tty: Hold termios_rwsem for tcflow(TCIxxx)

While transmitting a START/STOP char for tcflow(TCION/TCIOFF), prevent
a termios change. Otherwise, a garbage in-band flow control char
may be sent, if the termios change overlaps the transmission setup.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 7 insertions, 3 deletions

diff --git a/drivers/tty/tty_ioctl.c b/drivers/tty/tty_ioctl.c
index ad9120d1c0f1..62380ccf70fb 100644
--- a/drivers/tty/tty_ioctl.c
+++ b/drivers/tty/tty_ioctl.c
@@ -1164,17 +1164,21 @@ int n_tty_ioctl_helper(struct tty_struct *tty, struct file *file,
                         spin_unlock_irq(&tty->flow_lock);
                         break;
                 case TCIOFF:
+                        down_read(&tty->termios_rwsem);
                         if (STOP_CHAR(tty) != __DISABLED_CHAR)
-                                return tty_send_xchar(tty, STOP_CHAR(tty));
+                                retval = tty_send_xchar(tty, STOP_CHAR(tty));
+                        up_read(&tty->termios_rwsem);
                         break;
                 case TCION:
+                        down_read(&tty->termios_rwsem);
                         if (START_CHAR(tty) != __DISABLED_CHAR)
-                                return tty_send_xchar(tty, START_CHAR(tty));
+                                retval = tty_send_xchar(tty, START_CHAR(tty));
+                        up_read(&tty->termios_rwsem);
                         break;
                 default:
                         return -EINVAL;
                 }
-                return 0;
+                return retval;
         case TCFLSH:
                 retval = tty_check_change(tty);
                 if (retval)