb))
            goto discard;

    /* Prepare skb for adding to the session's reorder_q.  Hold
     * packets for max reorder_timeout or 1 second if not
     * reordering.
     */
    L2TP_SKB_CB(skb)->length = length;
    L2TP_SKB_CB(skb)->expires = jiffies +
        (session->reorder_timeout ? session->reorder_timeout : HZ);

    /* Add packet to the session's receive queue. Reordering is done here, if
     * enabled. Saved L2TP protocol info is stored in skb->sb[].
     */
    if (L2TP_SKB_CB(skb)->has_seq) {
        if (session->reorder_timeout != 0) {
            /* Packet reordering enabled. Add skb to session's
             * reorder queue, in order of ns.
             */
            l2tp_recv_queue_skb(session, skb);
        } else {
            /* Packet reordering disabled. Discard out-of-sequence
             * packets
             */
            if (L2TP_SKB_CB(skb)->ns != session->nr) {
                u64_stats_update_begin(&sstats->syncp);
                sstats->rx_seq_discards++;
                u64_stats_update_end(&sstats->syncp);
                l2tp_dbg(session, L2TP_MSG_SEQ,
                     "%s: oos pkt %u len %d discarded, waiting for %u, reorder_q_len=%d\n",
                     session->name, L2TP_SKB_CB(skb)->ns,
                     L2TP_SKB_CB(skb)->length, session->nr,
                     skb_queue_len(&session->reorder_q));
                goto discard;
            }
            skb_queue_tail(&session->reorder_q, skb);
        }
    } else {
        /* No sequence numbers. Add the skb to the tail of the
         * reorder queue. This ensures that it will be
         * delivered after all previous sequenced skbs.
         */
        skb_queue_tail(&session->reorder_q, skb);
    }

    /* Try to dequeue as many skbs from reorder_q as we can. */
    l2tp_recv_dequeue(session);

    l2tp_session_dec_refcount(session);

    return;

discard:
    u64_stats_update_begin(&sstats->syncp);
    sstats->rx_errors++;
    u64_stats_update_end(&sstats->syncp);
    kfree_skb(skb);

    if (session->deref)
        (*session->deref)(session);

    l2tp_session_dec_refcount(session);
}
EXPORT_SYMBOL(l2tp_recv_common);

/* Internal UDP receive frame. Do the real work of receiving an L2TP data frame
 * here. The skb is not on a list when we get here.
 * Returns 0 if the packet was a data packet and was successfully passed on.
 * Returns 1 if the packet was not a good data packet and could not be
 * forwarded.  All such packets are passed up to userspace to deal with.
 */
static int l2tp_udp_recv_core(struct l2tp_tunnel *tunnel, struct sk_buff *skb,
                  int (*payload_hook)(struct sk_buff *skb))
{
    struct l2tp_session *session = NULL;
    unsigned char *ptr, *optr;
    u16 hdrflags;
    u32 tunnel_id, session_id;
    u16 version;
    int length;
    struct l2tp_stats *tstats;

    if (tunnel->sock && l2tp_verify_udp_checksum(tunnel->sock, skb))
        goto discard_bad_csum;

    /* UDP always verifies the packet length. */
    __skb_pull(skb, sizeof(struct udphdr));

    /* Short packet? */
    if (!pskb_may_pull(skb, L2TP_HDR_SIZE_SEQ)) {
        l2tp_info(tunnel, L2TP_MSG_DATA,
              "%s: recv short packet (len=%d)\n",
              tunnel->name, skb->len);
        goto error;
    }

    /* Trace packet contents, if enabled */
    if (tunnel->debug & L2TP_MSG_DATA) {
        length = min(32u, skb->len);
        if (!pskb_may_pull(skb, length))
            goto error;

        pr_debug("%s: recv\n", tunnel->name);
        print_hex_dump_bytes("", DUMP_PREFIX_OFFSET, skb->data, length);
    }

    /* Point to L2TP header */
    optr = ptr = skb->data;

    /* Get L2TP header flags */
    hdrflags = ntohs(*(__be16 *) ptr);

    /* Check protocol version */
    version = hdrflags & L2TP_HDR_VER_MASK;
    if (version != tunnel->version) {
        l2tp_info(tunnel, L2TP_MSG_DATA,
              "%s: recv protocol version mismatch: got %d expected %d\n",
              tunnel->name, version, tunnel->version);
        goto error;
    }

    /* Get length of L2TP packet */
    length = skb->len;

    /* If type is control packet, it is handled by userspace. */
    if (hdrflags & L2TP_HDRFLAG_T) {
        l2tp_dbg(tunnel, L2TP_MSG_DATA,
             "%s: recv control packet, len=%d\n",
             tunnel->name, length);
        goto error;
    }

    /* Skip flags */
    ptr += 2;

    if (tunnel->version == L2TP_HDR_VER_2) {
        /* If length is present, skip it */
        if (hdrflags & L2TP_HDRFLAG_L)
            ptr += 2;

        /* Extract tunnel and session ID */
        tunnel_id = ntohs(*(__be16 *) ptr);
        ptr += 2;
        session_id = ntohs(*(__be16 *) ptr);
        ptr += 2;
    } else {
        ptr += 2;   /* skip reserved bits */
        tunnel_id = tunnel->tunnel_id;
        session_id = ntohl(*(__be32 *) ptr);
        ptr += 4;
    }

    /* Find the session context */
    session = l2tp_session_find(tunnel->l2tp_net, tunnel, session_id);
    if (!session || !session->recv_skb) {
        /* Not found? Pass to userspace to deal with */
        l2tp_info(tunnel, L2TP_MSG_DATA,
              "%s: no session found (%u/%u). Passing up.\n",
              tunnel->name, tunnel_id, session_id);
        goto error;
    }

    l2tp_recv_common(session, skb, ptr, optr, hdrflags, length, payload_hook);

    return 0;

discard_bad_csum:
    LIMIT_NETDEBUG("%s: UDP: bad checksum\n", tunnel->name);
    UDP_INC_STATS_USER(tunnel->l2tp_net, UDP_MIB_INERRORS, 0);
    tstats = &tunnel->stats;
    u64_stats_update_begin(&tstats->syncp);
    tstats->rx_errors++;
    u64_stats_update_end(&tstats->syncp);
    kfree_skb(skb);

    return 0;

error:
    /* Put UDP header back */
    __skb_push(skb, sizeof(struct udphdr));

    return 1;
}

/* UDP encapsulation receive handler. See net/ipv4/udp.c.
 * Return codes:
 * 0 : success.
 * <0: error
 * >0: skb should be passed up to userspace as UDP.
 */
int l2tp_udp_encap_recv(struct sock *sk, struct sk_buff *skb)
{
    struct l2tp_tunnel *tunnel;

    tunnel = l2tp_sock_to_tunnel(sk);
    if (tunnel == NULL)
        goto pass_up;

    l2tp_dbg(tunnel, L2TP_MSG_DATA, "%s: received %d bytes\n",
         tunnel->name, skb->len);

    if (l2tp_udp_recv_core(tunnel, skb, tunnel->recv_payload_hook))
        goto pass_up_put;

    sock_put(sk);
    return 0;

pass_up_put:
    sock_put(sk);
pass_up:
    return 1;
}
EXPORT_SYMBOL_GPL(l2tp_udp_encap_recv);

/************************************************************************
 * Transmit handling
 ***********************************************************************/

/* Build an L2TP header for the session into the buffer provided.
 */
static int l2tp_build_l2tpv2_header(struct l2tp_session *session, void *buf)
{
    struct l2tp_tunnel *tunnel = session->tunnel;
    __be16 *bufp = buf;
    __be16 *optr = buf;
    u16 flags = L2TP_HDR_VER_2;
    u32 tunnel_id = tunnel->peer_tunnel_id;
    u32 session_id = session->peer_session_id;

    if (session->send_seq)
        flags |= L2TP_HDRFLAG_S;

    /* Setup L2TP header. */
    *bufp++ = htons(flags);
    *bufp++ = htons(tunnel_id);
    *bufp++ = htons(session_id);
    if (session->send_seq) {
        *bufp++ = htons(session->ns);
        *bufp++ = 0;
        session->ns++;
        session->ns &= 0xffff;
        l2tp_dbg(session, L2TP_MSG_SEQ, "%s: updated ns to %u\n",
             session->name, session->ns);
    }

    return bufp - optr;
}

static int l2tp_build_l2tpv3_header(struct l2tp_session *session, void *buf)
{
    struct l2tp_tunnel *tunnel = session->tunnel;
    char *bufp = buf;
    char *optr = bufp;

    /* Setup L2TP header. The header differs slightly for UDP and
     * IP encapsulations. For UDP, there is 4 bytes of flags.
     */
    if (tunnel->encap == L2TP_ENCAPTYPE_UDP) {
        u16 flags = L2TP_HDR_VER_3;
        *((__be16 *) bufp) = htons(flags);
        bufp += 2;
        *((__be16 *) bufp) = 0;
        bufp += 2;
    }

    *((__be32 *) bufp) = htonl(session->peer_session_id);
    bufp += 4;
    if (session->cookie_len) {
        memcpy(bufp, &session->cookie[0], session->cookie_len);
        bufp += session->cookie_len;
    }
    if (session->l2specific_len) {
        if (session->l2specific_type == L2TP_L2SPECTYPE_DEFAULT) {
            u32 l2h = 0;
            if (session->send_seq) {
                l2h = 0x40000000 | session->ns;
                session->ns++;
                session->ns &= 0xffffff;
                l2tp_dbg(session, L2TP_MSG_SEQ,
                     "%s: updated ns to %u\n",
                     session->name, session->ns);
            }

            *((__be32 *) bufp) = htonl(l2h);
        }
        bufp += session->l2specific_len;
    }
    if (session->offset)
        bufp += session->offset;

    return bufp - optr;
}

static int l2tp_xmit_core(struct l2tp_session *session, struct sk_buff *skb,
              struct flowi *fl, size_t data_len)
{
    struct l2tp_tunnel *tunnel = session->tunnel;
    unsigned int len = skb->len;
    int error;
    struct l2tp_stats *tstats, *sstats;

    /* Debug */
    if (session->send_seq)
        l2tp_dbg(session, L2TP_MSG_DATA, "%s: send %Zd bytes, ns=%u\n",
             session->name, data_len, session->ns - 1);
    else
        l2tp_dbg(session, L2TP_MSG_DATA, "%s: send %Zd bytes\n",
             session->name, data_len);

    if (session->debug & L2TP_MSG_DATA) {
        int uhlen = (tunnel->encap == L2TP_ENCAPTYPE_UDP) ? sizeof(struct udphdr) : 0;
        unsigned char *datap = skb->data + uhlen;

        pr_debug("%s: xmit\n", session->name);
        print_hex_dump_bytes("", DUMP_PREFIX_OFFSET,
                     datap, min_t(size_t, 32, len - uhlen));
    }

    /* Queue the packet to IP for output */
    skb->local_df = 1;
#if IS_ENABLED(CONFIG_IPV6)
    if (skb->sk->sk_family == PF_INET6)
        error = inet6_csk_xmit(skb, NULL);
    else
#endif
        error = ip_queue_xmit(skb, fl);

    /* Update stats */
    tstats = &tunnel->stats;
    u64_stats_update_begin(&tstats->syncp);
    sstats = &session->stats;
    u64_stats_update_begin(&sstats->syncp);
    if (error >= 0) {
        tstats->tx_packets++;
        tstats->tx_bytes += len;
        sstats->tx_packets++;
        sstats->tx_bytes += len;
    } else {
        tstats->tx_errors++;
        sstats->tx_errors++;
    }
    u64_stats_update_end(&tstats->syncp);
    u64_stats_update_end(&sstats->syncp);

    return 0;
}

/* Automatically called when the skb is freed.
 */
static void l2tp_sock_wfree(struct sk_buff *skb)
{
    sock_put(skb->sk);
}

/* For data skbs that we transmit, we associate with the tunnel socket
 * but don't do accounting.
 */
static inline void l2tp_skb_set_owner_w(struct sk_buff *skb, struct sock *sk)
{
    sock_hold(sk);
    skb->sk = sk;
    skb->destructor = l2tp_sock_wfree;
}

#if IS_ENABLED(CONFIG_IPV6)
static void l2tp_xmit_ipv6_csum(struct sock *sk, struct sk_buff *skb,
                int udp_len)
{
    struct ipv6_pinfo *np = inet6_sk(sk);
    struct udphdr *uh = udp_hdr(skb);

    if (!skb_dst(skb) || !skb_dst(skb)->dev ||
        !(skb_dst(skb)->dev->features & NETIF_F_IPV6_CSUM)) {
        __wsum csum = skb_checksum(skb, 0, udp_len, 0);
        skb->ip_summed = CHECKSUM_UNNECESSARY;
        uh->check = csum_ipv6_magic(&np->saddr, &np->daddr, udp_len,
                        IPPROTO_UDP, csum);
        if (uh->check == 0)
            uh->check = CSUM_MANGLED_0;
    } else {
        skb->ip_summed = CHECKSUM_PARTIAL;
        skb->csum_start = skb_transport_header(skb) - skb->head;
        skb->csum_offset = offsetof(struct udphdr, check);
        uh->check = ~csum_ipv6_magic(&np->saddr, &np->daddr,
                         udp_len, IPPROTO_UDP, 0);
    }
}
#endif

/* If caller requires the skb to have a ppp header, the header must be
 * inserted in the skb data before calling this function.
 */
int l2tp_xmit_skb(struct l2tp_session *session, struct sk_buff *skb, int hdr_len)
{
    int data_len = skb->len;
    struct l2tp_tunnel *tunnel = session->tunnel;
    struct sock *sk = tunnel->sock;
    struct flowi *fl;
    struct udphdr *uh;
    struct inet_sock *inet;
    __wsum csum;
    int old_headroom;
    int new_headroom;
    int headroom;
    int uhlen = (tunnel->encap == L2TP_ENCAPTYPE_UDP) ? sizeof(struct udphdr) : 0;
    int udp_len;
    int ret = NET_XMIT_SUCCESS;

    /* Check that there's enough headroom in the skb to insert IP,
     * UDP and L2TP headers. If not enough, expand it to
     * make room. Adjust truesize.
     */
    headroom = NET_SKB_PAD + sizeof(struct iphdr) +
        uhlen + hdr_len;
    old_headroom = skb_headroom(skb);
    if (skb_cow_head(skb, headroom)) {
        kfree_skb(skb);
        return NET_XMIT_DROP;
    }

    new_headroom = skb_headroom(skb);
    skb_orphan(skb);
    skb->truesize += new_headroom - old_headroom;

    /* Setup L2TP header */
    session->build_header(session, __skb_push(skb, hdr_len));

    /* Reset skb netfilter state */
    memset(&(IPCB(skb)->opt), 0, sizeof(IPCB(skb)->opt));
    IPCB(skb)->flags &= ~(IPSKB_XFRM_TUNNEL_SIZE | IPSKB_XFRM_TRANSFORMED |
                  IPSKB_REROUTED);
    nf_reset(skb);

    bh_lock_sock(sk);
    if (sock_owned_by_user(sk)) {
        kfree_skb(skb);
        ret = NET_XMIT_DROP;
        goto out_unlock;
    }

    /* Get routing info from the tunnel socket */
    skb_dst_drop(skb);
    skb_dst_set(skb, dst_clone(__sk_dst_check(sk, 0)));

    inet = inet_sk(sk);
    fl = &inet->cork.fl;
    switch (tunnel->encap) {
    case L2TP_ENCAPTYPE_UDP:
        /* Setup UDP header */
        __skb_push(skb, sizeof(*uh));
        skb_reset_transport_header(skb);
        uh = udp_hdr(skb);
        uh->source = inet->inet_sport;
        uh->dest = inet->inet_dport;
        udp_len = uhlen + hdr_len + data_len;
        uh->len = htons(udp_len);
        uh->check = 0;

        /* Calculate UDP checksum if configured to do so */
#if IS_ENABLED(CONFIG_IPV6)
        if (sk->sk_family == PF_INET6)
            l2tp_xmit_ipv6_csum(sk, skb, udp_len);
        else
#endif
        if (sk->sk_no_check == UDP_CSUM_NOXMIT)
            skb->ip_summed = CHECKSUM_NONE;
        else if ((skb_dst(skb) && skb_dst(skb)->dev) &&
             (!(skb_dst(skb)->dev->features & NETIF_F_V4_CSUM))) {
            skb->ip_summed = CHECKSUM_COMPLETE;
            csum = skb_checksum(skb, 0, udp_len, 0);
            uh->check = csum_tcpudp_magic(inet->inet_saddr,
                              inet->inet_daddr,
                              udp_len, IPPROTO_UDP, csum);
            if (uh->check == 0)
                uh->check = CSUM_MANGLED_0;
        } else {
            skb->ip_summed = CHECKSUM_PARTIAL;
            skb->csum_start = skb_transport_header(skb) - skb->head;
            skb->csum_offset = offsetof(struct udphdr, check);
            uh->check = ~csum_tcpudp_magic(inet->inet_saddr,
                               inet->inet_daddr,
                               udp_len, IPPROTO_UDP, 0);
        }
        break;

    case L2TP_ENCAPTYPE_IP:
        break;
    }

    l2tp_skb_set_owner_w(skb, sk);

    l2tp_xmit_core(session, skb, fl, data_len);
out_unlock:
    bh_unlock_sock(sk);

    return ret;
}
EXPORT_SYMBOL_GPL(l2tp_xmit_skb);

/*****************************************************************************
 * Tinnel and session create/destroy.
 *****************************************************************************/

/* Tunnel socket destruct hook.
 * The tunnel context is deleted only when all session sockets have been
 * closed.
 */
static void l2tp_tunnel_destruct(struct sock *sk)
{
    struct l2tp_tunnel *tunnel;

    tunnel = sk->sk_user_data;
    if (tunnel == NULL)
        goto end;

    l2tp_info(tunnel, L2TP_MSG_CONTROL, "%s: closing...\n", tunnel->name);

    /* Close all sessions */
    l2tp_tunnel_closeall(tunnel);

    switch (tunnel->encap) {
    case L2TP_ENCAPTYPE_UDP:
        /* No longer an encapsulation socket. See net/ipv4/udp.c */
        (udp_sk(sk))->encap_type = 0;
        (udp_sk(sk))->encap_rcv = NULL;
        break;
    case L2TP_ENCAPTYPE_IP:
        break;
    }

    /* Remove hooks into tunnel socket */
    tunnel->sock = NULL;
    sk->sk_destruct = tunnel->old_sk_destruct;
    sk->sk_user_data = NULL;

    /* Call the original destructor */
    if (sk->sk_destruct)
        (*sk->sk_destruct)(sk);

    /* We're finished with the socket */
    l2tp_tunnel_dec_refcount(tunnel);

end:
    return;
}

/* When the tunnel is closed, all the attached sessions need to go too.
 */
static void l2tp_tunnel_closeall(struct l2tp_tunnel *tunnel)
{
    int hash;
    struct hlist_node *walk;
    struct hlist_node *tmp;
    struct l2tp_session *session;

    BUG_ON(tunnel == NULL);

    l2tp_info(tunnel, L2TP_MSG_CONTROL, "%s: closing all sessions...\n",
          tunnel->name);

    write_lock_bh(&tunnel->hlist_lock);
    for (hash = 0; hash < L2TP_HASH_SIZE; hash++) {
again:
        hlist_for_each_safe(walk, tmp, &tunnel->session_hlist[hash]) {
            session = hlist_entry(walk, struct l2tp_session, hlist);

            l2tp_info(session, L2TP_MSG_CONTROL,
                  "%s: closing session\n", session->name);

            hlist_del_init(&session->hlist);

            /* Since we should hold the sock lock while
             * doing any unbinding, we need to release the
             * lock we're holding before taking that lock.
             * Hold a reference to the sock so it doesn't
             * disappear as we're jumping between locks.
             */
            if (session->ref != NULL)
                (*session->ref)(session);

            write_unlock_bh(&tunnel->hlist_lock);

            if (tunnel->version != L2TP_HDR_VER_2) {
                struct l2tp_net *pn = l2tp_pernet(tunnel->l2tp_net);

                spin_lock_bh(&pn->l2tp_session_hlist_lock);
                hlist_del_init_rcu(&session->global_hlist);
                spin_unlock_bh(&pn->l2tp_session_hlist_lock);
                synchronize_rcu();
            }

            if (session->session_close != NULL)
                (*session->session_close)(session);

            if (session->deref != NULL)
                (*session->deref)(session);

            write_lock_bh(&tunnel->hlist_lock);

            /* Now restart from the beginning of this hash
             * chain.  We always remove a session from the
             * list so we are guaranteed to make forward
             * progress.
             */
            goto again;
        }
    }
    write_unlock_bh(&tunnel->hlist_lock);
}

/* Really kill the tunnel.
 * Come here only when all sessions have been cleared from the tunnel.
 */
static void l2tp_tunnel_free(struct l2tp_tunnel *tunnel)
{
    struct l2tp_net *pn = l2tp_pernet(tunnel->l2tp_net);

    BUG_ON(atomic_read(&tunnel->ref_count) != 0);
    BUG_ON(tunnel->sock != NULL);

    l2tp_info(tunnel, L2TP_MSG_CONTROL, "%s: free...\n", tunnel->name);

    /* Remove from tunnel list */
    spin_lock_bh(&pn->l2tp_tunnel_list_lock);
    list_del_rcu(&tunnel->list);
    kfree_rcu(tunnel, rcu);
    spin_unlock_bh(&pn->l2tp_tunnel_list_lock);

    atomic_dec(&l2tp_tunnel_count);
}

/* Create a socket for the tunnel, if one isn't set up by
 * userspace. This is used for static tunnels where there is no
 * managing L2TP daemon.
 */
static int l2tp_tunnel_sock_create(u32 tunnel_id, u32 peer_tunnel_id, struct l2tp_tunnel_cfg *cfg, struct socket **sockp)
{
    int err = -EINVAL;
    struct sockaddr_in udp_addr;
#if IS_ENABLED(CONFIG_IPV6)
    struct sockaddr_in6 udp6_addr;
    struct sockaddr_l2tpip6 ip6_addr;
#endif
    struct sockaddr_l2tpip ip_addr;
    struct socket *sock = NULL;

    switch (cfg->encap) {
    case L2TP_ENCAPTYPE_UDP:
#if IS_ENABLED(CONFIG_IPV6)
        if (cfg->local_ip6 && cfg->peer_ip6) {
            err = sock_create(AF_INET6, SOCK_DGRAM, 0, sockp);
            if (err < 0)
                goto out;

            sock = *sockp;

            memset(&udp6_addr, 0, sizeof(udp6_addr));
            udp6_addr.sin6_family = AF_INET6;
            memcpy(&udp6_addr.sin6_addr, cfg->local_ip6,
                   sizeof(udp6_addr.sin6_addr));
            udp6_addr.sin6_port = htons(cfg->local_udp_port);
            err = kernel_bind(sock, (struct sockaddr *) &udp6_addr,
                      sizeof(udp6_addr));
            if (err < 0)
                goto out;

            udp6_addr.sin6_family = AF_INET6;
            memcpy(&udp6_addr.sin6_addr, cfg->peer_ip6,
                   sizeof(udp6_addr.sin6_addr));
            udp6_addr.sin6_port = htons(cfg->peer_udp_port);
            err = kernel_connect(sock,
                         (struct sockaddr *) &udp6_addr,
                         sizeof(udp6_addr), 0);
            if (err < 0)
                goto out;
        } else
#endif
        {
            err = sock_create(AF_INET, SOCK_DGRAM, 0, sockp);
            if (err < 0)
                goto out;

            sock = *sockp;

            memset(&udp_addr, 0, sizeof(udp_addr));
            udp_addr.sin_family = AF_INET;
            udp_addr.sin_addr = cfg->local_ip;
            udp_addr.sin_port = htons(cfg->local_udp_port);
            err = kernel_bind(sock, (struct sockaddr *) &udp_addr,
                      sizeof(udp_addr));
            if (err < 0)
                goto out;

            udp_addr.sin_family = AF_INET;
            udp_addr.sin_addr = cfg->peer_ip;
            udp_addr.sin_port = htons(cfg->peer_udp_port);
            err = kernel_connect(sock,
                         (struct sockaddr *) &udp_addr,
                         sizeof(udp_addr), 0);
            if (err < 0)
                goto out;
        }

        if (!cfg->use_udp_checksums)
            sock->sk->sk_no_check = UDP_CSUM_NOXMIT;

        break;

    case L2TP_ENCAPTYPE_IP:
#if IS_ENABLED(CONFIG_IPV6)
        if (cfg->local_ip6 && cfg->peer_ip6) {
            err = sock_create(AF_INET6, SOCK_DGRAM, IPPROTO_L2TP,
                      sockp);
            if (err < 0)
                goto out;

            sock = *sockp;

            memset(&ip6_addr, 0, sizeof(ip6_addr));
            ip6_addr.l2tp_family = AF_INET6;
            memcpy(&ip6_addr.l2tp_addr, cfg->local_ip6,
                   sizeof(ip6_addr.l2tp_addr));
            ip6_addr.l2tp_conn_id = tunnel_id;
            err = kernel_bind(sock, (struct sockaddr *) &ip6_addr,
                      sizeof(ip6_addr));
            if (err < 0)
                goto out;

            ip6_addr.l2tp_family = AF_INET6;
            memcpy(&ip6_addr.l2tp_addr, cfg->peer_ip6,
                   sizeof(ip6_addr.l2tp_addr));
            ip6_addr.l2tp_conn_id = peer_tunnel_id;
            err = kernel_connect(sock,
                         (struct sockaddr *) &ip6_addr,
                         sizeof(ip6_addr), 0);
            if (err < 0)
                goto out;
        } else
#endif
        {
            err = sock_create(AF_INET, SOCK_DGRAM, IPPROTO_L2TP,
                      sockp);
            if (err < 0)
                goto out;

            sock = *sockp;

            memset(&ip_addr, 0, sizeof(ip_addr));
            ip_addr.l2tp_family = AF_INET;
            ip_addr.l2tp_addr = cfg->local_ip;
            ip_addr.l2tp_conn_id = tunnel_id;
            err = kernel_bind(sock, (struct sockaddr *) &ip_addr,
                      sizeof(ip_addr));
            if (err < 0)
                goto out;

            ip_addr.l2tp_family = AF_INET;
            ip_addr.l2tp_addr = cfg->peer_ip;
            ip_addr.l2tp_conn_id = peer_tunnel_id;
            err = kernel_connect(sock, (struct sockaddr *) &ip_addr,
                         sizeof(ip_addr), 0);
            if (err < 0)
                goto out;
        }
        break;

    default:
        goto out;
    }

out:
    if ((err < 0) && sock) {
        sock_release(sock);
        *sockp = NULL;
    }

    return err;
}

static struct lock_class_key l2tp_socket_class;

int l2tp_tunnel_create(struct net *net, int fd, int version, u32 tunnel_id, u32 peer_tunnel_id, struct l2tp_tunnel_cfg *cfg, struct l2tp_tunnel **tunnelp)
{
    struct l2tp_tunnel *tunnel = NULL;
    int err;
    struct socket *sock = NULL;
    struct sock *sk = NULL;
    struct l2tp_net *pn;
    enum l2tp_encap_type encap = L2TP_ENCAPTYPE_UDP;

    /* Get the tunnel socket from the fd, which was opened by
     * the userspace L2TP daemon. If not specified, create a
     * kernel socket.
     */
    if (fd < 0) {
        err = l2tp_tunnel_sock_create(tunnel_id, peer_tunnel_id, cfg, &sock);
        if (err < 0)
            goto err;
    } else {
        err = -EBADF;
        sock = sockfd_lookup(fd, &err);
        if (!sock) {
            pr_err("tunl %hu: sockfd_lookup(fd=%d) returned %d\n",
                   tunnel_id, fd, err);
            goto err;
        }
    }

    sk = sock->sk;

    if (cfg != NULL)
        encap = cfg->encap;

    /* Quick sanity checks */
    switch (encap) {
    case L2TP_ENCAPTYPE_UDP:
        err = -EPROTONOSUPPORT;
        if (sk->sk_protocol != IPPROTO_UDP) {
            pr_err("tunl %hu: fd %d wrong protocol, got %d, expected %d\n",
                   tunnel_id, fd, sk->sk_protocol, IPPROTO_UDP);
            goto err;
        }
        break;
    case L2TP_ENCAPTYPE_IP:
        err = -EPROTONOSUPPORT;
        if (sk->sk_protocol != IPPROTO_L2TP) {
            pr_err("tunl %hu: fd %d wrong protocol, got %d, expected %d\n",
                   tunnel_id, fd, sk->sk_protocol, IPPROTO_L2TP);
            goto err;
        }
        break;
    }

    /* Check if this socket has already been prepped */
    tunnel = (struct l2tp_tunnel *)sk->sk_user_data;
    if (tunnel != NULL) {
        /* This socket has already been prepped */
        err = -EBUSY;
        goto err;
    }

    tunnel = kzalloc(sizeof(struct l2tp_tunnel), GFP_KERNEL);
    if (tunnel == NULL) {
        err = -ENOMEM;
        goto err;
    }

    tunnel->version = version;
    tunnel->tunnel_id = tunnel_id;
    tunnel->peer_tunnel_id = peer_tunnel_id;
    tunnel->debug = L2TP_DEFAULT_DEBUG_FLAGS;

    tunnel->magic = L2TP_TUNNEL_MAGIC;
    sprintf(&tunnel->name[0], "tunl %u", tunnel_id);
    rwlock_init(&tunnel->hlist_lock);

    /* The net we belong to */
    tunnel->l2tp_net = net;
    pn = l2tp_pernet(net);

    if (cfg != NULL)
        tunnel->debug = cfg->debug;

    /* Mark socket as an encapsulation socket. See net/ipv4/udp.c */
    tunnel->encap = encap;
    if (encap == L2TP_ENCAPTYPE_UDP) {
        /* Mark socket as an encapsulation socket. See net/ipv4/udp.c */
        udp_sk(sk)->encap_type = UDP_ENCAP_L2TPINUDP;
        udp_sk(sk)->encap_rcv = l2tp_udp_encap_recv;
#if IS_ENABLED(CONFIG_IPV6)
        if (sk->sk_family == PF_INET6)
            udpv6_encap_enable();
        else
#endif
        udp_encap_enable();
    }

    sk->sk_user_data = tunnel;

    /* Hook on the tunnel socket destructor so that we can cleanup
     * if the tunnel socket goes away.
     */
    tunnel->old_sk_destruct = sk->sk_destruct;
    sk->sk_destruct = &l2tp_tunnel_destruct;
    tunnel->sock = sk;
    lockdep_set_class_and_name(&sk->sk_lock.slock, &l2tp_socket_class, "l2tp_sock");

    sk->sk_allocation = GFP_ATOMIC;

    /* Add tunnel to our list */
    INIT_LIST_HEAD(&tunnel->list);
    atomic_inc(&l2tp_tunnel_count);

    /* Bump the reference count. The tunnel context is deleted
     * only when this drops to zero. Must be done before list insertion
     */
    l2tp_tunnel_inc_refcount(tunnel);
    spin_lock_bh(&pn->l2tp_tunnel_list_lock);
    list_add_rcu(&tunnel->list, &pn->l2tp_tunnel_list);
    spin_unlock_bh(&pn->l2tp_tunnel_list_lock);

    err = 0;
err:
    if (tunnelp)
        *tunnelp = tunnel;

    /* If tunnel's socket was created by the kernel, it doesn't
     *  have a file.
     */
    if (sock && sock->file)
        sockfd_put(sock);

    return err;
}
EXPORT_SYMBOL_GPL(l2tp_tunnel_create);

/* This function is used by the netlink TUNNEL_DELETE command.
 */
int l2tp_tunnel_delete(struct l2tp_tunnel *tunnel)
{
    int err = 0;
    struct socket *sock = tunnel->sock ? tunnel->sock->sk_socket : NULL;

    /* Force the tunnel socket to close. This will eventually
     * cause the tunnel to be deleted via the normal socket close
     * mechanisms when userspace closes the tunnel socket.
     */
    if (sock != NULL) {
        err = inet_shutdown(sock, 2);

        /* If the tunnel's socket was created by the kernel,
         * close the socket here since the socket was not
         * created by userspace.
         */
        if (sock->file == NULL)
            err = inet_release(sock);
    }

    return err;
}
EXPORT_SYMBOL_GPL(l2tp_tunnel_delete);

/* Really kill the session.
 */
void l2tp_session_free(struct l2tp_session *session)
{
    struct l2tp_tunnel *tunnel;

    BUG_ON(atomic_read(&session->ref_count) != 0);

    tunnel = session->tunnel;
    if (tunnel != NULL) {
        BUG_ON(tunnel->magic != L2TP_TUNNEL_MAGIC);

        /* Delete the session from the hash */
        write_lock_bh(&tunnel->hlist_lock);
        hlist_del_init(&session->hlist);
        write_unlock_bh(&tunnel->hlist_lock);

        /* Unlink from the global hash if not L2TPv2 */
        if (tunnel->version != L2TP_HDR_VER_2) {
            struct l2tp_net *pn = l2tp_pernet(tunnel->l2tp_net);

            spin_lock_bh(&pn->l2tp_session_hlist_lock);
            hlist_del_init_rcu(&session->global_hlist);
            spin_unlock_bh(&pn->l2tp_session_hlist_lock);
            synchronize_rcu();
        }

        if (session->session_id != 0)
            atomic_dec(&l2tp_session_count);

        sock_put(tunnel->sock);

        /* This will delete the tunnel context if this
         * is the last session on the tunnel.
         */
        session->tunnel = NULL;
        l2tp_tunnel_dec_refcount(tunnel);
    }

    kfree(session);

    return;
}
EXPORT_SYMBOL_GPL(l2tp_session_free);

/* This function is used by the netlink SESSION_DELETE command and by
   pseudowire modules.
 */
int l2tp_session_delete(struct l2tp_session *session)
{
    if (session->session_close != NULL)
        (*session->session_close)(session);

    l2tp_session_dec_refcount(session);

    return 0;
}
EXPORT_SYMBOL_GPL(l2tp_session_delete);


/* We come here whenever a session's send_seq, cookie_len or
 * l2specific_len parameters are set.
 */
static void l2tp_session_set_header_len(struct l2tp_session *session, int version)
{
    if (version == L2TP_HDR_VER_2) {
        session->hdr_len = 6;
        if (session->send_seq)
            session->hdr_len += 4;
    } else {
        session->hdr_len = 4 + session->cookie_len + session->l2specific_len + session->offset;
        if (session->tunnel->encap == L2TP_ENCAPTYPE_UDP)
            session->hdr_len += 4;
    }

}

struct l2tp_session *l2tp_session_create(int priv_size, struct l2tp_tunnel *tunnel, u32 session_id, u32 peer_session_id, struct l2tp_session_cfg *cfg)
{
    struct l2tp_session *session;

    session = kzalloc(sizeof(struct l2tp_session) + priv_size, GFP_KERNEL);
    if (session != NULL) {
        session->magic = L2TP_SESSION_MAGIC;
        session->tunnel = tunnel;

        session->session_id = session_id;
        session->peer_session_id = peer_session_id;
        session->nr = 0;

        sprintf(&session->name[0], "sess %u/%u",
            tunnel->tunnel_id, session->session_id);

        skb_queue_head_init(&session->reorder_q);

        INIT_HLIST_NODE(&session->hlist);
        INIT_HLIST_NODE(&session->global_hlist);

        /* Inherit debug options from tunnel */
        session->debug = tunnel->debug;

        if (cfg) {
            session->pwtype = cfg->pw_type;
            session->debug = cfg->debug;
            session->mtu = cfg->mtu;
            session->mru = cfg->mru;
            session->send_seq = cfg->send_seq;
            session->recv_seq = cfg->recv_seq;
            session->lns_mode = cfg->lns_mode;
            session->reorder_timeout = cfg->reorder_timeout;
            session->offset = cfg->offset;
            session->l2specific_type = cfg->l2specific_type;
            session->l2specific_len = cfg->l2specific_len;
            session->cookie_len = cfg->cookie_len;
            memcpy(&session->cookie[0], &cfg->cookie[0], cfg->cookie_len);
            session->peer_cookie_len = cfg->peer_cookie_len;
            memcpy(&session->peer_cookie[0], &cfg->peer_cookie[0], cfg->peer_cookie_len);
        }

        if (tunnel->version == L2TP_HDR_VER_2)
            session->build_header = l2tp_build_l2tpv2_header;
        else
            session->build_header = l2tp_build_l2tpv3_header;

        l2tp_session_set_header_len(session, tunnel->version);

        /* Bump the reference count. The session context is deleted
         * only when this drops to zero.
         */
        l2tp_session_inc_refcount(session);
        l2tp_tunnel_inc_refcount(tunnel);

        /* Ensure tunnel socket isn't deleted */
        sock_hold(tunnel->sock);

        /* Add session to the tunnel's hash list */
        write_lock_bh(&tunnel->hlist_lock);
        hlist_add_head(&session->hlist,
                   l2tp_session_id_hash(tunnel, session_id));
        write_unlock_bh(&tunnel->hlist_lock);

        /* And to the global session list if L2TPv3 */
        if (tunnel->version != L2TP_HDR_VER_2) {
            struct l2tp_net *pn = l2tp_pernet(tunnel->l2tp_net);

            spin_lock_bh(&pn->l2tp_session_hlist_lock);
            hlist_add_head_rcu(&session->global_hlist,
                       l2tp_session_id_hash_2(pn, session_id));
            spin_unlock_bh(&pn->l2tp_session_hlist_lock);
        }

        /* Ignore management session in session count value */
        if (session->session_id != 0)
            atomic_inc(&l2tp_session_count);
    }

    return session;
}
EXPORT_SYMBOL_GPL(l2tp_session_create);

/*****************************************************************************
 * Init and cleanup
 *****************************************************************************/

static __net_init int l2tp_init_net(struct net *net)
{
    struct l2tp_net *pn = net_generic(net, l2tp_net_id);
    int hash;

    INIT_LIST_HEAD(&pn->l2tp_tunnel_list);
    spin_lock_init(&pn->l2tp_tunnel_list_lock);

    for (hash = 0; hash < L2TP_HASH_SIZE_2; hash++)
        INIT_HLIST_HEAD(&pn->l2tp_session_hlist[hash]);

    spin_lock_init(&pn->l2tp_session_hlist_lock);

    return 0;
}

static struct pernet_operations l2tp_net_ops = {
    .init = l2tp_init_net,
    .id   = &l2tp_net_id,
    .size = sizeof(struct l2tp_net),
};

static int __init l2tp_init(void)
{
    int rc = 0;

    rc = register_pernet_device(&l2tp_net_ops);
    if (rc)
        goto out;

    pr_info("L2TP core driver, %s\n", L2TP_DRV_VERSION);

out:
    return rc;
}

static void __exit l2tp_exit(void)
{
    unregister_pernet_device(&l2tp_net_ops);
}

module_init(l2tp_init);
module_exit(l2tp_exit);

MODULE_AUTHOR("James Chapman <[email protected]>");
MODULE_DESCRIPTION("L2TP core");
MODULE_LICENSE("GPL");
MODULE_VERSION(L2TP_DRV_VERSION);