Bluetooth: L2CAP: Fix calling sk_filter on non-socket based channel

Only sockets will have the chan->data set to an actual sk, channels
like A2MP would have its own data which would likely cause a crash when
calling sk_filter, in order to fix this a new callback has been
introduced so channels can implement their own filtering if necessary.

Signed-off-by: Luiz Augusto von Dentz <[e-mail]>
Signed-off-by: Marcel Holtmann <[e-mail]>

include/net/bluetooth/l2cap.h

···
 	struct sk_buff		*(*alloc_skb) (struct l2cap_chan *chan,
 					       unsigned long hdr_len,
 					       unsigned long len, int nb);
+	int			(*filter) (struct l2cap_chan * chan,
+					   struct sk_buff *skb);
 };
 
 struct l2cap_conn {

net/bluetooth/l2cap_core.c

···
 		goto drop;
 	}
 
-	if ((chan->mode == L2CAP_MODE_ERTM ||
-	     chan->mode == L2CAP_MODE_STREAMING) && sk_filter(chan->data, skb))
-		goto drop;
+	if (chan->ops->filter) {
+		if (chan->ops->filter(chan, skb))
+			goto drop;
+	}
 
 	if (!control->sframe) {
 		int err;

net/bluetooth/l2cap_sock.c

···
 	sk->sk_state_change(sk);
 }
 
+static int l2cap_sock_filter(struct l2cap_chan *chan, struct sk_buff *skb)
+{
+	struct sock *sk = chan->data;
+
+	switch (chan->mode) {
+	case L2CAP_MODE_ERTM:
+	case L2CAP_MODE_STREAMING:
+		return sk_filter(sk, skb);
+	}
+
+	return 0;
+}
+
 static const struct l2cap_ops l2cap_chan_ops = {
 	.name			= "L2CAP Socket Interface",
 	.new_connection		= l2cap_sock_new_connection_cb,
···
 	.get_sndtimeo		= l2cap_sock_get_sndtimeo_cb,
 	.get_peer_pid		= l2cap_sock_get_peer_pid_cb,
 	.alloc_skb		= l2cap_sock_alloc_skb_cb,
+	.filter			= l2cap_sock_filter,
 };
 
 static void l2cap_sock_destruct(struct sock *sk)