Re: [PATCH] PCI: ACPI: Allow internal devices to be marked as untrusted

[Bjorn Helgaas]

[+cc Greg, Jean-Philippe, Mika, Pavel, Oliver, Joerg since they
commented on previous "external-facing" discussion]

On Wed, Jan 19, 2022 at 04:04:09PM -0800, Rajat Jain wrote:
> Today the pci_dev->untrusted is set for any devices sitting downstream
> an external facing port (determined via "ExternalFacingPort" property).
> This however, disallows any internal devices to be marked as untrusted.

This isn't stated quite accurately.  "dev->untrusted" is currently set
only by set_pcie_untrusted(), when "dev" has an upstream bridge that
is either external-facing or untrusted.

But that doesn't disallow or prevent internal devices from being
marked as untrusted; it just doesn't implement that.

> There are use-cases though, where a platform would like to treat an
> internal device as untrusted (perhaps because it runs untrusted
> firmware, or offers an attack surface by handling untrusted network
> data etc).
> 
> This patch introduces a new "UntrustedDevice" property that can be used
> by the firmware to mark any device as untrusted.

I think I'm OK with this.  Write this last sentence in imperative
mood; see
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/process/maintainer-tip.rst?id=v5.16#n134
for examples.

> Signed-off-by: Rajat Jain <rajatja@xxxxxxxxxx>
> ---
>  drivers/pci/pci-acpi.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/drivers/pci/pci-acpi.c b/drivers/pci/pci-acpi.c
> index a42dbf448860..3d9e5fa49451 100644
> --- a/drivers/pci/pci-acpi.c
> +++ b/drivers/pci/pci-acpi.c
> @@ -1350,12 +1350,25 @@ static void pci_acpi_set_external_facing(struct pci_dev *dev)
>  		dev->external_facing = 1;
>  }
>  
> +static void pci_acpi_set_untrusted(struct pci_dev *dev)
> +{
> +	u8 val;
> +
> +	if (device_property_read_u8(&dev->dev, "UntrustedDevice", &val))
> +		return;
> +
> +	/* These PCI devices are not trustworthy */

Comment is probably superfluous since the code seems obvious without
it.

> +	if (val)
> +		dev->untrusted = 1;
> +}
> +
>  void pci_acpi_setup(struct device *dev, struct acpi_device *adev)
>  {
>  	struct pci_dev *pci_dev = to_pci_dev(dev);
>  
>  	pci_acpi_optimize_delay(pci_dev, adev->handle);
>  	pci_acpi_set_external_facing(pci_dev);
> +	pci_acpi_set_untrusted(pci_dev);
>  	pci_acpi_add_edr_notifier(pci_dev);
>  
>  	pci_acpi_add_pm_notifier(adev, pci_dev);
> -- 
> 2.34.1.703.g22d0c6ccf7-goog
>