[PATCH 3/4] n_tty: Fix unsafe update of available buffer space

Peter Hurley
2013-06-15T11:29:32Z

receive_room is used to control the amount of data the flip
buffer work can push to the read buffer. This update is unsafe:

CPU 0 | CPU 1
|
| n_tty_read()
| n_tty_set_room()
| left = <calc of space>
n_tty_receive_buf() |
<push data to buffer> |
n_tty_set_room() |
left = <calc of space> |
tty->receive_room = left |
| tty->receive_room = left

receive_room is now updated with a stale calculation of the
available buffer space, and the subsequent work loop will likely
overwrite unread data in the input buffer.

Update receive_room atomically with the calculation of the
available buffer space.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
---
drivers/tty/n_tty.c | 18 +++++++++++-------
1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index fa5cb46..0646165 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -115,13 +115,14 @@ static inline int tty_put_user(struct tty_struct *tty, unsigned char x,
}

/**
- * n_tty_set__room - receive space
+ * n_tty_set_room - receive space
* @tty: terminal
*
- * Called by the driver to find out how much data it is
- * permitted to feed to the line discipline without any being lost
- * and thus to manage flow control. Not serialized. Answers for the
- * "instant".
+ * Sets tty->receive_room to reflect the currently available space
+ * in the input buffer, and re-schedules the flip buffer work if space
+ * just became available.
+ *
+ * Locks: Concurrent update is protected with read_lock
*/

static void n_tty_set_room(struct tty_struct *tty)
@@ -129,8 +130,10 @@ static void n_tty_set_room(struct tty_struct *tty)
struct n_tty_data *ldata = tty->disc_data;
int left;
int old_left;
+ unsigned long flags;
+
+ raw_spin_lock_irqsave(&ldata->read_lock, flags);

- /* ldata->read_cnt is not read locked ? */
if (I_PARMRK(tty)) {
/* Multiply read_cnt by 3, since each byte might take up to
* three times as many spaces when PARMRK is set (depending on
@@ -150,6 +153,8 @@ static void n_tty_set_room(struct tty_struct *tty)
old_left = tty->receive_room;
tty->receive_room = left;

+ raw_spin_unlock_irqrestore(&ldata->read_lock, flags);
+
/* Did this open up the receive buffer? We may need to flip */
if (left && !old_left) {
WARN_RATELIMIT(tty->port->itty == NULL,
@@ -1872,7 +1877,6 @@ do_it_again:
retval = -ERESTARTSYS;
break;
}
- /* FIXME: does n_tty_set_room need locking ? */
n_tty_set_room(tty);
timeout = schedule_timeout(timeout);
continue;
--
1.8.1.2