linux-kernel - Re: [PATCH v6 1/3] revocable: Add fops replacement

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Message-ID: <20251107141509.GK1732817@nvidia.com>
Date: Fri, 7 Nov 2025 10:15:09 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: Tzung-Bi Shih <tzungbi@...nel.org>
Cc: Benson Leung <bleung@...omium.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Rafael J . Wysocki" <rafael@...nel.org>,
	Danilo Krummrich <dakr@...nel.org>,
	Jonathan Corbet <corbet@....net>, Shuah Khan <shuah@...nel.org>,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	chrome-platform@...ts.linux.dev, linux-kselftest@...r.kernel.org,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Bartosz Golaszewski <brgl@...ev.pl>,
	Wolfram Sang <wsa+renesas@...g-engineering.com>,
	Simona Vetter <simona.vetter@...ll.ch>,
	Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH v6 1/3] revocable: Add fops replacement

On Fri, Nov 07, 2025 at 05:07:54AM +0000, Tzung-Bi Shih wrote:
> On Thu, Nov 06, 2025 at 11:47:15AM -0400, Jason Gunthorpe wrote:
> > On Thu, Nov 06, 2025 at 11:27:10PM +0800, Tzung-Bi Shih wrote:
> > > +/*
> > > + * Recover the private_data to its original one.
> > > + */
> > > +static struct fops_replacement *_recover_private_data(struct file *filp)
> > > +{
> > > +	struct fops_replacement *fr = filp->private_data;
> > > +
> > > +	filp->private_data = fr->orig_private_data;
> > > +	return fr;
> > > +}
> > > +
> > > +/*
> > > + * Replace the private_data to fops_replacement.
> > > + */
> > > +static void _replace_private_data(struct fops_replacement *fr)
> > > +{
> > > +	fr->filp->private_data = fr;
> > > +}
> > 
> > This switching of private_data isn't reasonable, it breaks too much
> > stuff. I think I showed a better idea in my sketch.
> 
> The approach assumes the filp->private_data should be set once by the
> filp->f_op->open() if any.  Is it common that the filp->private_data
> be updated in other file operations?

You can set it once during open, but you can't change it around every
fops callback. This stuff is all concurrent.

> > This probably doesn't work out, is likely to make a memory leak.
> > It will be hard for the owning driver to free its per-file memory
> > without access to release.
> 
> Ah, I think this reveals a drawback of the approach.
> - Without calling ->release(), some memory may leak.
> - With calling ->release(), some UAF may happen. 

It just means the user of this needs to understand there are
limitations on what release can do. Usually release just frees memory,
that is fine.

I think it would be strange for a release to touch revocable data,
that might suggest some larger problem.

Jason