Re: [PATCH v3 2/2] PCI: Add sysfs "removable" attribute

[Rajat Jain <rajatja@google.com>]

On Thu, May 13, 2021 at 1:05 PM Bjorn Helgaas <helgaas@kernel.org> wrote:
>
> On Thu, May 13, 2021 at 11:02:10AM -0700, Rajat Jain wrote:
> > On Wed, May 12, 2021 at 2:35 PM Rajat Jain <rajatja@google.com> wrote:
> > >
> > > A PCI device is "external_facing" if it's a Root Port with the ACPI
> > > "ExternalFacingPort" property or if it has the DT "external-facing"
> > > property.  We consider everything downstream from such a device to
> > > be removable by user.
> > >
> > > We're mainly concerned with consumer platforms with user accessible
> > > thunderbolt ports that are vulnerable to DMA attacks, and we expect those
> > > ports to be identified as "ExternalFacingPort". Devices in traditional
> > > hotplug slots can technically be removed, but the expectation is that
> > > unless the port is marked with "ExternalFacingPort", such devices are less
> > > accessible to user / may not be removed by end user, and thus not exposed
> > > as "removable" to userspace.
>
> s/thunderbolt/Thunderbolt/ since I think it's a trademark
> s/identified as/identified by firmware as/

Ack, will do.

>
> > > Set pci_dev_type.supports_removable so the device core exposes the
> > > "removable" file in sysfs, and tell the device core about removable
> > > devices.
> > >
> > > This can be used by userspace to implment any policies it wants to,
> > > tailored specifically for user removable devices. Eg usage:
> > > https://chromium-review.googlesource.com/c/chromiumos/platform2/+/2591812
> > > https://chromium-review.googlesource.com/c/chromiumos/platform2/+/2795038
> > > (code uses such an attribute to remove external PCI devicces or disable
> > > features on them as needed by the policy desired)
>
> s/implment/implement/
> s/devicces/devices/
>
> Or maybe something like:
>
>   This can be used to implement userspace policies tailored for
>   user-removable devices.

Ack, will do.

>
> Not sure exactly what "remove external PCI devices" means.  You're
> talking about the *code* doing something, so I don't think it means
> physically unplugging the device from the system.  Maybe preventing a
> driver from binding to it or something similar?

echo 1 > /sys/bus/pci/devices/<device>/remove

>
> I hesitate slightly to rely on URLs like googlesource.com in commit
> logs because we don't know how long they will remain valid.  But I
> guess there's no real alternative here, since this code probably
> hasn't been posted to any public mailing lists like the ones archived
> at https://lore.kernel.org/lists.html, right?

Yes, chromium reviews (userspace code that shall use the new
attribute) happen over gerrit, and so the publicly available links
would be googlesource.com.

>
> > > Signed-off-by: Rajat Jain <rajatja@google.com>
>
> > > +static void pci_set_removable(struct pci_dev *dev)
> > > +{
> > > +       struct pci_dev *parent = pci_upstream_bridge(dev);
> > > +       if (parent &&
> > > +           (parent->external_facing || dev_is_removable(&parent->dev)))
> > > +               dev_set_removable(&dev->dev, DEVICE_REMOVABLE);
> > > +       else
> > > +               dev_set_removable(&dev->dev, DEVICE_FIXED);
> > > +}
> >
> > Copying comments from Krzysztof from another thread:
> >
> > [Krzysztof] We were also wondering if we should only set DEVICE_REMOVABLE for
> > devices known to be behind an external-facing port, and let everything
> > else be set to "unknown" (or whatever the default would be).
> >
> > [Rajat]: I think I'm fine with this proposal if Bjorn & PCI community
> > also sees this as a better idea. Essentially the question here is,
> > would it be better for the non-removable PCI devices to be shown as
> > "fixed" or "unknown"?
>
> I think I would rather see this as:
>
>   struct pci_dev *parent = pci_upstream_bridge(dev);
>
>   if (parent &&
>       (parent->external_facing || dev_is_removable(&parent->dev)))
>           dev_set_removable(&dev->dev, DEVICE_REMOVABLE);
>
> In other words, assume only that everything below an "external-facing"
> device is removable.
>
> In the absence of an "external-facing" property, we don't know
> anything about the connection, and I'd rather use the default
> (probably "unknown") instead of assuming "fixed."

Ack, will do.

One question: Under Greg's latest suggestion, the decision to show
this attribute does not have to be bus wide / device_type wide i.e.
subsystem can choose for this attribute to show up only under certain
devices. So if it is more preferable, I can have this attribute show
under ONLY PCI devices that attach below "external-facing" PCI devices
(and any other PCI devices will not have this attribute show up at
all). I guess this sounds better than having "unknown" show up on the
rest of the devices that are not removable?

>
> I don't think we have anything that depends on "fixed," so I don't
> think there's value in setting it.
>
> (Note the blank line between local variables and the "if"; maybe
> that's what Greg hinted at?)

Ack, will remove the blank line (didn't know that blank lines between
variables and code is not preferred).

Thanks,
Rajat

>
> Bjorn