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

[NeilBrown]

On Tue, 14 Apr 2015 08:50:01 +0200 Olivier Sobrie <olivier@xxxxxxxxx> wrote:

> Hello Neil,
> 
> On Tue, Apr 14, 2015 at 11:03:03AM +1000, NeilBrown wrote:
> > 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?
> 
> Indeed, if I undestand correctly the code in tty_io.c, cleanup() method
> is also called when the open fails while kref_get is only done if the
> open succeeds. Sorry for that mess.
> I assume you get that crash when hso_start_serial_device() returns
> an error?

Yes, that's correct.

> 
> At first sight, the patch below looks good to me.
> I'll test it in the next days.

Thanks.

NeilBrown

> 
> Thank you,
> 
> Olivier
> 
> > 
> > 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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