linux-kernel - Re: [PATCH 1/8] vfio/mdev: Fix to not do put_device on device

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Date:   Mon, 25 Mar 2019 13:21:27 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Kirti Wankhede <kwankhede@...dia.com>
Cc:     Parav Pandit <parav@...lanox.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/8] vfio/mdev: Fix to not do put_device on
 device_register failure

On Mon, 25 Mar 2019 23:47:30 +0530
Kirti Wankhede <kwankhede@...dia.com> wrote:

> On 3/23/2019 4:50 AM, Parav Pandit wrote:
> > device_register() performs put_device() if device_add() fails.
> > This balances with device_initialize().
> > 
> > mdev core performing put_device() when device_register() fails,
> > is an error that puts already released device again.
> > Therefore, don't put the device on error.
> >   
> 
> device_add() on all errors doesn't call put_device(dev). It releases
> reference to its parent, put_device(parent), but not the device itself,
> put_device(dev).

Sort of, device_initialize() initializes the reference count to 1,
device_add() increments the reference count to 2 via the get_device()
and then drops it back to 1 on all exit paths.  The oddity is the
failure path of get_device() itself, but that can only happen if passed
a NULL device, where put_device() is a no-op and not relevant here.  So
in all cases device_register() returns with a reference count of 1 and
we need to call put_device() to free the allocated object.  The below
change would leak the mdev on error.  Thanks,

Alex

> > Fixes: 7b96953bc640 ("vfio: Mediated device Core driver")
> > Signed-off-by: Parav Pandit <parav@...lanox.com>
> > ---
> >  drivers/vfio/mdev/mdev_core.c | 4 +---
> >  1 file changed, 1 insertion(+), 3 deletions(-)
> > 
> > diff --git a/drivers/vfio/mdev/mdev_core.c b/drivers/vfio/mdev/mdev_core.c
> > index 0212f0e..3e5880a 100644
> > --- a/drivers/vfio/mdev/mdev_core.c
> > +++ b/drivers/vfio/mdev/mdev_core.c
> > @@ -318,10 +318,8 @@ int mdev_device_create(struct kobject *kobj, struct device *dev, uuid_le uuid)
> >  	dev_set_name(&mdev->dev, "%pUl", uuid.b);
> >  
> >  	ret = device_register(&mdev->dev);
> > -	if (ret) {
> > -		put_device(&mdev->dev);
> > +	if (ret)
> >  		goto mdev_fail;
> > -	}
> >  
> >  	ret = mdev_device_create_ops(kobj, mdev);
> >  	if (ret)
> >