Bluetooth: hci_event: Detect if HCI_EV_NUM_COMP_PKTS is unbalanced

This attempts to detect if HCI_EV_NUM_COMP_PKTS contain an unbalanced
(more than currently considered outstanding) number of packets otherwise
it could cause the hcon->sent to underflow and loop around breaking the
tracking of the outstanding packets pending acknowledgment.

Fixes: f42809185896 ("Bluetooth: Simplify num_comp_pkts_evt function")
Signed-off-by: Luiz Augusto von Dentz <[e-mail]>

net/bluetooth/hci_event.c

···
 		if (!conn)
 			continue;
 
-		conn->sent -= count;
+		/* Check if there is really enough packets outstanding before
+		 * attempting to decrease the sent counter otherwise it could
+		 * underflow..
+		 */
+		if (conn->sent >= count) {
+			conn->sent -= count;
+		} else {
+			bt_dev_warn(hdev, "hcon %p sent %u < count %u",
+				    conn, conn->sent, count);
+			conn->sent = 0;
+		}
 
 		for (i = 0; i < count; ++i)
 			hci_conn_tx_dequeue(conn);