tty: n_gsm: fix race condition in gsmld_write()

The function may be used by the user directly and also by the n_gsm
internal functions. They can lead into a race condition which results in
interleaved frames if both are writing at the same time. The receiving side
is not able to decode those interleaved frames correctly.

Add a lock around the low side tty write to avoid race conditions and frame
interleaving between user originated writes and n_gsm writes.

Fixes: e1eaea46bb40 ("tty: n_gsm line discipline")
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20220701061652.39604-9-daniel.starke@siemens.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index 39359274096d459c16b17ef725c10ac039a16fee..9a3d7db333948f669afd2d5f7eb2c315f9e913e6 100644 (file)
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -2859,11 +2859,24 @@ static ssize_t gsmld_read(struct tty_struct *tty, struct file *file,
 static ssize_t gsmld_write(struct tty_struct *tty, struct file *file,
                           const unsigned char *buf, size_t nr)
 {
-       int space = tty_write_room(tty);
+       struct gsm_mux *gsm = tty->disc_data;
+       unsigned long flags;
+       int space;
+       int ret;
+
+       if (!gsm)
+               return -ENODEV;
+
+       ret = -ENOBUFS;
+       spin_lock_irqsave(&gsm->tx_lock, flags);
+       space = tty_write_room(tty);
        if (space >= nr)
-               return tty->ops->write(tty, buf, nr);
-       set_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
-       return -ENOBUFS;
+               ret = tty->ops->write(tty, buf, nr);
+       else
+               set_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
+       spin_unlock_irqrestore(&gsm->tx_lock, flags);
+
+       return ret;
 }
 
 /**