[PATCH] USB: serial: keyspan_pda: fix information leak

[Johan Hovold <johan@kernel.org>]

The write() callback is supposed to return the number of characters
accepted or a negative errno. Since the addition of write fifo support
the keyspan_pda implementation will however return the number characters
submitted to the device if the write urb is not already in use. If this
number is larger than the number of characters passed to write(), the
line discipline continues writing data from beyond the tty write buffer.

Fix the information leak by making sure that keyspan_pda_write_start()
returns zero on success as intended.

Fixes: 034e38e8f687 ("USB: serial: keyspan_pda: add write-fifo support")
Cc: stable@vger.kernel.org	# 5.11
Signed-off-by: Johan Hovold <johan@kernel.org>
---
 drivers/usb/serial/keyspan_pda.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/usb/serial/keyspan_pda.c b/drivers/usb/serial/keyspan_pda.c
index 3b99f9676c35..f05bcce60600 100644
--- a/drivers/usb/serial/keyspan_pda.c
+++ b/drivers/usb/serial/keyspan_pda.c
@@ -516,7 +516,7 @@ static int keyspan_pda_write_start(struct usb_serial_port *port)
 	if (count == room)
 		schedule_work(&priv->unthrottle_work);
 
-	return count;
+	return 0;
 }
 
 static void keyspan_pda_write_bulk_callback(struct urb *urb)
-- 
2.53.0