linux-kernel - Re: [PATCH v3] PCI: ACPI: Support Microsoft's "DmaProperty"

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Date:   Thu, 17 Feb 2022 07:16:22 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Rajat Jain <rajatja@...gle.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <lenb@...nel.org>, linux-pci@...r.kernel.org,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Rajat Jain <rajatxjain@...il.com>,
        Dmitry Torokhov <dtor@...gle.com>,
        Jesse Barnes <jsbarnes@...gle.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Pavel Machek <pavel@...x.de>,
        Oliver O'Halloran <oohall@...il.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH v3] PCI: ACPI: Support Microsoft's "DmaProperty"

On Wed, Feb 16, 2022 at 02:05:41PM -0800, Rajat Jain wrote:
> The "DmaProperty" is supported and documented by Microsoft here:
> https://docs.microsoft.com/en-us/windows-hardware/drivers/pci/dsd-for-pcie-root-ports
> They use this property for DMA protection:
> https://docs.microsoft.com/en-us/windows/security/information-protection/kernel-dma-protection-for-thunderbolt
> 
> Support the "DmaProperty" with the same semantics. Windows documents the
> property to apply to PCIe root ports only. Extend it to apply to any
> PCI device. This is useful for internal PCI devices that do not hang off
> a PCIe rootport, but offer an attack surface for DMA attacks (e.g.
> internal network devices).
> 
> Signed-off-by: Rajat Jain <rajatja@...gle.com>
> ---
> v3: * Use Microsoft's documented property "DmaProperty"
>     * Resctrict to ACPI only
> 
>  drivers/pci/pci-acpi.c | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/drivers/pci/pci-acpi.c b/drivers/pci/pci-acpi.c
> index a42dbf448860..660baa60c040 100644
> --- a/drivers/pci/pci-acpi.c
> +++ b/drivers/pci/pci-acpi.c
> @@ -1350,12 +1350,30 @@ static void pci_acpi_set_external_facing(struct pci_dev *dev)
>  		dev->external_facing = 1;
>  }
>  
> +static void pci_acpi_check_for_dma_protection(struct pci_dev *dev)
> +{
> +	u8 val;
> +
> +	/*
> +	 * Microsoft Windows uses this property, and is documented here:
> +	 * https://docs.microsoft.com/en-us/windows-hardware/drivers/pci/dsd-for-pcie-root-ports
> +	 * While Microsoft documents this property as only applicable to PCIe
> +	 * root ports, we expand it to be applicable to any PCI device.
> +	 */
> +	if (device_property_read_u8(&dev->dev, "DmaProperty", &val))
> +		return;

Why not continue to only do this for PCIe devices like it is actually
being used for?  Why expand it?

And what driver/device is going to use this?

thanks,

greg k-h