From 31e121284f90bf559618330e230b286f969b6b7f Mon Sep 17 00:00:00 2001 From: Jiri Slaby Date: Thu, 18 Oct 2012 22:26:33 +0200 Subject: [PATCH] TTY: ldisc, wait for idle ldisc in release We reintroduced tty_ldisc_wait_idle in 100eeae2c5c (TTY: restore tty_ldisc_wait_idle) and used in set_ldisc. Then we added it also to the hangup path in 92f6fa09bd453 (TTY: ldisc, do not close until there are readers). And we noted that there is one more path: ~ Before 65b770468e98 tty_ldisc_wait_idle was called also from ~ tty_ldisc_release. It is called from tty_release, so I don't think ~ we need to restore that one. Well, I was wrong. There might still be holders of an ldisc reference. Not from userspace, but drivers. If they take a reference and a user closes the device immediately after that, we have a problem. ldisc is halted and closed by TTY, but the driver still may call some ldisc's operation and cause a crash. So restore the tty_ldisc_wait_idle call also to the third location where it was before 65b770468e98 (tty-ldisc: turn ldisc user count into a proper refcount). Now we should be safe with respect to the ldisc reference counting as all* tty_ldisc_close paths are safely called with reference count of one. * Not the one in tty_ldisc_setup's fail path. But that is called before the first open finishes. So userspace does not see it yet. Even thought the driver is given the TTY already via ->install, it should not take a reference to the ldisc yet. If some driver is to do this, we should put one tty_ldisc_wait_idle also in the setup. Signed-off-by: Jiri Slaby Acked-by: Alan Cox Signed-off-by: Greg Kroah-Hartman --- drivers/tty/tty_ldisc.c | 5 +++++ 1 file changed, 5 insertions(+) diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c index 0f2a2c5e704c..47e3968df10b 100644 --- a/drivers/tty/tty_ldisc.c +++ b/drivers/tty/tty_ldisc.c @@ -897,6 +897,11 @@ int tty_ldisc_setup(struct tty_struct *tty, struct tty_struct *o_tty) static void tty_ldisc_kill(struct tty_struct *tty) { + /* There cannot be users from userspace now. But there still might be + * drivers holding a reference via tty_ldisc_ref. Do not steal them the + * ldisc until they are done. */ + tty_ldisc_wait_idle(tty, MAX_SCHEDULE_TIMEOUT); + mutex_lock(&tty->ldisc_mutex); /* * Now kill off the ldisc -- GitLab