Date:   Thu, 12 Oct 2017 17:46:45 -0700
From:   Darren Hart <dvhart@...radead.org>
To:     Alan Cox <gnomes@...rguk.ukuu.org.uk>
Cc:     Mario Limonciello <mario.limonciello@...l.com>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        platform-driver-x86@...r.kernel.org,
        Andy Lutomirski <luto@...nel.org>, quasisec@...gle.com,
        pali.rohar@...il.com, rjw@...ysocki.net, mjg59@...gle.com,
        hch@....de, Greg KH <greg@...ah.com>
Subject: Re: [PATCH v7 10/15] platform/x86: dell-smbios: add filtering
 capability for requests

On Thu, Oct 12, 2017 at 11:09:03AM +0100, One Thousand Gnomes wrote:
> On Wed, 11 Oct 2017 11:27:36 -0500
> Mario Limonciello <mario.limonciello@...l.com> wrote:
> 
> > There are some categories of tokens and SMBIOS calls that it makes
> > sense to protect userspace from accessing.  These are calls that
> > may write to one time use fields or activate hardware debugging
> > capabilities.  They are not intended for general purpose use.
> > 
> > This same functionality may be be later extended to also intercept
> > calls that may cause kernel functionality to get out of sync if
> > the same functions are used by other drivers.
> 
> This doesn't work. You are creating an API. If you then have to remove
> parts of the API because it messes stuff up you break your API guarantee.
> 
> As I said before, this needs to be a whitelist of stuff that is safe, and
> it needs to have a security model. When you make a feature available you
> can't nicely take it away again as you'll break people's user space.
> 
> Start with a whitelist of the ones you know people want to use, or that
> your existing tooling you want to enable uses. Add others as needed in
> future releases of the kernel.

Hi Alan,

I've attempted to get a focused discussion on this topic a few times,
but have failed to get it the focus in really needs. Thanks for bringing
it up. I look forward to your thoughts on the following:

The core of the issue is we are trying to achieve feature parity with
Windows using the Windows Management Instrumentation (WMI) mechanism.
I'm trying to support vendors like Dell in their attempts to do so, and
acknowledging that this involves supporting a platform which was
designed according to the norms and standards of the windows ecosystem.

This WMI mechanism was designed to enable vendors to create management
tools which could talk to interfaces their firmware exposed for this
purpose without any platform specific OS driver or vendor specific
knowledge on the part of the OS driver [1].

The result of this design decision is as you would expect: there is no
consistency in design, no guarantee of interfaces with functional
boundaries, across the various WMI interface implementations across
vendors (or even within the interface of a given vendor on a given
platform).

The result is a mechanism which is fundamentally incompatible with
standard Linux kernel approach to isolating userspace from the firmware.
WMI enumerates available data, methods, and events, and shepherds a
buffer of bits back and forth between firmware and userspace through
these methods.

Over the last few months, I've worked with Mario, Pali, Andy L, and
others to try and find an acceptable way to support Dell in their
efforts, while respecting Linux's design principles.

The first concession was to agree not to automatically publish a WMI
interface to userspace. This series includes the mechanism which
requires a GUID-specific driver to be written and to provide the file
ops structure, explicitly requesting the character device. This creates
a WMI GUID granular whitelist. I'm willing to deny any such driver which
is sent for review without the explicit collaboration of the vendor, to
ensure they are doing their due diligence with respect to their firmware
implementation. There has been some discussion in this thread regarding
Dell's firmware testing, and the access these interfaces have to
critical hardware resources.

The second concession was to acknowledge that some features implemented
in WMI are rightly the domain of the Linux kernel. LEDs, RFKill,
Backlight control, hotkey support, etc. When an exported GUID is used
for these purposes in addition to whatever purpose userspace requires
(see my comment above about a lack of functional boundaries in the
exported interfaces), we will provide a way to filter these usages. As
Mario said, we can either return an error, or we can attempt to handle
the request via the appropriate Linux kernel subsystem connections.

>From the Linux kernel side, the ask is that we acknowledge that it is
not practical to audit every WMI interface to present a set of
functional knobs to userspace - because they were specifically designed
without that requirement.

The platform was designed in this way specifically to optimize the
software support effort on the part of the vendor. They don't write
Windows OS drivers for these mechanisms, it is unrealistic to expect
them to write them for Linux.

I believe we need to arrive at a compromise which allows a vendor to
work upstream like Mario is doing on behalf of Dell to fully enable
their platforms as they were designed on Linux, while ensuring we don't
adversely affect the functionality or security of other platforms. I
believe the whitelist and blacklist above provides for this compromise.

Perhaps an additional concession we should consider is to make exporting
WMI interfaces a configuration option. If not set, the drivers will
continue to work, but the WMI bus driver will not create the character
device, even if requested. This would allow vendors to create a fully
supported platform using upstream code, while allowing individuals the
control to disable WMI userspace functionality if they don't trust the
mechanism.

The alternative seems to be that we accept these features will not be
available on Linux for these platforms, or that vendors will develop a
single wmi mapping driver out of tree, likely without the noted
concessions.

1. https://msdn.microsoft.com/en-us/library/windows/hardware/dn614028(v=vs.85).aspx
   Specifically: "ACPI-to-WMI Mapper Goals for Windows Instrumentation"

-- 
Darren Hart
VMware Open Source Technology Center

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