linux-kernel - Re: [PATCH v1] driver: base: Add driver filter support

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Date:   Thu, 5 Aug 2021 12:01:31 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>
Subject: Re: [PATCH v1] driver: base: Add driver filter support

On Thu, Aug 5, 2021 at 11:44 AM Andi Kleen <ak@...ux.intel.com> wrote:
>
>
> On 8/5/2021 11:09 AM, Greg Kroah-Hartman wrote:
> > On Thu, Aug 05, 2021 at 10:58:46AM -0700, Andi Kleen wrote:
> >> On 8/5/2021 10:51 AM, Greg Kroah-Hartman wrote:
> >>> It's controlled by whatever you want to use in userspace.  usbguard has
> >>> been handling this logic in userspace for over a decade now just fine.
> >>
> >> So how does that work with builtin USB drivers? Do you delay the USB binding
> >> until usbguard starts up?
> > Yes.
>
> That won't work for confidential guests, e.g. we need a virtio driver
> for the console and some other things.
>
>
> >
> >>>> This doesn't help us handle builtin drivers that initialize before user
> >>>> space is up.
> >>> Then have the default setting for your bus be "unauthorized" like we
> >>> allow for some busses today.
> >> We need some early boot drivers, just not all of them. For example in your
> >> scheme we would need to reject all early platform drivers, which would break
> >> booting. Or allow all early platform drivers, but then we exactly get into
> >> the problem that there are far too many of them to properly harden.
> > Define "harden" please.  Why not just allow all platform drivers, they
> > should all be trusted.  If not, well, you have bigger problems...
>
> Trusted here means someone audited them and also fuzzed them. That's all
> a lot of work and also needs to be maintained forever so we're trying to
> do only a minimum set. There are actually quite a few platform drivers,
> it's difficult to audit them all.
>

What's wrong with the generic authorized proposal? The core can
default to deauthorizing devices on the platform bus, or any bus,
unless on an allow list. It's a bit more work to uplevel the local
"authorized" implementations from USB and Thunderbolt to the core, but
it's functionally identical to the "filter" approach in terms of
protection, i.e. avoiding probe of unnecessary unvetted drivers.

[..]
> That's why I think the builtin allow list hook is still needed. Thoughts?

I see nothing that prevents a built-in allow list to augment the
driver-core default. Is there a gap I'm missing?