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Subject[PATCH 4.3 122/125] tty: Fix tty_send_xchar() lock order inversion
Date
4.3-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Peter Hurley <peter@hurleysoftware.com>

commit ee0c1a65cf95230d5eb3d9de94fd2ead9a428c67 upstream.

The correct lock order is atomic_write_lock => termios_rwsem, as
established by tty_write() => n_tty_write().

Fixes: c274f6ef1c666 ("tty: Hold termios_rwsem for tcflow(TCIxxx)")
Reported-and-Tested-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/tty/tty_io.c | 4 ++++
drivers/tty/tty_ioctl.c | 4 ----
2 files changed, 4 insertions(+), 4 deletions(-)

--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -1282,18 +1282,22 @@ int tty_send_xchar(struct tty_struct *tt
int was_stopped = tty->stopped;

if (tty->ops->send_xchar) {
+ down_read(&tty->termios_rwsem);
tty->ops->send_xchar(tty, ch);
+ up_read(&tty->termios_rwsem);
return 0;
}

if (tty_write_lock(tty, 0) < 0)
return -ERESTARTSYS;

+ down_read(&tty->termios_rwsem);
if (was_stopped)
start_tty(tty);
tty->ops->write(tty, &ch, 1);
if (was_stopped)
stop_tty(tty);
+ up_read(&tty->termios_rwsem);
tty_write_unlock(tty);
return 0;
}
--- a/drivers/tty/tty_ioctl.c
+++ b/drivers/tty/tty_ioctl.c
@@ -1147,16 +1147,12 @@ int n_tty_ioctl_helper(struct tty_struct
spin_unlock_irq(&tty->flow_lock);
break;
case TCIOFF:
- down_read(&tty->termios_rwsem);
if (STOP_CHAR(tty) != __DISABLED_CHAR)
retval = tty_send_xchar(tty, STOP_CHAR(tty));
- up_read(&tty->termios_rwsem);
break;
case TCION:
- down_read(&tty->termios_rwsem);
if (START_CHAR(tty) != __DISABLED_CHAR)
retval = tty_send_xchar(tty, START_CHAR(tty));
- up_read(&tty->termios_rwsem);
break;
default:
return -EINVAL;



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