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

[Greg Kroah-Hartman]

On Thu, May 13, 2021 at 09:39:58AM -0700, Rajat Jain wrote:
> Hello,
> 
> On Thu, May 13, 2021 at 6:58 AM Greg Kroah-Hartman
> <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> >
> > On Wed, May 12, 2021 at 02:34:57PM -0700, Rajat Jain 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.
> > >
> > > 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)
> > >
> > > Signed-off-by: Rajat Jain <rajatja@xxxxxxxxxx>
> > > ---
> > > v3: - commit log updated
> > >     - Rename set_pci_dev_removable() -> pci_set_removable()
> > >     - Call it after applying early PCI quirks.
> > > v2: Add documentation
> > >
> > >  Documentation/ABI/testing/sysfs-devices-removable |  3 ++-
> > >  drivers/pci/pci-sysfs.c                           |  1 +
> > >  drivers/pci/probe.c                               | 12 ++++++++++++
> > >  3 files changed, 15 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/Documentation/ABI/testing/sysfs-devices-removable b/Documentation/ABI/testing/sysfs-devices-removable
> > > index 9dabcad7cdcd..ec0b243f5db4 100644
> > > --- a/Documentation/ABI/testing/sysfs-devices-removable
> > > +++ b/Documentation/ABI/testing/sysfs-devices-removable
> > > @@ -14,4 +14,5 @@ Description:
> > >
> > >               Currently this is only supported by USB (which infers the
> > >               information from a combination of hub descriptor bits and
> > > -             platform-specific data such as ACPI).
> > > +             platform-specific data such as ACPI) and PCI (which gets this
> > > +             from ACPI / device tree).
> > > diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c
> > > index beb8d1f4fafe..38b3259ba333 100644
> > > --- a/drivers/pci/pci-sysfs.c
> > > +++ b/drivers/pci/pci-sysfs.c
> > > @@ -1541,4 +1541,5 @@ static const struct attribute_group *pci_dev_attr_groups[] = {
> > >
> > >  const struct device_type pci_dev_type = {
> > >       .groups = pci_dev_attr_groups,
> > > +     .supports_removable = true,
> > >  };
> > > diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> > > index 3a62d09b8869..3515afeeaba8 100644
> > > --- a/drivers/pci/probe.c
> > > +++ b/drivers/pci/probe.c
> > > @@ -1575,6 +1575,16 @@ static void set_pcie_untrusted(struct pci_dev *dev)
> > >               dev->untrusted = true;
> > >  }
> > >
> > > +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);
> > > +}
> >
> > Always run checkpatch.pl so you don't get grumpy maintainers telling you
> > to run checkpatch.pl :(
> 
> Yes, I did (it gave me 0 errors and 0 warnings). Please let me know if
> I need to fix something and I'll be happy to fix that.
> 
> >
> > And why does external_facing come into play here?  I know you say it
> > above, but you should also put it here into the code for when we need to
> > look at it in a few months and wonder what in the world this is doing.
> 
> Ack, will do.
> 
> >
> > Also, are you SURE this is correct and will handle the hotpluggable PCI
> > devices in things like drawers and the like?
> 
> Yes, me and Bjorn discussed this in the v2 of this patch
> (https://patchwork.kernel.org/project/linux-usb/patch/20210424021631.1972022-2-rajatja@xxxxxxxxxx/),
> and yes, this can take care of the hot-pluggable trays if the firmware
> marks the slots external-facing.

Ok, I'll trust you two :)

> > What is the goal here in exposing this information to userspace, who is
> > going to use it and what is it going to be used for?
> 
> The goal here is to implement policies regarding usage of external PCI
> devices, in userspace. ChromeOS is using it for things like:
> - Remove external PCI devices when a user logs out.

remove them how?  disconnect the device from the system through what
method?

> - Don't allow new external PCI devices while the screen is locked.

Don't allow how?  Don't allow the binding of a driver to a device, or
the device to be discovered at all?  What controls this?

> - collect metrics about usage of external PCI devices (how many users
> actually use it etc).
> - disable certain features (that are deemed to be dangerous) for
> external PCI network cards.

What is a "dangerous" network feature, RDMA?

thanks,

greg k-h