pty: Fix packet mode setting race

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@hurleysoftware.com>
Reviewed-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
index e554393d5551192997dc1048194b76bcc70b62e4..bcec4c71a408cd60832d1f1bcf4fdc32e174c777 100644 (file)
--- 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;