From: Peter Hurley <peter@hurleysoftware.com>
Date: Thu, 16 Oct 2014 19:33:27 +0000 (-0400)
Subject: pty: Fix packet mode setting race
X-Git-Tag: v3.19-rc1~79^2~199
X-Git-Url: https://asedeno.scripts.mit.edu/gitweb/?a=commitdiff_plain;h=2622d73e51acfe1dbeb21bf2299066bdead85163;p=linux.git

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 e554393d5551..bcec4c71a408 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;