[PATCH] speakup: Reject setting the speakup line discipline outside of speakup

[Samuel Thibault]

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: sta...@vger.kernel.org
Reported-by: Shisong Qin <qinshisong1...@gmail.com>
Signed-off-by: Samuel Thibault <samuel.thiba...@ens-lyon.org>
Tested-by: Shisong Qin <qinshisong1...@gmail.com>

Index: linux-5.9/drivers/accessibility/speakup/spk_ttyio.c
===================================================================
--- linux-5.9.orig/drivers/accessibility/speakup/spk_ttyio.c
+++ linux-5.9/drivers/accessibility/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(*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(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;
 }