tty: Allow safe access to termios for set_ldisc() handlers

Allow a tty driver to safely access termios settings while handling
the set_ldisc() notification. UART drivers use the set_ldisc()
notification to check if the N_PPS line discipline is being enabled;
if so, modem status interrupts may also need to be enabled. Conversely,
modem status interrupts may need to be disabled if switching away
from the N_PPS line discipline.

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

1 files changed, 4 insertions, 1 deletions

diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
index b66a81d0549e..3737f55272d2 100644
--- a/drivers/tty/tty_ldisc.c
+++ b/drivers/tty/tty_ldisc.c
@@ -572,8 +572,11 @@ int tty_set_ldisc(struct tty_struct *tty, int ldisc)
                 tty_ldisc_restore(tty, old_ldisc);
         }
 
-        if (tty->ldisc->ops->num != old_ldisc->ops->num && tty->ops->set_ldisc)
+        if (tty->ldisc->ops->num != old_ldisc->ops->num && tty->ops->set_ldisc) {
+                down_read(&tty->termios_rwsem);
                 tty->ops->set_ldisc(tty);
+                up_read(&tty->termios_rwsem);
+        }
 
         /* At this point we hold a reference to the new ldisc and a
            reference to the old ldisc, or we hold two references to