[PATCH v2 4/7] tty: Fix ioctl(FIOASYNC) on hungup file

[Peter Hurley]

A small race window exists which allows signal-driven async i/o to be
enabled for the tty when the file ptr has already been hungup and
signal-driven i/o has been disabled:

CPU 0                                CPU 1
-----                                ------
ioctl_fioasync(on)
  filp->f_op->fasync(on)             __tty_hangup()
    tty_fasync(on)                     tty_lock()
      tty_lock()                       ...
        .                              filp->f_op = &hung_up_tty_fops;
      (waiting)                       __tty_fasync(off)
        .                              tty_unlock()
      /* gets tty lock  */
      /* enables FASYNC */

Check the tty has not been hungup while holding tty_lock.

Signed-off-by: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>
---
 drivers/tty/tty_io.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 05b0a9c..7113bfa 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -2260,10 +2260,11 @@ out:
 static int tty_fasync(int fd, struct file *filp, int on)
 {
 	struct tty_struct *tty = file_tty(filp);
-	int retval;
+	int retval = -ENOTTY;
 
 	tty_lock(tty);
-	retval = __tty_fasync(fd, filp, on);
+	if (!tty_hung_up_p(filp))
+		retval = __tty_fasync(fd, filp, on);
 	tty_unlock(tty);
 
 	return retval;
-- 
2.7.0