speakup: Reject setting the speakup line discipline outside of speakup

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/accessibility/speakup/spk_ttyio.c

@@ -47,27 +47,20 @@ static int spk_ttyio_ldisc_open(struct tty_struct *tty)
 {
 	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(*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;
 }
@@ -191,9 +184,25 @@ static int spk_ttyio_initialise_ldisc(struct spk_synth *synth)
 
 	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;
 }