[PATCH 5.4 36/54] speakup: Reject setting the speakup line discipline outside of speakup

Greg Kroah-Hartman
2020-12-10T15:33:19Z

From: Samuel Thibault <samuel.thibault@ens-lyon.org>

commit f0992098cadb4c9c6a00703b66cafe604e178fea upstream.

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>

---
drivers/staging/speakup/spk_ttyio.c | 37 ++++++++++++++++++++++--------------
1 file changed, 23 insertions(+), 14 deletions(-)

--- a/drivers/staging/speakup/spk_ttyio.c
+++ b/drivers/staging/speakup/spk_ttyio.c
@@ -47,27 +47,20 @@ static int spk_ttyio_ldisc_open(struct t
{
struct spk_ldisc_data *ldisc_data;

+ if (tty != speakup_tty)
+ /* Somebody tried to use this line discipline outside speakup */
+ return -ENODEV;
+
if (!tty->ops->write)
return -EOPNOTSUPP;

- mutex_lock(&speakup_tty_mutex);
- if (speakup_tty) {
- mutex_unlock(&speakup_tty_mutex);
- return -EBUSY;
- }
- speakup_tty = tty;
-
ldisc_data = kmalloc(sizeof(struct spk_ldisc_data), GFP_KERNEL);
- if (!ldisc_data) {
- speakup_tty = NULL;
- mutex_unlock(&speakup_tty_mutex);
+ if (!ldisc_data)
return -ENOMEM;
- }

init_completion(&ldisc_data->completion);
ldisc_data->buf_free = true;
- speakup_tty->disc_data = ldisc_data;
- mutex_unlock(&speakup_tty_mutex);
+ tty->disc_data = ldisc_data;

return 0;
}
@@ -189,9 +182,25 @@ static int spk_ttyio_initialise_ldisc(st

tty_unlock(tty);

+ mutex_lock(&speakup_tty_mutex);
+ speakup_tty = tty;
ret = tty_set_ldisc(tty, N_SPEAKUP);
if (ret)
- pr_err("speakup: Failed to set N_SPEAKUP on tty\n");
+ speakup_tty = NULL;
+ mutex_unlock(&speakup_tty_mutex);
+
+ if (!ret)
+ /* Success */
+ return 0;
+
+ pr_err("speakup: Failed to set N_SPEAKUP on tty\n");
+
+ tty_lock(tty);
+ if (tty->ops->close)
+ tty->ops->close(tty, NULL);
+ tty_unlock(tty);
+
+ tty_kclose(tty);

return ret;
}