l2tp: fix duplicate session creation

l2tp_session_create() relies on its caller for checking for duplicate
sessions. This is racy since a session can be concurrently inserted
after the caller's verification.

Fix this by letting l2tp_session_create() verify sessions uniqueness
upon insertion. Callers need to be adapted to check for
l2tp_session_create()'s return code instead of calling
l2tp_session_find().

pppol2tp_connect() is a bit special because it has to work on existing
sessions (if they're not connected) or to create a new session if none
is found. When acting on a preexisting session, a reference must be
held or it could go away on us. So we have to use l2tp_session_get()
instead of l2tp_session_find() and drop the reference before exiting.

Fixes: d9e31d17ceba ("l2tp: Add L2TP ethernet pseudowire support")
Fixes: fd558d186df2 ("l2tp: Split pppol2tp patch into separate l2tp and ppp parts")
Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 53 insertions, 17 deletions

diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
index 8a067536d15c..46b450a1bc21 100644
--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c
@@ -374,6 +374,48 @@ struct l2tp_session *l2tp_session_find_by_ifname(struct net *net, char *ifname)
 }
 EXPORT_SYMBOL_GPL(l2tp_session_find_by_ifname);
 
+static int l2tp_session_add_to_tunnel(struct l2tp_tunnel *tunnel,
+				      struct l2tp_session *session)
+{
+	struct l2tp_session *session_walk;
+	struct hlist_head *g_head;
+	struct hlist_head *head;
+	struct l2tp_net *pn;
+
+	head = l2tp_session_id_hash(tunnel, session->session_id);
+
+	write_lock_bh(&tunnel->hlist_lock);
+	hlist_for_each_entry(session_walk, head, hlist)
+		if (session_walk->session_id == session->session_id)
+			goto exist;
+
+	if (tunnel->version == L2TP_HDR_VER_3) {
+		pn = l2tp_pernet(tunnel->l2tp_net);
+		g_head = l2tp_session_id_hash_2(l2tp_pernet(tunnel->l2tp_net),
+						session->session_id);
+
+		spin_lock_bh(&pn->l2tp_session_hlist_lock);
+		hlist_for_each_entry(session_walk, g_head, global_hlist)
+			if (session_walk->session_id == session->session_id)
+				goto exist_glob;
+
+		hlist_add_head_rcu(&session->global_hlist, g_head);
+		spin_unlock_bh(&pn->l2tp_session_hlist_lock);
+	}
+
+	hlist_add_head(&session->hlist, head);
+	write_unlock_bh(&tunnel->hlist_lock);
+
+	return 0;
+
+exist_glob:
+	spin_unlock_bh(&pn->l2tp_session_hlist_lock);
+exist:
+	write_unlock_bh(&tunnel->hlist_lock);
+
+	return -EEXIST;
+}
+
 /* Lookup a tunnel by id
  */
 struct l2tp_tunnel *l2tp_tunnel_find(struct net *net, u32 tunnel_id)
@@ -1785,6 +1827,7 @@ EXPORT_SYMBOL_GPL(l2tp_session_set_header_len);
 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;
+	int err;
 
 	session = kzalloc(sizeof(struct l2tp_session) + priv_size, GFP_KERNEL);
 	if (session != NULL) {
@@ -1840,6 +1883,13 @@ struct l2tp_session *l2tp_session_create(int priv_size, struct l2tp_tunnel *tunn
 
 		l2tp_session_set_header_len(session, tunnel->version);
 
+		err = l2tp_session_add_to_tunnel(tunnel, session);
+		if (err) {
+			kfree(session);
+
+			return ERR_PTR(err);
+		}
+
 		/* Bump the reference count. The session context is deleted
 		 * only when this drops to zero.
 		 */
@@ -1849,28 +1899,14 @@ struct l2tp_session *l2tp_session_create(int priv_size, struct l2tp_tunnel *tunn
 		/* 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;
 	}
 
-	return session;
+	return ERR_PTR(-ENOMEM);
 }
 EXPORT_SYMBOL_GPL(l2tp_session_create);