pptp.c

// SPDX-License-Identifier: GPL-2.0-or-later
/*
 *  Point-to-Point Tunneling Protocol for Linux
 *
 *	Authors: Dmitry Kozlov <xeb@mail.ru>
 */

#include <linux/string.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/slab.h>
#include <linux/errno.h>
#include <linux/netdevice.h>
#include <linux/net.h>
#include <linux/skbuff.h>
#include <linux/vmalloc.h>
#include <linux/init.h>
#include <linux/ppp_channel.h>
#include <linux/ppp_defs.h>
#include <linux/if_pppox.h>
#include <linux/ppp-ioctl.h>
#include <linux/notifier.h>
#include <linux/file.h>
#include <linux/in.h>
#include <linux/ip.h>
#include <linux/rcupdate.h>
#include <linux/security.h>
#include <linux/spinlock.h>

#include <net/sock.h>
#include <net/protocol.h>
#include <net/ip.h>
#include <net/icmp.h>
#include <net/route.h>
#include <net/gre.h>
#include <net/pptp.h>

#include <linux/uaccess.h>

#define PPTP_DRIVER_VERSION "0.8.5"

#define MAX_CALLID 65535

static DECLARE_BITMAP(callid_bitmap, MAX_CALLID + 1);
static struct pppox_sock __rcu **callid_sock;

static DEFINE_SPINLOCK(chan_lock);

static struct proto pptp_sk_proto __read_mostly;
static const struct ppp_channel_ops pptp_chan_ops;
static const struct proto_ops pptp_ops;

static struct pppox_sock *lookup_chan(u16 call_id, __be32 s_addr)
{
	struct pppox_sock *sock;
	struct pptp_opt *opt;

	rcu_read_lock();
	sock = rcu_dereference(callid_sock[call_id]);
	if (sock) {
		opt = &sock->proto.pptp;
		if (opt->dst_addr.sin_addr.s_addr != s_addr)
			sock = NULL;
		else
			sock_hold(sk_pppox(sock));
	}
	rcu_read_unlock();

	return sock;
}

static int lookup_chan_dst(u16 call_id, __be32 d_addr)
{
	struct pppox_sock *sock;
	struct pptp_opt *opt;
	int i;

	rcu_read_lock();
	i = 1;
	for_each_set_bit_from(i, callid_bitmap, MAX_CALLID) {
		sock = rcu_dereference(callid_sock[i]);
		if (!sock)
			continue;
		opt = &sock->proto.pptp;
		if (opt->dst_addr.call_id == call_id &&
			  opt->dst_addr.sin_addr.s_addr == d_addr)
			break;
	}
	rcu_read_unlock();

	return i < MAX_CALLID;
}

static int add_chan(struct pppox_sock *sock,
		    struct pptp_addr *sa)
{
	static int call_id;

	spin_lock(&chan_lock);
	if (!sa->call_id)	{
		call_id = find_next_zero_bit(callid_bitmap, MAX_CALLID, call_id + 1);
		if (call_id == MAX_CALLID) {
			call_id = find_next_zero_bit(callid_bitmap, MAX_CALLID, 1);
			if (call_id == MAX_CALLID)
				goto out_err;
		}
		sa->call_id = call_id;
	} else if (test_bit(sa->call_id, callid_bitmap)) {
		goto out_err;
	}

	sock->proto.pptp.src_addr = *sa;
	set_bit(sa->call_id, callid_bitmap);
	rcu_assign_pointer(callid_sock[sa->call_id], sock);
	spin_unlock(&chan_lock);

	return 0;

out_err:
	spin_unlock(&chan_lock);
	return -1;
}

static void del_chan(struct pppox_sock *sock)
{
	spin_lock(&chan_lock);
	clear_bit(sock->proto.pptp.src_addr.call_id, callid_bitmap);
	RCU_INIT_POINTER(callid_sock[sock->proto.pptp.src_addr.call_id], NULL);
	spin_unlock(&chan_lock);
}

static struct rtable *pptp_route_output(const struct pppox_sock *po,
					struct flowi4 *fl4)
{
	const struct sock *sk = &po->sk;
	struct net *net;

	net = sock_net(sk);
	flowi4_init_output(fl4, sk->sk_bound_dev_if, sk->sk_mark, 0,
			   RT_SCOPE_UNIVERSE, IPPROTO_GRE, 0,
			   po->proto.pptp.dst_addr.sin_addr.s_addr,
			   po->proto.pptp.src_addr.sin_addr.s_addr,
			   0, 0, sock_net_uid(net, sk));
	security_sk_classify_flow(sk, flowi4_to_flowi_common(fl4));

	return ip_route_output_flow(net, fl4, sk);
}

static int pptp_xmit(struct ppp_channel *chan, struct sk_buff *skb)
{
	struct sock *sk = chan->private;
	struct pppox_sock *po = pppox_sk(sk);
	struct net *net = sock_net(sk);
	struct pptp_opt *opt = &po->proto.pptp;
	struct pptp_gre_header *hdr;
	unsigned int header_len = sizeof(*hdr);
	struct flowi4 fl4;
	int islcp;
	int len;
	unsigned char *data;
	__u32 seq_recv;


	struct rtable *rt;
	struct net_device *tdev;
	struct iphdr  *iph;
	int    max_headroom;

	if (sk_pppox(po)->sk_state & PPPOX_DEAD)
		goto tx_error;

	rt = pptp_route_output(po, &fl4);
	if (IS_ERR(rt))
		goto tx_error;

	tdev = rt->dst.dev;

	max_headroom = LL_RESERVED_SPACE(tdev) + sizeof(*iph) + sizeof(*hdr) + 2;

	if (skb_headroom(skb) < max_headroom || skb_cloned(skb) || skb_shared(skb)) {
		struct sk_buff *new_skb = skb_realloc_headroom(skb, max_headroom);
		if (!new_skb) {
			ip_rt_put(rt);
			goto tx_error;
		}
		if (skb->sk)
			skb_set_owner_w(new_skb, skb->sk);
		consume_skb(skb);
		skb = new_skb;
	}

	data = skb->data;
	islcp = ((data[0] << 8) + data[1]) == PPP_LCP && 1 <= data[2] && data[2] <= 7;

	/* compress protocol field */
	if ((opt->ppp_flags & SC_COMP_PROT) && data[0] == 0 && !islcp)
		skb_pull(skb, 1);

	/* Put in the address/control bytes if necessary */
	if ((opt->ppp_flags & SC_COMP_AC) == 0 || islcp) {
		data = skb_push(skb, 2);
		data[0] = PPP_ALLSTATIONS;
		data[1] = PPP_UI;
	}

	len = skb->len;

	seq_recv = opt->seq_recv;

	if (opt->ack_sent == seq_recv)
		header_len -= sizeof(hdr->ack);

	/* Push down and install GRE header */
	skb_push(skb, header_len);
	hdr = (struct pptp_gre_header *)(skb->data);

	hdr->gre_hd.flags = GRE_KEY | GRE_VERSION_1 | GRE_SEQ;
	hdr->gre_hd.protocol = GRE_PROTO_PPP;
	hdr->call_id = htons(opt->dst_addr.call_id);

	hdr->seq = htonl(++opt->seq_sent);
	if (opt->ack_sent != seq_recv)	{
		/* send ack with this message */
		hdr->gre_hd.flags |= GRE_ACK;
		hdr->ack  = htonl(seq_recv);
		opt->ack_sent = seq_recv;
	}
	hdr->payload_len = htons(len);

	/*	Push down and install the IP header. */

	skb_reset_transport_header(skb);
	skb_push(skb, sizeof(*iph));
	skb_reset_network_header(skb);
	memset(&(IPCB(skb)->opt), 0, sizeof(IPCB(skb)->opt));
	IPCB(skb)->flags &= ~(IPSKB_XFRM_TUNNEL_SIZE | IPSKB_XFRM_TRANSFORMED | IPSKB_REROUTED);

	iph =	ip_hdr(skb);
	iph->version =	4;
	iph->ihl =	sizeof(struct iphdr) >> 2;
	if (ip_dont_fragment(sk, &rt->dst))
		iph->frag_off	=	htons(IP_DF);
	else
		iph->frag_off	=	0;
	iph->protocol = IPPROTO_GRE;
	iph->tos      = 0;
	iph->daddr    = fl4.daddr;
	iph->saddr    = fl4.saddr;
	iph->ttl      = ip4_dst_hoplimit(&rt->dst);
	iph->tot_len  = htons(skb->len);

	skb_dst_drop(skb);
	skb_dst_set(skb, &rt->dst);

	nf_reset_ct(skb);

	skb->ip_summed = CHECKSUM_NONE;
	ip_select_ident(net, skb, NULL);
	ip_send_check(iph);

	ip_local_out(net, skb->sk, skb);
	return 1;

tx_error:
	kfree_skb(skb);
	return 1;
}

static int pptp_rcv_core(struct sock *sk, struct sk_buff *skb)
{
	struct pppox_sock *po = pppox_sk(sk);
	struct pptp_opt *opt = &po->proto.pptp;
	int headersize, payload_len, seq;
	__u8 *payload;
	struct pptp_gre_header *header;

	if (!(sk->sk_state & PPPOX_CONNECTED)) {
		if (sock_queue_rcv_skb(sk, skb))
			goto drop;
		return NET_RX_SUCCESS;
	}

	header = (struct pptp_gre_header *)(skb->data);
	headersize  = sizeof(*header);

	/* test if acknowledgement present */
	if (GRE_IS_ACK(header->gre_hd.flags)) {
		__u32 ack;

		if (!pskb_may_pull(skb, headersize))
			goto drop;
		header = (struct pptp_gre_header *)(skb->data);

		/* ack in different place if S = 0 */
		ack = GRE_IS_SEQ(header->gre_hd.flags) ? ntohl(header->ack) :
							 ntohl(header->seq);
		if (ack > opt->ack_recv)
			opt->ack_recv = ack;
		/* also handle sequence number wrap-around  */
		if (WRAPPED(ack, opt->ack_recv))
			opt->ack_recv = ack;
	} else {
		headersize -= sizeof(header->ack);
	}
	/* test if payload present */
	if (!GRE_IS_SEQ(header->gre_hd.flags))
		goto drop;

	payload_len = ntohs(header->payload_len);
	seq         = ntohl(header->seq);

	/* check for incomplete packet (length smaller than expected) */
	if (!pskb_may_pull(skb, headersize + payload_len))
		goto drop;

	payload = skb->data + headersize;
	/* check for expected sequence number */
	if (seq < opt->seq_recv + 1 || WRAPPED(opt->seq_recv, seq)) {
		if ((payload[0] == PPP_ALLSTATIONS) && (payload[1] == PPP_UI) &&
				(PPP_PROTOCOL(payload) == PPP_LCP) &&
				((payload[4] == PPP_LCP_ECHOREQ) || (payload[4] == PPP_LCP_ECHOREP)))
			goto allow_packet;
	} else {
		opt->seq_recv = seq;
allow_packet:
		skb_pull(skb, headersize);

		if (payload[0] == PPP_ALLSTATIONS && payload[1] == PPP_UI) {
			/* chop off address/control */
			if (skb->len < 3)
				goto drop;
			skb_pull(skb, 2);
		}

		skb->ip_summed = CHECKSUM_NONE;
		skb_set_network_header(skb, skb->head-skb->data);
		ppp_input(&po->chan, skb);

		return NET_RX_SUCCESS;
	}
drop:
	kfree_skb(skb);
	return NET_RX_DROP;
}

static int pptp_rcv(struct sk_buff *skb)
{
	struct pppox_sock *po;
	struct pptp_gre_header *header;
	struct iphdr *iph;

	if (skb->pkt_type != PACKET_HOST)
		goto drop;

	if (!pskb_may_pull(skb, 12))
		goto drop;

	iph = ip_hdr(skb);

	header = (struct pptp_gre_header *)skb->data;

	if (header->gre_hd.protocol != GRE_PROTO_PPP || /* PPTP-GRE protocol for PPTP */
		GRE_IS_CSUM(header->gre_hd.flags) ||    /* flag CSUM should be clear */
		GRE_IS_ROUTING(header->gre_hd.flags) || /* flag ROUTING should be clear */
		!GRE_IS_KEY(header->gre_hd.flags) ||    /* flag KEY should be set */
		(header->gre_hd.flags & GRE_FLAGS))     /* flag Recursion Ctrl should be clear */
		/* if invalid, discard this packet */
		goto drop;

	po = lookup_chan(ntohs(header->call_id), iph->saddr);
	if (po) {
		skb_dst_drop(skb);
		nf_reset_ct(skb);
		return sk_receive_skb(sk_pppox(po), skb, 0);
	}
drop:
	kfree_skb(skb);
	return NET_RX_DROP;
}

static int pptp_bind(struct socket *sock, struct sockaddr *uservaddr,
	int sockaddr_len)
{
	struct sock *sk = sock->sk;
	struct sockaddr_pppox *sp = (struct sockaddr_pppox *) uservaddr;
	struct pppox_sock *po = pppox_sk(sk);
	int error = 0;

	if (sockaddr_len < sizeof(struct sockaddr_pppox))
		return -EINVAL;

	lock_sock(sk);

	if (sk->sk_state & PPPOX_DEAD) {
		error = -EALREADY;
		goto out;
	}

	if (sk->sk_state & PPPOX_BOUND) {
		error = -EBUSY;
		goto out;
	}

	if (add_chan(po, &sp->sa_addr.pptp))
		error = -EBUSY;
	else
		sk->sk_state |= PPPOX_BOUND;

out:
	release_sock(sk);
	return error;
}

static int pptp_connect(struct socket *sock, struct sockaddr *uservaddr,
	int sockaddr_len, int flags)
{
	struct sock *sk = sock->sk;
	struct sockaddr_pppox *sp = (struct sockaddr_pppox *) uservaddr;
	struct pppox_sock *po = pppox_sk(sk);
	struct pptp_opt *opt = &po->proto.pptp;
	struct rtable *rt;
	struct flowi4 fl4;
	int error = 0;

	if (sockaddr_len < sizeof(struct sockaddr_pppox))
		return -EINVAL;

	if (sp->sa_protocol != PX_PROTO_PPTP)
		return -EINVAL;

	if (lookup_chan_dst(sp->sa_addr.pptp.call_id, sp->sa_addr.pptp.sin_addr.s_addr))
		return -EALREADY;

	lock_sock(sk);
	/* Check for already bound sockets */
	if (sk->sk_state & PPPOX_CONNECTED) {
		error = -EBUSY;
		goto end;
	}

	/* Check for already disconnected sockets, on attempts to disconnect */
	if (sk->sk_state & PPPOX_DEAD) {
		error = -EALREADY;
		goto end;
	}

	if (!opt->src_addr.sin_addr.s_addr || !sp->sa_addr.pptp.sin_addr.s_addr) {
		error = -EINVAL;
		goto end;
	}

	po->chan.private = sk;
	po->chan.ops = &pptp_chan_ops;

	rt = pptp_route_output(po, &fl4);
	if (IS_ERR(rt)) {
		error = -EHOSTUNREACH;
		goto end;
	}
	sk_setup_caps(sk, &rt->dst);

	po->chan.mtu = dst_mtu(&rt->dst);
	if (!po->chan.mtu)
		po->chan.mtu = PPP_MRU;
	po->chan.mtu -= PPTP_HEADER_OVERHEAD;

	po->chan.hdrlen = 2 + sizeof(struct pptp_gre_header);
	error = ppp_register_channel(&po->chan);
	if (error) {
		pr_err("PPTP: failed to register PPP channel (%d)\n", error);
		goto end;
	}

	opt->dst_addr = sp->sa_addr.pptp;
	sk->sk_state |= PPPOX_CONNECTED;

 end:
	release_sock(sk);
	return error;
}

static int pptp_getname(struct socket *sock, struct sockaddr *uaddr,
	int peer)
{
	int len = sizeof(struct sockaddr_pppox);
	struct sockaddr_pppox sp;

	memset(&sp.sa_addr, 0, sizeof(sp.sa_addr));

	sp.sa_family    = AF_PPPOX;
	sp.sa_protocol  = PX_PROTO_PPTP;
	sp.sa_addr.pptp = pppox_sk(sock->sk)->proto.pptp.src_addr;

	memcpy(uaddr, &sp, len);

	return len;
}

static int pptp_release(struct socket *sock)
{
	struct sock *sk = sock->sk;
	struct pppox_sock *po;
	int error = 0;

	if (!sk)
		return 0;

	lock_sock(sk);

	if (sock_flag(sk, SOCK_DEAD)) {
		release_sock(sk);
		return -EBADF;
	}

	po = pppox_sk(sk);
	del_chan(po);
	synchronize_rcu();

	pppox_unbind_sock(sk);
	sk->sk_state = PPPOX_DEAD;

	sock_orphan(sk);
	sock->sk = NULL;

	release_sock(sk);
	sock_put(sk);

	return error;
}

static void pptp_sock_destruct(struct sock *sk)
{
	if (!(sk->sk_state & PPPOX_DEAD)) {
		del_chan(pppox_sk(sk));
		pppox_unbind_sock(sk);
	}
	skb_queue_purge(&sk->sk_receive_queue);
	dst_release(rcu_dereference_protected(sk->sk_dst_cache, 1));
}

static int pptp_create(struct net *net, struct socket *sock, int kern)
{
	int error = -ENOMEM;
	struct sock *sk;
	struct pppox_sock *po;
	struct pptp_opt *opt;

	sk = sk_alloc(net, PF_PPPOX, GFP_KERNEL, &pptp_sk_proto, kern);
	if (!sk)
		goto out;

	sock_init_data(sock, sk);

	sock->state = SS_UNCONNECTED;
	sock->ops   = &pptp_ops;

	sk->sk_backlog_rcv = pptp_rcv_core;
	sk->sk_state       = PPPOX_NONE;
	sk->sk_type        = SOCK_STREAM;
	sk->sk_family      = PF_PPPOX;
	sk->sk_protocol    = PX_PROTO_PPTP;
	sk->sk_destruct    = pptp_sock_destruct;

	po = pppox_sk(sk);
	opt = &po->proto.pptp;

	opt->seq_sent = 0; opt->seq_recv = 0xffffffff;
	opt->ack_recv = 0; opt->ack_sent = 0xffffffff;

	error = 0;
out:
	return error;
}

static int pptp_ppp_ioctl(struct ppp_channel *chan, unsigned int cmd,
	unsigned long arg)
{
	struct sock *sk = chan->private;
	struct pppox_sock *po = pppox_sk(sk);
	struct pptp_opt *opt = &po->proto.pptp;
	void __user *argp = (void __user *)arg;
	int __user *p = argp;
	int err, val;

	err = -EFAULT;
	switch (cmd) {
	case PPPIOCGFLAGS:
		val = opt->ppp_flags;
		if (put_user(val, p))
			break;
		err = 0;
		break;
	case PPPIOCSFLAGS:
		if (get_user(val, p))
			break;
		opt->ppp_flags = val & ~SC_RCV_BITS;
		err = 0;
		break;
	default:
		err = -ENOTTY;
	}

	return err;
}

static const struct ppp_channel_ops pptp_chan_ops = {
	.start_xmit = pptp_xmit,
	.ioctl      = pptp_ppp_ioctl,
};

static struct proto pptp_sk_proto __read_mostly = {
	.name     = "PPTP",
	.owner    = THIS_MODULE,
	.obj_size = sizeof(struct pppox_sock),
};

static const struct proto_ops pptp_ops = {
	.family     = AF_PPPOX,
	.owner      = THIS_MODULE,
	.release    = pptp_release,
	.bind       = pptp_bind,
	.connect    = pptp_connect,
	.socketpair = sock_no_socketpair,
	.accept     = sock_no_accept,
	.getname    = pptp_getname,
	.listen     = sock_no_listen,
	.shutdown   = sock_no_shutdown,
	.sendmsg    = sock_no_sendmsg,
	.recvmsg    = sock_no_recvmsg,
	.mmap       = sock_no_mmap,
	.ioctl      = pppox_ioctl,
#ifdef CONFIG_COMPAT
	.compat_ioctl = pppox_compat_ioctl,
#endif
};

static const struct pppox_proto pppox_pptp_proto = {
	.create = pptp_create,
	.owner  = THIS_MODULE,
};

static const struct gre_protocol gre_pptp_protocol = {
	.handler = pptp_rcv,
};

static int __init pptp_init_module(void)
{
	int err = 0;
	pr_info("PPTP driver version " PPTP_DRIVER_VERSION "\n");

	callid_sock = vzalloc(array_size(sizeof(void *), (MAX_CALLID + 1)));
	if (!callid_sock)
		return -ENOMEM;

	err = gre_add_protocol(&gre_pptp_protocol, GREPROTO_PPTP);
	if (err) {
		pr_err("PPTP: can't add gre protocol\n");
		goto out_mem_free;
	}

	err = proto_register(&pptp_sk_proto, 0);
	if (err) {
		pr_err("PPTP: can't register sk_proto\n");
		goto out_gre_del_protocol;
	}

	err = register_pppox_proto(PX_PROTO_PPTP, &pppox_pptp_proto);
	if (err) {
		pr_err("PPTP: can't register pppox_proto\n");
		goto out_unregister_sk_proto;
	}

	return 0;

out_unregister_sk_proto:
	proto_unregister(&pptp_sk_proto);
out_gre_del_protocol:
	gre_del_protocol(&gre_pptp_protocol, GREPROTO_PPTP);
out_mem_free:
	vfree(callid_sock);

	return err;
}

static void __exit pptp_exit_module(void)
{
	unregister_pppox_proto(PX_PROTO_PPTP);
	proto_unregister(&pptp_sk_proto);
	gre_del_protocol(&gre_pptp_protocol, GREPROTO_PPTP);
	vfree(callid_sock);
}

module_init(pptp_init_module);
module_exit(pptp_exit_module);

MODULE_DESCRIPTION("Point-to-Point Tunneling Protocol");
MODULE_AUTHOR("D. Kozlov <xeb@mail.ru>");
MODULE_LICENSE("GPL");
MODULE_ALIAS_NET_PF_PROTO(PF_PPPOX, PX_PROTO_PPTP);