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

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

On Sat, Sep 13, 2025 at 11:55:45PM +0800, Tzung-Bi Shih wrote:
> 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 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.

Why shouldn't we ? We have enough precedents where driver authors rushed
to adopt brand new APIs without understand the implications.
devm_kzalloc() is a prime example of a small new API that very quickly
got misused everywhere. If we had taken the time to clearly explain when
it should be used and when it should *not* be used, we wouldn't be
plagued by as many device removal race conditions today. Let's not
repeat the same mistake, I'd like this new API to make things better,
not worse.

> - 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.

I'm looking forward to that. Please let me know if there's anything you
would like to discuss. I didn't dive deep in technical details in this
thread, and I don't expect anyone to guess what I have in mind if I
failed to express it :-) I'm very confident the cdev race condition can
be fixed in a neat way, so let's do that.

> > > 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.

-- 
Regards,

Laurent Pinchart