Bluetooth: 6lowpan: fix use after free in chan_suspend/resume

A status field in the skb_cb struct was storing a channel status
based on channel suspend/resume events.  This stored status was
then used to return EAGAIN if there were packet sending issues
in snd_pkt().

The issue is that the skb has been freed by the time the callback
to 6lowpan's suspend/resume was called.  So, this generates a
"use after free" issue that was noticed while running kernel tests
with KASAN debug enabled.

Let's eliminate the status field entirely as we can use the channel
tx_credits to indicate whether we should return EAGAIN when handling
packets.
Signed-off-by: Michael Scott <michael.scott@linaro.org>
Acked-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

net/bluetooth/6lowpan.c

@@ -38,7 +38,6 @@ struct skb_cb {
 	struct in6_addr addr;
 	struct in6_addr gw;
 	struct l2cap_chan *chan;
-	int status;
 };
 #define lowpan_cb(skb) ((struct skb_cb *)((skb)->cb))
 
@@ -485,7 +484,7 @@ static int send_pkt(struct l2cap_chan *chan, struct sk_buff *skb,
 	}
 
 	if (!err)
-		err = lowpan_cb(skb)->status;
+		err = (!chan->tx_credits ? -EAGAIN : 0);
 
 	if (err < 0) {
 		if (err == -EAGAIN)
@@ -880,26 +879,12 @@ static struct sk_buff *chan_alloc_skb_cb(struct l2cap_chan *chan,
 
 static void chan_suspend_cb(struct l2cap_chan *chan)
 {
-	struct sk_buff *skb = chan->data;
-
-	BT_DBG("chan %p conn %p skb %p", chan, chan->conn, skb);
-
-	if (!skb)
-		return;
-
-	lowpan_cb(skb)->status = -EAGAIN;
+	BT_DBG("chan %p suspend", chan);
 }
 
 static void chan_resume_cb(struct l2cap_chan *chan)
 {
-	struct sk_buff *skb = chan->data;
-
-	BT_DBG("chan %p conn %p skb %p", chan, chan->conn, skb);
-
-	if (!skb)
-		return;
-
-	lowpan_cb(skb)->status = 0;
+	BT_DBG("chan %p resume", chan);
 }
 
 static long chan_get_sndtimeo_cb(struct l2cap_chan *chan)