tty: n_gsm: fix reset fifo race condition

gsmtty_write() and gsm_dlci_data_output() properly guard the fifo access.
However, gsm_dlci_close() and gsmtty_flush_buffer() modifies the fifo but
do not guard this.
Add a guard here to prevent race conditions on parallel writes to the fifo.

Fixes: e1eaea4 ("tty: n_gsm line discipline")
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20220414094225.4527-17-daniel.starke@siemens.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/n_gsm.c

@@ -1442,13 +1442,17 @@ static int gsm_control_wait(struct gsm_mux *gsm, struct gsm_control *control)
 
 static void gsm_dlci_close(struct gsm_dlci *dlci)
 {
+	unsigned long flags;
+
 	del_timer(&dlci->t1);
 	if (debug & 8)
 		pr_debug("DLCI %d goes closed.\n", dlci->addr);
 	dlci->state = DLCI_CLOSED;
 	if (dlci->addr != 0) {
 		tty_port_tty_hangup(&dlci->port, false);
+		spin_lock_irqsave(&dlci->lock, flags);
 		kfifo_reset(&dlci->fifo);
+		spin_unlock_irqrestore(&dlci->lock, flags);
 		/* Ensure that gsmtty_open() can return. */
 		tty_port_set_initialized(&dlci->port, 0);
 		wake_up_interruptible(&dlci->port.open_wait);
@@ -3148,13 +3152,17 @@ static unsigned int gsmtty_chars_in_buffer(struct tty_struct *tty)
 static void gsmtty_flush_buffer(struct tty_struct *tty)
 {
 	struct gsm_dlci *dlci = tty->driver_data;
+	unsigned long flags;
+
 	if (dlci->state == DLCI_CLOSED)
 		return;
 	/* Caution needed: If we implement reliable transport classes
 	   then the data being transmitted can't simply be junked once
 	   it has first hit the stack. Until then we can just blow it
 	   away */
+	spin_lock_irqsave(&dlci->lock, flags);
 	kfifo_reset(&dlci->fifo);
+	spin_unlock_irqrestore(&dlci->lock, flags);
 	/* Need to unhook this DLCI from the transmit queue logic */
 }