Re: [PATCH] hso: fix refcnt leak in recent patch.

[NeilBrown]

On Tue, 14 Apr 2015 09:36:34 +1000 NeilBrown <neilb@xxxxxxx> wrote:

> 
> 
> Prior to
> commit 29bd3bc1194c624ce863cab2a7da9bc1f0c3b47b
>     hso: fix crash when device disappears while serial port is open
> 
> hso_serial_open would always kref_get(&serial->parent->ref) before
> returning zero.
> Since that commit, it only calls kref_get when returning 0 if
> serial->port.count was zero.
> 
> This results in calls to
>        kref_put(&serial->parent->ref, hso_serial_ref_free);
> 
> after hso_serial_ref_free has been called, which dereferences a freed
> pointer.
> 
> This patch adds the missing kref_get().
> 
> Fixes: commit 29bd3bc1194c624ce863cab2a7da9bc1f0c3b47b
> Cc: stable@xxxxxxxxxxxxxxx (v4.0)
> Cc: Olivier Sobrie <olivier@xxxxxxxxx>
> Signed-off-by: NeilBrown <neilb@xxxxxxx>
> 
> diff --git a/drivers/net/usb/hso.c b/drivers/net/usb/hso.c
> index 75befc1bd816..6848fc903340 100644
> --- a/drivers/net/usb/hso.c
> +++ b/drivers/net/usb/hso.c
> @@ -1299,6 +1299,7 @@ static int hso_serial_open(struct tty_struct *tty, struct file *filp)
>  		}
>  	} else {
>  		D1("Port was already open");
> +		kref_get(&serial->parent->ref);
>  	}
>  
>  	usb_autopm_put_interface(serial->parent->interface);


Sorry - that was wrong.
I'm getting crashes which strongly suggest the kref_put is being called extra
times, but I misunderstood the code and was hasty.

Maybe this instead?

Thanks,
NeilBrown

From: NeilBrown <neil@xxxxxxxxxx>
Date: Tue, 14 Apr 2015 09:33:03 +1000
Subject: [PATCH] hso: fix refcnt leak in recent patch.

Prior to
commit 29bd3bc1194c624ce863cab2a7da9bc1f0c3b47b
    hso: fix crash when device disappears while serial port is open

a kref_get on serial->parent->ref would be taken on each open,
and it would be kref_put on each close.

Now the kref_put happens when the tty_struct is finally put (via
the 'cleanup') providing tty->driver_data has been set.
So the kref_get must be called exact once when tty->driver_data is
set.

With the current code, if the first open fails the kref_get() is never
called, but the kref_put() is called, leaving to a crash.

So change the kref_get call to happen exactly when ->driver_data is
changed from NULL to non-NULL.

Fixes: commit 29bd3bc1194c624ce863cab2a7da9bc1f0c3b47b
Cc: stable@xxxxxxxxxxxxxxx (v4.0)
Cc: Olivier Sobrie <olivier@xxxxxxxxx>
Signed-off-by: NeilBrown <neil@xxxxxxxxxx>

diff --git a/drivers/net/usb/hso.c b/drivers/net/usb/hso.c
index 75befc1bd816..17fd3820263a 100644
--- a/drivers/net/usb/hso.c
+++ b/drivers/net/usb/hso.c
@@ -1278,6 +1278,8 @@ static int hso_serial_open(struct tty_struct *tty, struct file *filp)
 	D1("Opening %d", serial->minor);
 
 	/* setup */
+	if (tty->driver_data == NULL)
+		kref_get(&serial->parent->ref);
 	tty->driver_data = serial;
 	tty_port_tty_set(&serial->port, tty);
 
@@ -1294,8 +1296,6 @@ static int hso_serial_open(struct tty_struct *tty, struct file *filp)
 		if (result) {
 			hso_stop_serial_device(serial->parent);
 			serial->port.count--;
-		} else {
-			kref_get(&serial->parent->ref);
 		}
 	} else {
 		D1("Port was already open");

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