linux-kernel - Re: [PATCH v3 0/5] platform/chrome: Fix a possible UAF via revocable

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Message-ID: <aMWUAS9jReia0jhy@tzungbi-laptop>
Date: Sat, 13 Sep 2025 23:55:45 +0800
From: Tzung-Bi Shih <tzungbi@...nel.org>
To: Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc: Bartosz Golaszewski <brgl@...ev.pl>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Bartosz Golaszewski <bartosz.golaszewski@...aro.org>,
	Krzysztof Kozlowski <krzk@...nel.org>,
	Benson Leung <bleung@...omium.org>,
	"Rafael J . Wysocki" <rafael@...nel.org>,
	Danilo Krummrich <dakr@...nel.org>,
	Jonathan Corbet <corbet@....net>, Shuah Khan <shuah@...nel.org>,
	Dawid Niedzwiecki <dawidn@...gle.com>, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, chrome-platform@...ts.linux.dev,
	linux-kselftest@...r.kernel.org,
	Wolfram Sang <wsa+renesas@...g-engineering.com>,
	Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH v3 0/5] platform/chrome: Fix a possible UAF via revocable

On Fri, Sep 12, 2025 at 05:54:16PM +0300, Laurent Pinchart wrote:
> On Fri, Sep 12, 2025 at 04:44:56PM +0200, Bartosz Golaszewski wrote:
> > On Fri, Sep 12, 2025 at 4:40 PM Greg Kroah-Hartman
> > <gregkh@...uxfoundation.org> wrote:
> > >
> > > Dan's proposal here is a good start, but the "sleep in cdev_del() until
> > > the device drains all existing opens" is going to not really work well
> > > for what we want.
> > >
> > > So sure, make a new cdev api to use this, that's fine, then we will have
> > > what, 5 different ways to use a cdev?  :)
> > >
> > > Seriously, that would be good, then we can work to convert things over,
> > > but I think overall it will look much the same as what patch 5/5 does
> > > here.  But details matter, I don't really known for sure...
> > >
> > > Either way, I think this patch series stands on its own, it doesn't
> > > require cdev to implement it, drivers can use it to wrap a cdev if they
> > > want to.  We have other structures that want to do this type of thing
> > > today as is proof with the rust implementation for the devm api.
> > 
> > Yeah, I'm not against this going upstream. If more development is
> > needed for this to be usable in other parts of the kernel, that can be
> > done gradually. Literally no subsystem ever was perfect on day 1.
> 
> To be clear, I'm not against the API being merged for the use cases that
> would benefit from it, but I don't want to see drivers using it to
> protect from the cdev/unregistration race.

Based on the discussion thread, my main takeaways are:

- Current `revocable` is considered a low level API.  We shouldn't (and
  likely can't) stop drivers, like the one in patch 5/5 in the series,
  from using it directly to fix UAFs.

- Subsystems (like cdev) should build on this API to provide an easier
  interface for their drivers to manage revocable resources.

I'll create a PoC based on this.

> > Tzung-Bi: I'm not sure if you did submit anything but I'd love to see
> > this discussed during Linux Plumbers in Tokyo, it's the perfect fit
> > for the kernel summit.

Yes, and I just realized that in addition to the website submission, a
separate email is also required (or at least encouraged).  I've just sent
that email and am hoping it's not too late.