[PATCH -next 6/9] pty: Fix packet mode setting race

[Peter Hurley]

Because pty_set_pktmode() does not claim the slave's ctrl_lock
to clear ->ctrl_status (to avoid unnecessary lock nesting),
pty_set_pktmode() may accidentally erase new ->ctrl_status updates.
For example,

CPU 0                             | CPU 1
pty_set_pktmode()                 | pty_start()
  spin_lock(master's ctrl_lock)   |
  tty->packet = 1                 |
                                  |   if (tty->link->packet)
                                  |     spin_lock(slave's ctrl_lock)
                                  |     tty->ctrl_status = TIOCPKT_START
  tty->link->ctrl_status = 0      |

Ensure the clear of ->ctrl_status occurs before packet mode is set
(and observable on another cpu).

Signed-off-by: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>
---
 drivers/tty/pty.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
index e554393..bcec4c7 100644
--- a/drivers/tty/pty.c
+++ b/drivers/tty/pty.c
@@ -186,8 +186,9 @@ static int pty_set_pktmode(struct tty_struct *tty, int __user *arg)
 	spin_lock_irq(&tty->ctrl_lock);
 	if (pktmode) {
 		if (!tty->packet) {
-			tty->packet = 1;
 			tty->link->ctrl_status = 0;
+			smp_mb();
+			tty->packet = 1;
 		}
 	} else
 		tty->packet = 0;
-- 
2.1.1

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