[PATCH net 0/2] l2tp: tunnel creation fixes

[PATCH net 0/2] l2tp: tunnel creation fixes

[Guillaume Nault]

L2TP tunnel creation is racy. We need to make sure that the tunnel
returned by l2tp_tunnel_create() isn't going to be freed while the
caller is using it. This is done in patch #1, by separating tunnel
creation from tunnel registration.

With the tunnel registration code in place, we can now check for
duplicate tunnels in a race-free way. This is done in patch #2, which
incidentally removes the last use of l2tp_tunnel_find().

Guillaume Nault (2):
  l2tp: fix races in tunnel creation
  l2tp: fix race in duplicate tunnel detection

 net/l2tp/l2tp_core.c    | 225 +++++++++++++++++-----------------------
 net/l2tp/l2tp_core.h    |   4 +-
 net/l2tp/l2tp_netlink.c |  22 ++--
 net/l2tp/l2tp_ppp.c     |   9 ++
 4 files changed, 123 insertions(+), 137 deletions(-)

-- 
2.17.0

[PATCH net 1/2] l2tp: fix races in tunnel creation

[Guillaume Nault]

l2tp_tunnel_create() inserts the new tunnel into the namespace's tunnel
list and sets the socket's ->sk_user_data field, before returning it to
the caller. Therefore, there are two ways the tunnel can be accessed
and freed, before the caller even had the opportunity to take a
reference. In practice, syzbot could crash the module by closing the
socket right after a new tunnel was returned to pppol2tp_create().

This patch moves tunnel registration out of l2tp_tunnel_create(), so
that the caller can safely hold a reference before publishing the
tunnel. This second step is done with the new l2tp_tunnel_register()
function, which is now responsible for associating the tunnel to its
socket and for inserting it into the namespace's list.

While moving the code to l2tp_tunnel_register(), a few modifications
have been done. First, the socket validation tests are done in a helper
function, for clarity. Also, modifying the socket is now done after
having inserted the tunnel to the namespace's tunnels list. This will
allow insertion to fail, without having to revert theses modifications
in the error path (a followup patch will check for duplicate tunnels
before insertion). Either the socket is a kernel socket which we
control, or it is a user-space socket for which we have a reference on
the file descriptor. In any case, the socket isn't going to be closed
from under us.

Reported-by: syzbot+fbeeb5c3b538e8545644@syzkaller.appspotmail.com
Fixes: fd558d186df2 ("l2tp: Split pppol2tp patch into separate l2tp and ppp parts")
Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
---
 net/l2tp/l2tp_core.c    | 192 ++++++++++++++++++----------------------
 net/l2tp/l2tp_core.h    |   3 +
 net/l2tp/l2tp_netlink.c |  16 +++-
 net/l2tp/l2tp_ppp.c     |   9 ++
 4 files changed, 110 insertions(+), 110 deletions(-)

diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
index 14b67dfacc4b..afb42d142807 100644
--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c
@@ -1436,74 +1436,11 @@ int l2tp_tunnel_create(struct net *net, int fd, int version, u32 tunnel_id, u32
 {
 	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(net, tunnel_id, peer_tunnel_id,
-				cfg, &sock);
-		if (err < 0)
-			goto err;
-	} else {
-		sock = sockfd_lookup(fd, &err);
-		if (!sock) {
-			pr_err("tunl %u: sockfd_lookup(fd=%d) returned %d\n",
-			       tunnel_id, fd, err);
-			err = -EBADF;
-			goto err;
-		}
-
-		/* Reject namespace mismatches */
-		if (!net_eq(sock_net(sock->sk), net)) {
-			pr_err("tunl %u: netns mismatch\n", tunnel_id);
-			err = -EINVAL;
-			goto err;
-		}
-	}
-
-	sk = sock->sk;
-
 	if (cfg != NULL)
 		encap = cfg->encap;
 
-	/* Quick sanity checks */
-	err = -EPROTONOSUPPORT;
-	if (sk->sk_type != SOCK_DGRAM) {
-		pr_debug("tunl %hu: fd %d wrong socket type\n",
-			 tunnel_id, fd);
-		goto err;
-	}
-	switch (encap) {
-	case L2TP_ENCAPTYPE_UDP:
-		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:
-		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 = l2tp_tunnel(sk);
-	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;
@@ -1520,72 +1457,113 @@ int l2tp_tunnel_create(struct net *net, int fd, int version, u32 tunnel_id, u32
 	rwlock_init(&tunnel->hlist_lock);
 	tunnel->acpt_newsess = true;
 
-	/* 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) {
-		struct udp_tunnel_sock_cfg udp_cfg = { };
-
-		udp_cfg.sk_user_data = tunnel;
-		udp_cfg.encap_type = UDP_ENCAP_L2TPINUDP;
-		udp_cfg.encap_rcv = l2tp_udp_encap_recv;
-		udp_cfg.encap_destroy = l2tp_udp_encap_destroy;
-
-		setup_udp_tunnel_sock(net, sock, &udp_cfg);
-	} else {
-		sk->sk_user_data = tunnel;
-	}
 
-	/* Bump the reference count. The tunnel context is deleted
-	 * only when this drops to zero. A reference is also held on
-	 * the tunnel socket to ensure that it is not released while
-	 * the tunnel is extant. Must be done before sk_destruct is
-	 * set.
-	 */
 	refcount_set(&tunnel->ref_count, 1);
-	sock_hold(sk);
-	tunnel->sock = sk;
 	tunnel->fd = fd;
 
-	/* 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;
-	lockdep_set_class_and_name(&sk->sk_lock.slock, &l2tp_socket_class, "l2tp_sock");
-
-	sk->sk_allocation = GFP_ATOMIC;
-
 	/* Init delete workqueue struct */
 	INIT_WORK(&tunnel->del_work, l2tp_tunnel_del_work);
 
-	/* Add tunnel to our list */
 	INIT_LIST_HEAD(&tunnel->list);
-	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);
 
+static int l2tp_validate_socket(const struct sock *sk, const struct net *net,
+				enum l2tp_encap_type encap)
+{
+	if (!net_eq(sock_net(sk), net))
+		return -EINVAL;
+
+	if (sk->sk_type != SOCK_DGRAM)
+		return -EPROTONOSUPPORT;
+
+	if ((encap == L2TP_ENCAPTYPE_UDP && sk->sk_protocol != IPPROTO_UDP) ||
+	    (encap == L2TP_ENCAPTYPE_IP && sk->sk_protocol != IPPROTO_L2TP))
+		return -EPROTONOSUPPORT;
+
+	if (sk->sk_user_data)
+		return -EBUSY;
+
+	return 0;
+}
+
+int l2tp_tunnel_register(struct l2tp_tunnel *tunnel, struct net *net,
+			 struct l2tp_tunnel_cfg *cfg)
+{
+	struct l2tp_net *pn;
+	struct socket *sock;
+	struct sock *sk;
+	int ret;
+
+	if (tunnel->fd < 0) {
+		ret = l2tp_tunnel_sock_create(net, tunnel->tunnel_id,
+					      tunnel->peer_tunnel_id, cfg,
+					      &sock);
+		if (ret < 0)
+			goto err;
+	} else {
+		sock = sockfd_lookup(tunnel->fd, &ret);
+		if (!sock)
+			goto err;
+
+		ret = l2tp_validate_socket(sock->sk, net, tunnel->encap);
+		if (ret < 0)
+			goto err_sock;
+	}
+
+	sk = sock->sk;
+
+	sock_hold(sk);
+	tunnel->sock = sk;
+	tunnel->l2tp_net = net;
+
+	pn = l2tp_pernet(net);
+	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);
+
+	if (tunnel->encap == L2TP_ENCAPTYPE_UDP) {
+		struct udp_tunnel_sock_cfg udp_cfg = {
+			.sk_user_data = tunnel,
+			.encap_type = UDP_ENCAP_L2TPINUDP,
+			.encap_rcv = l2tp_udp_encap_recv,
+			.encap_destroy = l2tp_udp_encap_destroy,
+		};
+
+		setup_udp_tunnel_sock(net, sock, &udp_cfg);
+	} else {
+		sk->sk_user_data = tunnel;
+	}
+
+	tunnel->old_sk_destruct = sk->sk_destruct;
+	sk->sk_destruct = &l2tp_tunnel_destruct;
+	lockdep_set_class_and_name(&sk->sk_lock.slock, &l2tp_socket_class,
+				   "l2tp_sock");
+	sk->sk_allocation = GFP_ATOMIC;
+
+	if (tunnel->fd >= 0)
+		sockfd_put(sock);
+
+	return 0;
+
+err_sock:
+	sockfd_put(sock);
+err:
+	return ret;
+}
+EXPORT_SYMBOL_GPL(l2tp_tunnel_register);
+
 /* This function is used by the netlink TUNNEL_DELETE command.
  */
 void l2tp_tunnel_delete(struct l2tp_tunnel *tunnel)
diff --git a/net/l2tp/l2tp_core.h b/net/l2tp/l2tp_core.h
index 2718d0b284d0..12f0fa82f162 100644
--- a/net/l2tp/l2tp_core.h
+++ b/net/l2tp/l2tp_core.h
@@ -226,6 +226,9 @@ struct l2tp_tunnel *l2tp_tunnel_find_nth(const struct net *net, int nth);
 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);
+int l2tp_tunnel_register(struct l2tp_tunnel *tunnel, struct net *net,
+			 struct l2tp_tunnel_cfg *cfg);
+
 void l2tp_tunnel_closeall(struct l2tp_tunnel *tunnel);
 void l2tp_tunnel_delete(struct l2tp_tunnel *tunnel);
 struct l2tp_session *l2tp_session_create(int priv_size,
diff --git a/net/l2tp/l2tp_netlink.c b/net/l2tp/l2tp_netlink.c
index e7ea9c4b89ff..45db9b73eb1a 100644
--- a/net/l2tp/l2tp_netlink.c
+++ b/net/l2tp/l2tp_netlink.c
@@ -251,9 +251,19 @@ static int l2tp_nl_cmd_tunnel_create(struct sk_buff *skb, struct genl_info *info
 		break;
 	}
 
-	if (ret >= 0)
-		ret = l2tp_tunnel_notify(&l2tp_nl_family, info,
-					 tunnel, L2TP_CMD_TUNNEL_CREATE);
+	if (ret < 0)
+		goto out;
+
+	l2tp_tunnel_inc_refcount(tunnel);
+	ret = l2tp_tunnel_register(tunnel, net, &cfg);
+	if (ret < 0) {
+		kfree(tunnel);
+		goto out;
+	}
+	ret = l2tp_tunnel_notify(&l2tp_nl_family, info, tunnel,
+				 L2TP_CMD_TUNNEL_CREATE);
+	l2tp_tunnel_dec_refcount(tunnel);
+
 out:
 	return ret;
 }
diff --git a/net/l2tp/l2tp_ppp.c b/net/l2tp/l2tp_ppp.c
index d6deca11da19..896bbca9bdaa 100644
--- a/net/l2tp/l2tp_ppp.c
+++ b/net/l2tp/l2tp_ppp.c
@@ -698,6 +698,15 @@ static int pppol2tp_connect(struct socket *sock, struct sockaddr *uservaddr,
 			error = l2tp_tunnel_create(sock_net(sk), fd, ver, tunnel_id, peer_tunnel_id, &tcfg, &tunnel);
 			if (error < 0)
 				goto end;
+
+			l2tp_tunnel_inc_refcount(tunnel);
+			error = l2tp_tunnel_register(tunnel, sock_net(sk),
+						     &tcfg);
+			if (error < 0) {
+				kfree(tunnel);
+				goto end;
+			}
+			drop_tunnel = true;
 		}
 	} else {
 		/* Error if we can't find the tunnel */
-- 
2.17.0

[PATCH net 2/2] l2tp: fix race in duplicate tunnel detection

[Guillaume Nault]

We can't use l2tp_tunnel_find() to prevent l2tp_nl_cmd_tunnel_create()
from creating a duplicate tunnel. A tunnel can be concurrently
registered after l2tp_tunnel_find() returns. Therefore, searching for
duplicates must be done at registration time.

Finally, remove l2tp_tunnel_find() entirely as it isn't use anywhere
anymore.

Fixes: 309795f4bec2 ("l2tp: Add netlink control API for L2TP")
Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
---
 net/l2tp/l2tp_core.c    | 35 ++++++++++++++---------------------
 net/l2tp/l2tp_core.h    |  1 -
 net/l2tp/l2tp_netlink.c |  6 ------
 3 files changed, 14 insertions(+), 28 deletions(-)

diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
index afb42d142807..0fbd3ee26165 100644
--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c
@@ -335,26 +335,6 @@ int l2tp_session_register(struct l2tp_session *session,
 }
 EXPORT_SYMBOL_GPL(l2tp_session_register);
 
-/* Lookup a tunnel by id
- */
-struct l2tp_tunnel *l2tp_tunnel_find(const 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(const struct net *net, int nth)
 {
 	struct l2tp_net *pn = l2tp_pernet(net);
@@ -1501,6 +1481,7 @@ static int l2tp_validate_socket(const struct sock *sk, const struct net *net,
 int l2tp_tunnel_register(struct l2tp_tunnel *tunnel, struct net *net,
 			 struct l2tp_tunnel_cfg *cfg)
 {
+	struct l2tp_tunnel *tunnel_walk;
 	struct l2tp_net *pn;
 	struct socket *sock;
 	struct sock *sk;
@@ -1529,7 +1510,16 @@ int l2tp_tunnel_register(struct l2tp_tunnel *tunnel, struct net *net,
 	tunnel->l2tp_net = net;
 
 	pn = l2tp_pernet(net);
+
 	spin_lock_bh(&pn->l2tp_tunnel_list_lock);
+	list_for_each_entry(tunnel_walk, &pn->l2tp_tunnel_list, list) {
+		if (tunnel_walk->tunnel_id == tunnel->tunnel_id) {
+			spin_unlock_bh(&pn->l2tp_tunnel_list_lock);
+
+			ret = -EEXIST;
+			goto err_sock;
+		}
+	}
 	list_add_rcu(&tunnel->list, &pn->l2tp_tunnel_list);
 	spin_unlock_bh(&pn->l2tp_tunnel_list_lock);
 
@@ -1558,7 +1548,10 @@ int l2tp_tunnel_register(struct l2tp_tunnel *tunnel, struct net *net,
 	return 0;
 
 err_sock:
-	sockfd_put(sock);
+	if (tunnel->fd < 0)
+		sock_release(sock);
+	else
+		sockfd_put(sock);
 err:
 	return ret;
 }
diff --git a/net/l2tp/l2tp_core.h b/net/l2tp/l2tp_core.h
index 12f0fa82f162..ba33cbec71eb 100644
--- a/net/l2tp/l2tp_core.h
+++ b/net/l2tp/l2tp_core.h
@@ -220,7 +220,6 @@ struct l2tp_session *l2tp_session_get(const struct net *net,
 struct l2tp_session *l2tp_session_get_nth(struct l2tp_tunnel *tunnel, int nth);
 struct l2tp_session *l2tp_session_get_by_ifname(const struct net *net,
 						const char *ifname);
-struct l2tp_tunnel *l2tp_tunnel_find(const struct net *net, u32 tunnel_id);
 struct l2tp_tunnel *l2tp_tunnel_find_nth(const struct net *net, int nth);
 
 int l2tp_tunnel_create(struct net *net, int fd, int version, u32 tunnel_id,
diff --git a/net/l2tp/l2tp_netlink.c b/net/l2tp/l2tp_netlink.c
index 45db9b73eb1a..b05dbd9ffcb2 100644
--- a/net/l2tp/l2tp_netlink.c
+++ b/net/l2tp/l2tp_netlink.c
@@ -236,12 +236,6 @@ static int l2tp_nl_cmd_tunnel_create(struct sk_buff *skb, struct genl_info *info
 	if (info->attrs[L2TP_ATTR_DEBUG])
 		cfg.debug = nla_get_u32(info->attrs[L2TP_ATTR_DEBUG]);
 
-	tunnel = l2tp_tunnel_find(net, tunnel_id);
-	if (tunnel != NULL) {
-		ret = -EEXIST;
-		goto out;
-	}
-
 	ret = -EINVAL;
 	switch (cfg.encap) {
 	case L2TP_ENCAPTYPE_UDP:
-- 
2.17.0

Re: [PATCH net 0/2] l2tp: tunnel creation fixes

[David Miller]

From: Guillaume Nault <g.nault@alphalink.fr>
Date: Tue, 10 Apr 2018 21:01:07 +0200

> L2TP tunnel creation is racy. We need to make sure that the tunnel
> returned by l2tp_tunnel_create() isn't going to be freed while the
> caller is using it. This is done in patch #1, by separating tunnel
> creation from tunnel registration.
> 
> With the tunnel registration code in place, we can now check for
> duplicate tunnels in a race-free way. This is done in patch #2, which
> incidentally removes the last use of l2tp_tunnel_find().

Series applied and queued up for -stable, thanks.