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

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Date:   Wed, 16 Feb 2022 14:05:41 -0800
From:   Rajat Jain <rajatja@...gle.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <lenb@...nel.org>, linux-pci@...r.kernel.org,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        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>
Cc:     Rajat Jain <rajatja@...gle.com>
Subject: [PATCH v3] PCI: ACPI: Support Microsoft's "DmaProperty"

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;
+
+	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_check_for_dma_protection(pci_dev);
 	pci_acpi_add_edr_notifier(pci_dev);
 
 	pci_acpi_add_pm_notifier(adev, pci_dev);
-- 
2.35.1.265.g69c8d7142f-goog