Re: [PATCH v2 5/7] driver core: Add device location to "struct device" and expose it in sysfs

["Oliver O'Halloran" <oohall@gmail.com>]

On Thu, Jul 2, 2020 at 4:07 AM Rajat Jain <rajatja@google.com> wrote:
>
> *snip*
>
> > > I guess it would make sense to have an attribute for user space to
> > > write to in order to make the kernel reject device plug-in events
> > > coming from a given port or connector, but the kernel has no reliable
> > > means to determine *which* ports or connectors are "safe", and even if
> > > there was a way for it to do that, it still may not agree with user
> > > space on which ports or connectors should be regarded as "safe".
> >
> > Again, we have been doing this for USB devices for a very long time, PCI
> > shouldn't be any different.  Why people keep ignoring working solutions
> > is beyond me, there's nothing "special" about PCI devices here for this
> > type of "worry" or reasoning to try to create new solutions.
> >
> > So, again, I ask, go do what USB does, and to do that, take the logic
> > out of the USB core, make it bus-agnositic, and _THEN_ add it to the PCI
> > code. Why the original submitter keeps ignoring my request to do this
> > is beyond me, I guess they like making patches that will get rejected :(
>
> IMHO I'm actually trying to precisely do what I think was the
> conclusion of our discussion, and then some changes because of the
> further feedback I received on those patches. Let's take a step back
> and please allow me to explain how I got here (my apologies but this
> spans a couple of threads, and I"m trying to tie them all together
> here):

The previous thread had some suggestions, but no real conclusions.
That's probably why we're still arguing about it...

> GOAL: To allow user space to control what (PCI) drivers he wants to
> allow on external (thunderbolt) ports. There was a lot of debate about
> the need for such a policy at
> https://lore.kernel.org/linux-pci/CACK8Z6GR7-wseug=TtVyRarVZX_ao2geoLDNBwjtB+5Y7VWNEQ@mail.gmail.com/
> with the final conclusion that it should be OK to implement such a
> policy in userspace, as long as the policy is not implemented in the
> kernel. The kernel only needs to expose bits & info that is needed by
> the userspace to implement such a policy, and it can be used in
> conjunction with "drivers_autoprobe" to implement this policy:
> --------------------------------------------------------------------
> ....
> That's an odd thing, but sure, if you want to write up such a policy for
> your systems, great.  But that policy does not belong in the kernel, it
> belongs in userspace.
> ....
> --------------------------------------------------------------------
> 1) The post https://lore.kernel.org/linux-pci/20200609210400.GA1461839@bjorn-Precision-5520/
> lists out the approach that was agreed on. Replicating it here:
> -----------------------------------------------------------------------
>   - Expose the PCI pdev->untrusted bit in sysfs.  We don't expose this
>     today, but doing so would be trivial.  I think I would prefer a
>     sysfs name like "external" so it's more descriptive and less of a
>     judgment.
>
>     This comes from either the DT "external-facing" property or the
>     ACPI "ExternalFacingPort" property.
>
>   - All devices present at boot are enumerated.  Any statically built
>     drivers will bind to them before any userspace code runs.
>
>     If you want to keep statically built drivers from binding, you'd
>     need to invent some mechanism so pci_driver_init() could clear
>     drivers_autoprobe after registering pci_bus_type.
>
>   - Early userspace code prevents modular drivers from automatically
>     binding to PCI devices:
>
>       echo 0 > /sys/bus/pci/drivers_autoprobe
>
>     This prevents modular drivers from binding to all devices, whether
>     present at boot or hot-added.
>
>   - Userspace code uses the sysfs "bind" file to control which drivers
>     are loaded and can bind to each device, e.g.,
>
>       echo 0000:02:00.0 > /sys/bus/pci/drivers/nvme/bind

I think this is a reasonable suggestion. However, as Greg pointed out
it's gratuitously different to what USB does for no real reason.

> -----------------------------------------------------------------------
> 2) As part of implementing the above agreed approach, when I exposed
> PCI "untrusted" attribute to userspace, it ran into discussion that
> concluded that instead of this, the device core should be enhanced
> with a location attribute.
> https://lore.kernel.org/linux-pci/20200618184621.GA446639@kroah.com/
> -----------------------------------------------------------------------
> ...
> The attribute should be called something like "location" or something
> like that (naming is hard), as you don't always know if something is
> external or not (it could be internal, it could be unknown, it could be
> internal to an external device that you trust (think PCI drawers for
> "super" computers that are hot pluggable but yet really part of the
> internal bus).
> ....
> "trust" has no direct relation to the location, except in a policy of
> what you wish to do with that device, so as long as you keep them
> separate that way, I am fine with it.
> ...
> -----------------------------------------------------------------------
>
> And hence this patch. I don't see an attribute in USB comparable to
> this new attribute, except for the boolean "removable" may be. Are you
> suggesting to pull that into the device core instead of adding this
> "physical_location" attribute?

He's suggesting you pull the "authorized" attribute into the driver
core. That's the mechanism USB uses to block drivers binding unless
userspace authorizes them. I don't see any reason why we can't re-use
that sysfs interface for PCI devices since the problem being solved is
fundamentally the same. The main question is what we should do as a
default policy in the kernel. For USB the default comes from the
"authorized_default" module param of usbcore:

> /* authorized_default behaviour:
>  * -1 is authorized for all devices except wireless (old behaviour)
>  * 0 is unauthorized for all devices
>  * 1 is authorized for all devices
>  * 2 is authorized for internal devices
>  */
> #define USB_AUTHORIZE_WIRED   -1
> #define USB_AUTHORIZE_NONE    0
> #define USB_AUTHORIZE_ALL     1
> #define USB_AUTHORIZE_INTERNAL        2
>
> static int authorized_default = USB_AUTHORIZE_WIRED;
> module_param(authorized_default, int, S_IRUGO|S_IWUSR);

So the default policy for USB is to authorize any wired USB device and
we can optionally restrict that to just integrated devices. Sounding
familiar?

The internal / external status is still useful to know so we might
want to make a sysfs attribute for that too. However, I'd like to
point out that internal / external isn't the whole story. As I
mentioned in the last thread if I have a BMC device I *really* don't
want it to be authorized by default even though it's an internal
device. Similarly, if I know all my internal cards support PCIe
Component Authentication then I might choose not to trust any PCI
devices unless they authenticate successfully.

> 3) The one deviation from the agreed approach in (1) is
> https://patchwork.kernel.org/patch/11633095/ . The reason is I
> realized that contrary to what I earlier believed, we might not be
> able to disable the PCI link to all external PCI devices at boot. So
> external PCI devices may actually bind to drivers before userspace
> comes up and does "echo 0 > /sys/bus/pci/drivers_autoprobe").

Yep, that's a problem. If we want to provide a useful mechanism to
userspace then the default behaviour of the kernel can't undermine
that mechanism. If that means we need another kernel command line
parameter then I guess we just have to live with it.

Oliver