linux-kernel - Re: [PATCH] fs/char_dev: fix cdev_put() vs f

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Date:	Thu, 11 Aug 2016 06:16:50 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-nvdimm@...ts.01.org
Subject: Re: [PATCH] fs/char_dev: fix cdev_put() vs f_op->release()
 use-after-free

On Wed, Aug 10, 2016 at 09:49:22PM -0700, Dan Williams wrote:

> Where dax_dev_release() is the f_op->release() method, and is
> implemented to simply drop the final references on our driver objects:
> 
>         struct dax_dev *dax_dev = filp->private_data;
>         struct device *dev = dax_dev->dev;
> 
>         dev_dbg(dax_dev->dev, "%s\n", __func__);
>         put_device(dev);
>         dax_dev_put(dax_dev);
> 
> The dax_dev object embeds a 'struct cdev' which means f_op->release()
> may free cdev, so __fput() needs to drop the cdev reference before
> calling f_op->release().

NAK.  You *can't* free a structure that contains kobj with currently
positive refcount.  Ever.  If you embed a struct kobj into something,
you must use the refcount of that kobj (or one of its ancestors) to
control the lifetime of containing object.  If your dax_dev_put() can
trigger freeing of dax_dev despite the still-positive refcount of
embedded cdev.kobj, it is fundamentally broken.