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

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Date:   Thu, 13 May 2021 11:02:10 -0700
From:   Rajat Jain <rajatja@...gle.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Alan Stern <stern@...land.harvard.edu>,
        Rajat Jain <rajatja@...gle.com>,
        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>, Bjorn Helgaas <helgaas@...nel.org>,
        Oliver Neukum <oneukum@...e.com>,
        David Laight <David.Laight@...lab.com>,
        Krzysztof Wilczyński <kw@...ux.com>
Cc:     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

Hi

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.
>
> 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@...gle.com>
> ---
> 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);
> +}

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"?

Thanks,

Rajat

> +
>  /**
>   * pci_ext_cfg_is_aliased - Is ext config space just an alias of std config?
>   * @dev: PCI device
> @@ -1822,6 +1832,8 @@ int pci_setup_device(struct pci_dev *dev)
>         /* Early fixups, before probing the BARs */
>         pci_fixup_device(pci_fixup_early, dev);
>
> +       pci_set_removable(dev);
> +
>         pci_info(dev, "[%04x:%04x] type %02x class %#08x\n",
>                  dev->vendor, dev->device, dev->hdr_type, dev->class);
>
> --
> 2.31.1.607.g51e8a6a459-goog
>