linux-kernel - Re: [PATCH] Fix race/oops in tty layer after BKL pushdown

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Message-Id: <200808072242.20422.borntraeger@de.ibm.com>
Date:	Thu, 7 Aug 2008 22:42:20 +0200
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Olaf Schnapper <os@...ibm.com>
Subject: Re: [PATCH] Fix race/oops in tty layer after BKL pushdown

Am Donnerstag, 7. August 2008 schrieb Alan Cox:
> > It seems that tty was already free in disassocate_ctty when it tries
> > to dereference tty->driver.
> > 
> > After moving the lock_kernel before the mutex_unlock, I can no longer
> > reproduce the problem.
> > 
> > Please review and consider to apply:
> 
> This doesn't help as the BKL doesn't protect tty here - we don't have
> refcounting on the ttys yet so this bug (which goes back forever) simply
> becomes a different sized race if you swap the lock ordering.
> 
> Given tty_vhangup just fires off a wait queue which is killed on the tty
> destruction you should be able for now at least to move the mutex_unlock
> to after the call to tty_vhangup().
> 
> Does that also fix the problem ?

You mean something like the below patch? Yes, that works as well.

Signed-off-by: Christian Borntraeger <borntraeger@...ibm.com>

---
 drivers/char/tty_io.c |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

Index: kvm/drivers/char/tty_io.c
===================================================================
--- kvm.orig/drivers/char/tty_io.c
+++ kvm/drivers/char/tty_io.c
@@ -1161,12 +1161,11 @@ void disassociate_ctty(int on_exit)
 	tty = get_current_tty();
 	if (tty) {
 		tty_pgrp = get_pid(tty->pgrp);
-		mutex_unlock(&tty_mutex);
 		lock_kernel();
-		/* XXX: here we race, there is nothing protecting tty */
 		if (on_exit && tty->driver->type != TTY_DRIVER_TYPE_PTY)
 			tty_vhangup(tty);
 		unlock_kernel();
+		mutex_unlock(&tty_mutex);
 	} else if (on_exit) {
 		struct pid *old_pgrp;
 		spin_lock_irq(&current->sighand->siglock);
--
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