In the Linux kernel, the following vulnerability has been resolved: Bluetooth: L2CAP: use chan timer to close channels in cleanup_listen() l2cap_chan_close() removes the channel from conn-chan_l, which must be done under conn-lock. cleanup_listen() runs under the parent sk_lock, so acquiring conn-lock would invert the established conn-lock - chan-lock - sk_lock order. Instead of calling l2cap_chan_close() directly, schedule l2cap_chan_timeout with delay 0 to close the channel asynchronously. The timeout handler already acquires conn-lock and chan-lock in the correct order. The timer is only armed when chan-conn is still set: if it is already NULL, l2cap_conn_del() has already processed this channel (l2cap_chan_del + l2cap_sock_teardown_cb + l2cap_sock_close_cb), so there is nothing left to do. If l2cap_conn_del() races in after the timer is armed, __clear_chan_timer() inside l2cap_chan_del() cancels it.

if the timer has already fired, the handler returns harmlessly because chan-conn was cleared.