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n_tty: Fix unsafe update of available buffer space

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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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Peter Hurley 2013-06-15 07:28:30 -04:00 committed by Greg Kroah-Hartman
parent a6e54319a7
commit b848305276
1 changed files with 11 additions and 7 deletions
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18
drivers/tty/n_tty.c
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@ -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;