net: hso: fix NULL-deref on disconnect regression

stable inclusion
from stable-v4.19.189
commit 5c17cfe155d21954b4c7e2a78fa771cebcd86725
category: bugfix
bugzilla: https://gitee.com/src-openeuler/kernel/issues/I93LMH
CVE: CVE-2021-46904

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5c17cfe155d21954b4c7e2a78fa771cebcd86725



---------------------------

commit 2ad5692d upstream.

Commit 8a12f883 ("net: hso: fix null-ptr-deref during tty device
unregistration") fixed the racy minor allocation reported by syzbot, but
introduced an unconditional NULL-pointer dereference on every disconnect
instead.

Specifically, the serial device table must no longer be accessed after
the minor has been released by hso_serial_tty_unregister().

Fixes: 8a12f883 ("net: hso: fix null-ptr-deref during tty device unregistration")
Cc: stable@vger.kernel.org
Cc: Anirudh Rayabharam <mail@anirudhrb.com>
Reported-by: Leonardo Antoniazzi <leoanto@aruba.it>
Signed-off-by: Johan Hovold <johan@kernel.org>
Reviewed-by: Anirudh Rayabharam <mail@anirudhrb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Liu Jian <liujian56@huawei.com>