tty: fix data race between tty_init_dev and flush of buf

There can be a race, if receive_buf call comes before
tty initialization completes in n_tty_open and tty->disc_data
may be NULL.

CPU0					CPU1
----					----
 000|n_tty_receive_buf_common()   	n_tty_open()
-001|n_tty_receive_buf2()		tty_ldisc_open.isra.3()
-002|tty_ldisc_receive_buf(inline)	tty_ldisc_setup()

Using ldisc semaphore lock in tty_init_dev till disc_data
initializes completely.

Signed-off-by: Gaurav Kohli <gkohli@codeaurora.org>
Reviewed-by: Alan Cox <alan@linux.intel.com>
Cc: stable <stable@vger.kernel.org>

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 2 insertions, 2 deletions

diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
index 24ec5c7e6b20..4e7946c0484b 100644
--- a/drivers/tty/tty_ldisc.c
+++ b/drivers/tty/tty_ldisc.c
@@ -337,7 +337,7 @@ static inline void __tty_ldisc_unlock(struct tty_struct *tty)
 	ldsem_up_write(&tty->ldisc_sem);
 }
 
-static int tty_ldisc_lock(struct tty_struct *tty, unsigned long timeout)
+int tty_ldisc_lock(struct tty_struct *tty, unsigned long timeout)
 {
 	int ret;
 
@@ -348,7 +348,7 @@ static int tty_ldisc_lock(struct tty_struct *tty, unsigned long timeout)
 	return 0;
 }
 
-static void tty_ldisc_unlock(struct tty_struct *tty)
+void tty_ldisc_unlock(struct tty_struct *tty)
 {
 	clear_bit(TTY_LDISC_HALTED, &tty->flags);
 	__tty_ldisc_unlock(tty);