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

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Date:   Thu, 13 May 2021 15:05:50 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Rajat Jain <rajatja@...gle.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Alan Stern <stern@...land.harvard.edu>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-pci <linux-pci@...r.kernel.org>,
        "open list:ULTRA-WIDEBAND (UWB) SUBSYSTEM:" 
        <linux-usb@...r.kernel.org>, Oliver Neukum <oneukum@...e.com>,
        David Laight <David.Laight@...lab.com>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Rajat Jain <rajatxjain@...il.com>,
        Jesse Barnes <jsbarnes@...gle.com>,
        Dmitry Torokhov <dtor@...gle.com>
Subject: Re: [PATCH v3 2/2] PCI: Add sysfs "removable" attribute

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@...gle.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/

> > 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.

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?

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?

> > Signed-off-by: Rajat Jain <rajatja@...gle.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."

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?)

Bjorn