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pty: Fix packet mode setting race

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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>
This commit is contained in:
Peter Hurley
2014-10-16 15:33:27 -04:00
committed by Greg Kroah-Hartman
parent 54e8e5fcaa
commit 2622d73e51
1 changed files with 2 additions and 1 deletions
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drivers/tty/pty.c
+2 -1
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@@ -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;