Linux: Bluetooth: ISO: release unused CIS holds after channel attach

In the Linux kernel, the following vulnerability has been resolved:

Bluetooth: ISO: release unused CIS holds after channel attach

hci_bind_cis() and hci_connect_cis() return one hci_conn hold for the ISO layer. A new channel association consumes that hold, which is eventually released by iso_conn_free().

There are two cases where iso_chan_add() does not create an association: it returns success when the socket is already attached to the same iso_conn, and it returns -EBUSY when another socket is attached. The hold returned for the current call is unused in both cases. This occurs when deferred setup calls iso_connect_cis() again for its existing socket, or when another socket attempts to reuse the CIS.

Detect the idempotent case while the connection is locked and release the unused hold after iso_chan_add(). Also release it on -EBUSY. Do not drop it for other errors: a newly allocated iso_conn releases the transferred hold when its last temporary reference is put.