Commit 24256415 authored by James Chapman's avatar James Chapman Committed by David S. Miller
Browse files

l2tp: prevent possible tunnel refcount underflow 



When a session is created, it sets a backpointer to its tunnel. When
the session refcount drops to 0, l2tp_session_free drops the tunnel
refcount if session->tunnel is non-NULL. However, session->tunnel is
set in l2tp_session_create, before the tunnel refcount is incremented
by l2tp_session_register, which leaves a small window where
session->tunnel is non-NULL when the tunnel refcount hasn't been
bumped.

Moving the assignment to l2tp_session_register is trivial but
l2tp_session_create calls l2tp_session_set_header_len which uses
session->tunnel to get the tunnel's encap. Add an encap arg to
l2tp_session_set_header_len to avoid using session->tunnel.

If l2tpv3 sessions have colliding IDs, it is possible for
l2tp_v3_session_get to race with l2tp_session_register and fetch a
session which doesn't yet have session->tunnel set. Add a check for
this case.

Signed-off-by: default avatarJames Chapman <jchapman@katalix.com>
Signed-off-by: default avatarTom Parkin <tparkin@katalix.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent c5cbaef9

net/l2tp/l2tp_core.c

+17 −7
Original line number Diff line number Diff line
@@ -279,7 +279,14 @@ struct l2tp_session *l2tp_v3_session_get(const struct net *net, struct sock *sk, 


		hash_for_each_possible_rcu(pn->l2tp_v3_session_htable, session,

					   hlist, key) {

			if (session->tunnel->sock == sk &&

			/* session->tunnel may be NULL if another thread is in

			 * l2tp_session_register and has added an item to

			 * l2tp_v3_session_htable but hasn't yet added the

			 * session to its tunnel's session_list.

			 */

			struct l2tp_tunnel *tunnel = READ_ONCE(session->tunnel);



			if (tunnel && tunnel->sock == sk &&

			    refcount_inc_not_zero(&session->ref_count)) {

				rcu_read_unlock_bh();

				return session;
@@ -507,6 +514,7 @@ int l2tp_session_register(struct l2tp_session *session, 
	}



	l2tp_tunnel_inc_refcount(tunnel);

	WRITE_ONCE(session->tunnel, tunnel);

	list_add(&session->list, &tunnel->session_list);



	if (tunnel->version == L2TP_HDR_VER_3) {
@@ -822,7 +830,8 @@ void l2tp_recv_common(struct l2tp_session *session, struct sk_buff *skb, 
		if (!session->lns_mode && !session->send_seq) {

			trace_session_seqnum_lns_enable(session);

			session->send_seq = 1;

			l2tp_session_set_header_len(session, tunnel->version);

			l2tp_session_set_header_len(session, tunnel->version,

						    tunnel->encap);

		}

	} else {

		/* No sequence numbers.
@@ -843,7 +852,8 @@ void l2tp_recv_common(struct l2tp_session *session, struct sk_buff *skb, 
		if (!session->lns_mode && session->send_seq) {

			trace_session_seqnum_lns_disable(session);

			session->send_seq = 0;

			l2tp_session_set_header_len(session, tunnel->version);

			l2tp_session_set_header_len(session, tunnel->version,

						    tunnel->encap);

		} else if (session->send_seq) {

			pr_debug_ratelimited("%s: recv data has no seq numbers when required. Discarding.\n",

					     session->name);
@@ -1648,7 +1658,8 @@ static void l2tp_session_del_work(struct work_struct *work) 
/* We come here whenever a session's send_seq, cookie_len or

 * l2specific_type parameters are set.

 */

void l2tp_session_set_header_len(struct l2tp_session *session, int version)

void l2tp_session_set_header_len(struct l2tp_session *session, int version,

				 enum l2tp_encap_type encap)

{

	if (version == L2TP_HDR_VER_2) {

		session->hdr_len = 6;
@@ -1657,7 +1668,7 @@ void l2tp_session_set_header_len(struct l2tp_session *session, int version) 
	} else {

		session->hdr_len = 4 + session->cookie_len;

		session->hdr_len += l2tp_get_l2specific_len(session);

		if (session->tunnel->encap == L2TP_ENCAPTYPE_UDP)

		if (encap == L2TP_ENCAPTYPE_UDP)

			session->hdr_len += 4;

	}

}
@@ -1671,7 +1682,6 @@ struct l2tp_session *l2tp_session_create(int priv_size, struct l2tp_tunnel *tunn 
	session = kzalloc(sizeof(*session) + priv_size, GFP_KERNEL);

	if (session) {

		session->magic = L2TP_SESSION_MAGIC;

		session->tunnel = tunnel;



		session->session_id = session_id;

		session->peer_session_id = peer_session_id;
@@ -1710,7 +1720,7 @@ struct l2tp_session *l2tp_session_create(int priv_size, struct l2tp_tunnel *tunn 
			memcpy(&session->peer_cookie[0], &cfg->peer_cookie[0], cfg->peer_cookie_len);

		}



		l2tp_session_set_header_len(session, tunnel->version);

		l2tp_session_set_header_len(session, tunnel->version, tunnel->encap);



		refcount_set(&session->ref_count, 1);

net/l2tp/l2tp_core.h

+2 −1
Original line number Diff line number Diff line
@@ -258,7 +258,8 @@ void l2tp_recv_common(struct l2tp_session *session, struct sk_buff *skb, 
int l2tp_udp_encap_recv(struct sock *sk, struct sk_buff *skb);



/* Transmit path helpers for sending packets over the tunnel socket. */

void l2tp_session_set_header_len(struct l2tp_session *session, int version);

void l2tp_session_set_header_len(struct l2tp_session *session, int version,

				 enum l2tp_encap_type encap);

int l2tp_xmit_skb(struct l2tp_session *session, struct sk_buff *skb);



/* Pseudowire management.

net/l2tp/l2tp_netlink.c

+3 −1
Original line number Diff line number Diff line
@@ -692,8 +692,10 @@ static int l2tp_nl_cmd_session_modify(struct sk_buff *skb, struct genl_info *inf 
		session->recv_seq = nla_get_u8(info->attrs[L2TP_ATTR_RECV_SEQ]);



	if (info->attrs[L2TP_ATTR_SEND_SEQ]) {

		struct l2tp_tunnel *tunnel = session->tunnel;



		session->send_seq = nla_get_u8(info->attrs[L2TP_ATTR_SEND_SEQ]);

		l2tp_session_set_header_len(session, session->tunnel->version);

		l2tp_session_set_header_len(session, tunnel->version, tunnel->encap);

	}



	if (info->attrs[L2TP_ATTR_LNS_MODE])

net/l2tp/l2tp_ppp.c

+2 −1
Original line number Diff line number Diff line
@@ -1189,7 +1189,8 @@ static int pppol2tp_session_setsockopt(struct sock *sk, 
			po->chan.hdrlen = val ? PPPOL2TP_L2TP_HDR_SIZE_SEQ :

				PPPOL2TP_L2TP_HDR_SIZE_NOSEQ;

		}

		l2tp_session_set_header_len(session, session->tunnel->version);

		l2tp_session_set_header_len(session, session->tunnel->version,

					    session->tunnel->encap);

		break;



	case PPPOL2TP_SO_LNSMODE: