Bluetooth: L2CAP: Fix potential user-after-free

This fixes all instances of which requires to allocate a buffer calling
alloc_skb which may release the chan lock and reacquire later which
makes it possible that the chan is disconnected in the meantime.

Fixes: a6a5568c ("Bluetooth: Lock the L2CAP channel when sending")
Reported-by: Alexander Coffin <alex.coffin@matician.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>

net/bluetooth/l2cap_core.c

@@ -2683,14 +2683,6 @@ int l2cap_chan_send(struct l2cap_chan *chan, struct msghdr *msg, size_t len)
 		if (IS_ERR(skb))
 			return PTR_ERR(skb);
 
-		/* Channel lock is released before requesting new skb and then
-		 * reacquired thus we need to recheck channel state.
-		 */
-		if (chan->state != BT_CONNECTED) {
-			kfree_skb(skb);
-			return -ENOTCONN;
-		}
-
 		l2cap_do_send(chan, skb);
 		return len;
 	}
@@ -2735,14 +2727,6 @@ int l2cap_chan_send(struct l2cap_chan *chan, struct msghdr *msg, size_t len)
 		if (IS_ERR(skb))
 			return PTR_ERR(skb);
 
-		/* Channel lock is released before requesting new skb and then
-		 * reacquired thus we need to recheck channel state.
-		 */
-		if (chan->state != BT_CONNECTED) {
-			kfree_skb(skb);
-			return -ENOTCONN;
-		}
-
 		l2cap_do_send(chan, skb);
 		err = len;
 		break;
@@ -2763,14 +2747,6 @@ int l2cap_chan_send(struct l2cap_chan *chan, struct msghdr *msg, size_t len)
 		 */
 		err = l2cap_segment_sdu(chan, &seg_queue, msg, len);
 
-		/* The channel could have been closed while segmenting,
-		 * check that it is still connected.
-		 */
-		if (chan->state != BT_CONNECTED) {
-			__skb_queue_purge(&seg_queue);
-			err = -ENOTCONN;
-		}
-
 		if (err)
 			break;
 

net/bluetooth/l2cap_sock.c

@@ -1624,6 +1624,14 @@ static struct sk_buff *l2cap_sock_alloc_skb_cb(struct l2cap_chan *chan,
 	if (!skb)
 		return ERR_PTR(err);
 
+	/* Channel lock is released before requesting new skb and then
+	 * reacquired thus we need to recheck channel state.
+	 */
+	if (chan->state != BT_CONNECTED) {
+		kfree_skb(skb);
+		return ERR_PTR(-ENOTCONN);
+	}
+
 	skb->priority = sk->sk_priority;
 
 	bt_cb(skb)->l2cap.chan = chan;