linux-kernel - Re: [PATCH] tty: add lockdep annotations

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Date:	Sat, 02 Jun 2012 09:17:18 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Alan Cox <alan@...ux.intel.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jens Axboe <jaxboe@...ionio.com>
Subject: Re: [PATCH] tty: add lockdep annotations

On Fri, 2012-06-01 at 22:59 +0200, Eric Dumazet wrote:
> On Fri, 2012-06-01 at 21:56 +0100, Alan Cox wrote:
> > > Yes, tty->driver deref is ok (tty points to valid memory), but crash is
> > > on tty->driver->ops (driver points to freed/illegal memory)
> > > 
> > > using slub_debug=FZPU, I can indeed see RDI=6b6b6b6b6b6b6b6b
> > 
> > driver and driver->ops is basically const and it's not what you'd expect
> > from a tty refcount bug. The driver side puts shouldn't have changed but
> > I'll take a look over that patch and the error paths closely again just
> > in case.
> 
> right
> 
> The code looks like :
> ...
> 	call mcount
> 	mov  %rdi,%rbx
> 	mov  0x10(%rdi),%rdi   tty->driver
> <>	mov  0xf8(%rdi),%rax   CRASH
> 
> So tty was freed an tty->driver contains 6b6b6b6b6b6b6b6b
> 
> 

Here is the patch I am currently testing (need to boot the machine ~50
times to make sure it is the right fix)

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 9e930c0..128a95b 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -1479,13 +1479,14 @@ static void release_one_tty(struct work_struct *work)
 	struct tty_struct *tty =
 		container_of(work, struct tty_struct, hangup_work);
 	struct tty_driver *driver = tty->driver;
+	struct module *module = driver->owner;
 
 	if (tty->ops->cleanup)
 		tty->ops->cleanup(tty);
 
 	tty->magic = 0;
 	tty_driver_kref_put(driver);
-	module_put(driver->owner);
+	module_put(module);
 
 	spin_lock(&tty_files_lock);
 	list_del_init(&tty->tty_files);
@@ -2005,11 +2006,15 @@ retry_open:
 			filp->f_op = &tty_fops;
 		goto retry_open;
 	}
-	tty_unlock(tty);
-
 
+	/*
+	 * Must acquire both mutexes in right order,
+	 * and keep a reference on tty, so dont call tty_unlock() !
+	 */
+	mutex_unlock(&tty->legacy_mutex);
 	mutex_lock(&tty_mutex);
-	tty_lock(tty);
+	mutex_lock(&tty->legacy_mutex);
+
 	spin_lock_irq(&current->sighand->siglock);
 	if (!noctty &&
 	    current->signal->leader &&


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