speakup: Reject setting the speakup line discipline outside of speakup

mainline inclusion
from mainline-v5.10-rc7
commit f0992098
category: bugfix
bugzilla: NA
CVE: CVE-2020-27830

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

Speakup exposing a line discipline allows userland to try to use it,
while it is deemed to be useless, and thus uselessly exposes potential
bugs. One of them is simply that in such a case if the line sends data,
spk_ttyio_receive_buf2 is called and crashes since spk_ttyio_synth
is NULL.

This change restricts the use of the speakup line discipline to
speakup drivers, thus avoiding such kind of issues altogether.

Cc: stable@vger.kernel.org
Reported-by: Shisong Qin <qinshisong1205@gmail.com>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Tested-by: Shisong Qin <qinshisong1205@gmail.com>
Link: https://lore.kernel.org/r/20201129193523.hm3f6n5xrn6fiyyc@function


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Conflicts:
  drivers/accessibility/speakup/spk_ttyio.c
[yyl: spk_ttyio.c is in drivers/staging/speakup/ in kernel-4.19]
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>