Bluetooth: L2CAP: fix race l2cap_sock_cleanup_listen() vs. put_chan

For L2CAP sockets without owning sk->sk_socket, reading
l2cap_pi(sk)->chan may race against concurrent l2cap_sock_kill() ->
l2cap_sock_put_chan().  This excludes simultaneous proto_ops callbacks,
but access in l2cap_sock_cleanup_listen() has unsafe lockless read.

 [Task 1]                         [Task 2 (hdev->workqueue)]
 l2cap_sock_release(parent)       l2cap_disconn_cfm
   l2cap_sock_cleanup_listen        l2cap_conn_del
     bt_accept_dequeue                l2cap_chan_del
       lock_sock(sk)                    l2cap_sock_teardown_cb
       bt_accept_unlink
         bt_sk(sk)->parent = NULL
       release_sock(sk) ----------------> lock_sock(sk)
                                          parent = /* NULL */
     lock_sock(sk) <--------------------- release_sock(sk)
                                          sock_set_flag(sk, SOCK_ZAPPED)
                                      l2cap_sock_close_cb
                                        l2cap_sock_kill(sk)
                                          l2cap_sock_put_chan
     chan = READ l2cap_pi(sk)->chan         l2cap_pi(sk)->chan = NULL
     l2cap_chan_hold_unless_zero            l2cap_put_chan(chan)
       kref_get_unless_zero(&chan->ref)

Task 1 may observe NULL which causes null-ptr-deref.

Fix the race by taking lock_sock() in l2cap_sock_kill() to
synchronize with l2cap_sock_cleanup_listen().  hold_unless_zero() is not
needed here, l2cap_pi(sk)->chan owns reference if it is non-NULL.

Clarify code comments vs. locking.

Fixes: 6fef032af009 ("Bluetooth: L2CAP: Fix use-after-free in l2cap_sock_new_connection_cb()")
Reported-by: syzbot+e6382a2f53f5fc7453ac@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=e6382a2f53f5fc7453ac
Signed-off-by: Pauli Virtanen <pav@iki.fi>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>