linux-kernel - Re: [PATCH] Allow userspace to request device probing even if defer_all

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Message-ID: <20170104200319.GA26503@kroah.com>
Date:   Wed, 4 Jan 2017 21:03:19 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Matthew Garrett <mjg59@...eos.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH] Allow userspace to request device probing even if
 defer_all_probes is true

On Wed, Jan 04, 2017 at 01:53:45PM -0600, Matthew Garrett wrote:
> On Wed, Jan 4, 2017 at 1:42 PM, Greg Kroah-Hartman
> <gregkh@...uxfoundation.org> wrote:
> > On Wed, Jan 04, 2017 at 12:11:49PM -0600, Matthew Garrett wrote:
> >> Userspace doesn't know the order that the kernel will use when
> >> attempting to bind drivers, so punting binding out to userspace may
> >> result in different behaviour.
> >
> > How can the order in which drivers are bound result in different
> > behavior?
> 
> If you have two loaded drivers that could bind to the device then the
> order you attempt to bind them in will matter.

If you have that, you are screwed no matter what.  The driver model
never guarantees any order in which a driver and device is matched up,
sorry, and if that's the goal of this patch somehow, then I'll strongly
object to it.

What in-kernel drivers do we have that bind to the same device?  We
shouldn't have that, because of this very issue.

> >> The kernel already has the code to do this, so we should just reuse
> >> it.
> >
> > That's fine, but I don't understand the problem you are trying to solve,
> > please explain better.  What am I missing here?
> 
> If you plug in a device while defer_all_probes is true, it won't be
> bound - that's the point. But if you have a USB keyboard and unplug it
> and plug it, you'd then end up with no keyboard. So you want userspace
> to be able to make an appropriate policy decision around which devices
> should be bound, and you need a mechanism to allow userspace to
> trigger that binding.

Use the in-place mechanism for that, userspace gets notification that
the device was plugged in, it can authorize it or not.  That's what
systems have been doing for a while now, and is what that api was
created for.

I'm getting the impression that somehow these two different patches are
a series and related to each other which is even more confusing...

thanks,

greg k-h