linux-kernel - Re: [PATCH v3 7/9] vfio: Use driver_override to avert binding to compromising drivers

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Date:   Mon, 10 Jul 2017 15:34:12 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
Cc:     Greg KH <greg@...ah.com>, kvm@...r.kernel.org,
        eric.auger@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 7/9] vfio: Use driver_override to avert binding to
 compromising drivers

On Mon, 26 Jun 2017 10:08:55 +0100
Russell King - ARM Linux <linux@...linux.org.uk> wrote:

> On Tue, Jun 20, 2017 at 09:48:31AM -0600, Alex Williamson wrote:
> > If a device is bound to a non-vfio, non-whitelisted driver while a
> > group is in use, then the integrity of the group is compromised and
> > will result in hitting a BUG_ON.  This code tries to avoid this case
> > by mangling driver_override to force a no-match for the driver.  The
> > driver-core will either follow-up with a DRIVER_NOT_BOUND (preferred)
> > or BOUND_DRIVER, at which point we can remove the driver_override
> > mangling.  
> 
> Rather than mangling the driver override string to prevent driver binding,
> I wonder if it would make more sense to allow the BUS_NOTIFY_BIND_DRIVER
> notifier to fail the device probe?

Well, it seemed like a good idea, but I don't think we're getting any
traction here, the thread has gone cold:

https://lkml.org/lkml/2017/6/27/1002

Greg, any further comments?

> The driver override strings are, after all, exposed to userspace, and
> it strikes me that this kind of mangling is racy - userspace can read
> or change the override string at any time.

As an alternative, I think we can make this not racy.  BIND_DRIVER is
notified through device_bind_driver() which specifies that the device
lock is held.  This covers not only BIND_DRIVER, but also BOUND_DRIVER
and DRIVER_NOT_BOUND.  So if the user entry points in sysfs were to
require the device lock, we could easily mangle and de-mangle without
interference from a user.  It also seems like a rather good idea in
general to exclude the user from changing driver_override while we're
evaluating a match using it.  Do you still have an objection to
mangling driver_override if we can avoid the user race?  Thanks,

Alex