[PATCH] Bluetooth: bnep: pin L2CAP connection during netdev registration

[Yousef Alhouseen <alhouseenyousef@gmail.com>]

bnep_add_connection() reads the L2CAP connection without holding the
channel lock, then passes its HCI device to register_netdev(). Controller
teardown can clear and release that connection concurrently, leaving the
network device registration path to dereference a freed parent device.

Take a reference to the L2CAP connection while holding the channel lock.
Retain it until register_netdev() has taken the parent device reference.

Fixes: 65f53e9802db ("Bluetooth: Access BNEP session addresses through L2CAP channel")
Reported-by: syzbot+fed5dce4553262f3b35c@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=fed5dce4553262f3b35c
Cc: stable@vger.kernel.org
Signed-off-by: Yousef Alhouseen <alhouseenyousef@gmail.com>
---
 net/bluetooth/bnep/core.c | 27 +++++++++++++++++++++------
 1 file changed, 21 insertions(+), 6 deletions(-)

diff --git a/net/bluetooth/bnep/core.c b/net/bluetooth/bnep/core.c
index add9a8f7535d..f7d88c33e23e 100644
--- a/net/bluetooth/bnep/core.c
+++ b/net/bluetooth/bnep/core.c
@@ -559,14 +559,18 @@ static int bnep_session(void *arg)
 	return 0;
 }
 
-static struct device *bnep_get_device(struct bnep_session *session)
+static struct l2cap_conn *bnep_get_conn(struct bnep_session *session)
 {
-	struct l2cap_conn *conn = l2cap_pi(session->sock->sk)->chan->conn;
+	struct l2cap_chan *chan = l2cap_pi(session->sock->sk)->chan;
+	struct l2cap_conn *conn;
 
-	if (!conn || !conn->hcon)
-		return NULL;
+	l2cap_chan_lock(chan);
+	conn = chan->conn;
+	if (conn)
+		l2cap_conn_get(conn);
+	l2cap_chan_unlock(chan);
 
-	return &conn->hcon->dev;
+	return conn;
 }
 
 static const struct device_type bnep_type = {
@@ -578,6 +582,7 @@ int bnep_add_connection(struct bnep_connadd_req *req, struct socket *sock)
 	u32 valid_flags = BIT(BNEP_SETUP_RESPONSE);
 	struct net_device *dev;
 	struct bnep_session *s, *ss;
+	struct l2cap_conn *conn = NULL;
 	u8 dst[ETH_ALEN], src[ETH_ALEN];
 	int err;
 
@@ -637,10 +642,18 @@ int bnep_add_connection(struct bnep_connadd_req *req, struct socket *sock)
 	bnep_set_default_proto_filter(s);
 #endif
 
-	SET_NETDEV_DEV(dev, bnep_get_device(s));
+	conn = bnep_get_conn(s);
+	if (!conn) {
+		err = -ENOTCONN;
+		goto failed;
+	}
+
+	SET_NETDEV_DEV(dev, &conn->hcon->dev);
 	SET_NETDEV_DEVTYPE(dev, &bnep_type);
 
 	err = register_netdev(dev);
+	l2cap_conn_put(conn);
+	conn = NULL;
 	if (err)
 		goto failed;
 
@@ -662,6 +675,8 @@ int bnep_add_connection(struct bnep_connadd_req *req, struct socket *sock)
 	return 0;
 
 failed:
+	if (conn)
+		l2cap_conn_put(conn);
 	up_write(&bnep_session_sem);
 	free_netdev(dev);
 	return err;
-- 
2.54.0