Re: [PATCH v6 1/2] PCI/ACPI: Support Microsoft's "DmaProperty"

[Robin Murphy <robin.murphy@arm.com>]

On 2022-04-26 01:06, Rajat Jain via iommu wrote:
> The "DmaProperty" is supported and currently documented and used by
> Microsoft [link 1 below], to flag internal PCIe root ports that need
> DMA protection [link 2 below]. We have discussed with them and reached
> a common understanding that they shall change their MSDN documentation
> to say that the same property can be used to protect any PCI device,
> and not just internal PCIe root ports (since there is no point
> introducing yet another property for arbitrary PCI devices). This helps
> with security from internal devices that offer an attack surface for
> DMA attacks (e.g. internal network devices).
> 
> Support DmaProperty to mark DMA from a PCI device as untrusted.
> 
> Link: [1] https://docs.microsoft.com/en-us/windows-hardware/drivers/pci/dsd-for-pcie-root-ports#identifying-internal-pcie-ports-accessible-to-users-and-requiring-dma-protection
> Link: [2] https://docs.microsoft.com/en-us/windows/security/information-protection/kernel-dma-protection-for-thunderbolt
> Signed-off-by: Rajat Jain <rajatja@google.com>
> Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
> Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> ---
> v6: * Take care of Bjorn's comments:
>         - Update the commit log
>         - Rename to pci_dev_has_dma_property()
>         - Use acpi_dev_get_property()
> v5: * Reorder the patches in the series
> v4: * Add the GUID.
>      * Update the comment and commitlog.
> v3: * Use Microsoft's documented property "DmaProperty"
>      * Resctrict to ACPI only
> 
>   drivers/acpi/property.c |  3 +++
>   drivers/pci/pci-acpi.c  | 21 +++++++++++++++++++++
>   2 files changed, 24 insertions(+)
> 
> diff --git a/drivers/acpi/property.c b/drivers/acpi/property.c
> index 12bbfe833609..bafe35c301ac 100644
> --- a/drivers/acpi/property.c
> +++ b/drivers/acpi/property.c
> @@ -48,6 +48,9 @@ static const guid_t prp_guids[] = {
>   	/* Storage device needs D3 GUID: 5025030f-842f-4ab4-a561-99a5189762d0 */
>   	GUID_INIT(0x5025030f, 0x842f, 0x4ab4,
>   		  0xa5, 0x61, 0x99, 0xa5, 0x18, 0x97, 0x62, 0xd0),
> +	/* DmaProperty for PCI devices GUID: 70d24161-6dd5-4c9e-8070-705531292865 */
> +	GUID_INIT(0x70d24161, 0x6dd5, 0x4c9e,
> +		  0x80, 0x70, 0x70, 0x55, 0x31, 0x29, 0x28, 0x65),
>   };
>   
>   /* ACPI _DSD data subnodes GUID: dbb8e3e6-5886-4ba6-8795-1319f52a966b */
> diff --git a/drivers/pci/pci-acpi.c b/drivers/pci/pci-acpi.c
> index 3ae435beaf0a..d7c6ba48486f 100644
> --- a/drivers/pci/pci-acpi.c
> +++ b/drivers/pci/pci-acpi.c
> @@ -1369,12 +1369,33 @@ static void pci_acpi_set_external_facing(struct pci_dev *dev)
>   		dev->external_facing = 1;
>   }
>   
> +static int pci_dev_has_dma_property(struct pci_dev *dev)
> +{
> +	struct acpi_device *adev;
> +	const union acpi_object *obj;
> +
> +	adev = ACPI_COMPANION(&dev->dev);
> +	if (!adev)
> +		return 0;
> +
> +	/*
> +	 * Property also used by Microsoft Windows for same purpose,
> +	 * (to implement DMA protection from a device, using the IOMMU).

Nit: there is no context for "same purpose" here, so this comment is 
more confusing than helpful. Might I suggest a rewording like:

	/*
	 * Property used by Microsoft Windows to enforce IOMMU DMA
	 * protection for any device that the system might not fully
	 * trust; we'll honour it the same way.
	 */

?

Personally I think it would have been more logical to handle this in 
pci_set_dma_untrusted(), but I see some of those implementation aspects 
have already been discussed, and Bjorn's preference definitely wins over 
mine here :)

Thanks,
Robin.

> +	 */
> +	if (!acpi_dev_get_property(adev, "DmaProperty", ACPI_TYPE_INTEGER,
> +				   &obj) && obj->integer.value == 1)
> +		return 1;
> +
> +	return 0;
> +}
> +
>   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_dev->untrusted |= pci_dev_has_dma_property(pci_dev); >   	pci_acpi_add_edr_notifier(pci_dev);
>   
>   	pci_acpi_add_pm_notifier(adev, pci_dev);
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