tty: make sure to flush any pending work when halting the ldisc

When I rewrote tty ldisc code to use proper reference counts (commits
65b770468e98 and cbe9352fa08f) in order to avoid a race with hangup, the
test-program that Eric Biederman used to trigger the original problem
seems to have exposed another long-standing bug: the hangup code did the
'tty_ldisc_halt()' to stop any buffer flushing activity, but unlike the
other call sites it never actually flushed any pending work.

As a result, if you get just the right timing, the pending work may be
just about to execute (ie the timer has already triggered and thus
cancel_delayed_work() was a no-op), when we then re-initialize the ldisc
from under it.

That, in turn, results in various random problems, usually seen as a
NULL pointer dereference in run_timer_softirq() or a BUG() in
worker_thread (but it can be almost anything).

Fix it by adding the required 'flush_scheduled_work()' after doing the
tty_ldisc_halt() (this also requires us to move the ldisc halt to before
taking the ldisc mutex in order to avoid a deadlock with the workqueue
executing do_tty_hangup, which requires the mutex).

The locking should be cleaned up one day (the requirement to do this
outside the ldisc_mutex is very annoying, and weakens the lock), but
that's a larger and separate undertaking.

Reported-by: Eric W. Biederman <ebiederm@xmission.com>
Tested-by: Xiaotian Feng <xtfeng@gmail.com>
Tested-by: Yanmin Zhang <yanmin_zhang@linux.intel.com>
Tested-by: Dave Young <hidave.darkstar@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Greg Kroah-Hartman <gregkh@suse.de>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 7 insertions, 3 deletions

diff --git a/drivers/char/tty_ldisc.c b/drivers/char/tty_ldisc.c
index 1733d3439ad2..e48af9f79219 100644
--- a/drivers/char/tty_ldisc.c
+++ b/drivers/char/tty_ldisc.c
@@ -508,8 +508,9 @@ static void tty_ldisc_restore(struct tty_struct *tty, struct tty_ldisc *old)
  *      be obtained while the delayed work queue halt ensures that no more
  *      data is fed to the ldisc.
  *
- *      In order to wait for any existing references to complete see
- *      tty_ldisc_wait_idle.
+ *      You need to do a 'flush_scheduled_work()' (outside the ldisc_mutex)
+ *      in order to make sure any currently executing ldisc work is also
+ *      flushed.
  */
 
 static int tty_ldisc_halt(struct tty_struct *tty)
@@ -753,11 +754,14 @@ void tty_ldisc_hangup(struct tty_struct *tty)
          * N_TTY.
          */
         if (tty->driver->flags & TTY_DRIVER_RESET_TERMIOS) {
+                /* Make sure the old ldisc is quiescent */
+                tty_ldisc_halt(tty);
+                flush_scheduled_work();
+
                 /* Avoid racing set_ldisc or tty_ldisc_release */
                 mutex_lock(&tty->ldisc_mutex);
                 if (tty->ldisc) {       /* Not yet closed */
                         /* Switch back to N_TTY */
-                        tty_ldisc_halt(tty);
                         tty_ldisc_reinit(tty);
                         /* At this point we have a closed ldisc and we want to
                            reopen it. We could defer this to the next open but