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[2/5] PCI / ACPI: Identify untrusted PCI devices

Message ID 20190315050744.7952-3-aaron.ma@canonical.com
State New
Headers show
Series iommu: add kernel dma protection | expand

Commit Message

Aaron Ma March 15, 2019, 5:07 a.m. UTC 
From: Mika Westerberg <mika.westerberg@linux.intel.com>

BugLink: https://bugs.launchpad.net/bugs/1820153

A malicious PCI device may use DMA to attack the system. An external
Thunderbolt port is a convenient point to attach such a device. The OS
may use IOMMU to defend against DMA attacks.

Some BIOSes mark these externally facing root ports with this
ACPI _DSD [1]:

  Name (_DSD, Package () {
      ToUUID ("efcc06cc-73ac-4bc3-bff0-76143807c389"),
      Package () {
          Package () {"ExternalFacingPort", 1},
	  Package () {"UID", 0 }
      }
  })

If we find such a root port, mark it and all its children as untrusted.
The rest of the OS may use this information to enable DMA protection
against malicious devices. For instance the device may be put behind an
IOMMU to keep it from accessing memory outside of what the driver has
allocated for it.

While at it, add a comment on top of prp_guids array explaining the
possible caveat resulting when these GUIDs are treated equivalent.

[1] https://docs.microsoft.com/en-us/windows-hardware/drivers/pci/dsd-for-pcie-root-ports#identifying-externally-exposed-pcie-root-ports

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
(backported from commit 617654aae50eb59dd98aa53fb562e850937f4cde)
Signed-off-by: Aaron Ma <aaron.ma@canonical.com>
---
 drivers/acpi/property.c | 11 +++++++++++
 drivers/pci/pci-acpi.c  | 19 +++++++++++++++++++
 drivers/pci/probe.c     | 15 +++++++++++++++
 include/linux/pci.h     |  8 ++++++++
 4 files changed, 53 insertions(+)
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Patch

diff --git a/drivers/acpi/property.c b/drivers/acpi/property.c
index b5e1d8025649..f7a2bd07f76e 100644
--- a/drivers/acpi/property.c
+++ b/drivers/acpi/property.c
@@ -24,10 +24,21 @@  static int acpi_data_get_property_array(const struct acpi_device_data *data,
 					acpi_object_type type,
 					const union acpi_object **obj);
 
+/*
+ * The GUIDs here are made equivalent to each other in order to avoid extra
+ * complexity in the properties handling code, with the caveat that the
+ * kernel will accept certain combinations of GUID and properties that are
+ * not defined without a warning. For instance if any of the properties
+ * from different GUID appear in a property list of another, it will be
+ * accepted by the kernel. Firmware validation tools should catch these.
+ */
 static const guid_t prp_guids[] = {
 	/* ACPI _DSD device properties GUID: daffd814-6eba-4d8c-8a91-bc9bbf4aa301 */
 	GUID_INIT(0xdaffd814, 0x6eba, 0x4d8c,
 		  0x8a, 0x91, 0xbc, 0x9b, 0xbf, 0x4a, 0xa3, 0x01),
+	/* External facing port GUID: efcc06cc-73ac-4bc3-bff0-76143807c389 */
+	GUID_INIT(0xefcc06cc, 0x73ac, 0x4bc3,
+		  0xbf, 0xf0, 0x76, 0x14, 0x38, 0x07, 0xc3, 0x89),
 };
 
 static const guid_t ads_guid =
diff --git a/drivers/pci/pci-acpi.c b/drivers/pci/pci-acpi.c
index d2b04ab37308..2d0ad7e58b31 100644
--- a/drivers/pci/pci-acpi.c
+++ b/drivers/pci/pci-acpi.c
@@ -764,6 +764,24 @@  static void pci_acpi_optimize_delay(struct pci_dev *pdev,
 	ACPI_FREE(obj);
 }
 
+static void pci_acpi_set_untrusted(struct pci_dev *dev)
+{
+	u8 val;
+
+	if (pci_pcie_type(dev) != PCI_EXP_TYPE_ROOT_PORT)
+		return;
+	if (device_property_read_u8(&dev->dev, "ExternalFacingPort", &val))
+		return;
+
+	/*
+	 * These root ports expose PCIe (including DMA) outside of the
+	 * system so make sure we treat them and everything behind as
+	 * untrusted.
+	 */
+	if (val)
+		dev->untrusted = 1;
+}
+
 static void pci_acpi_setup(struct device *dev)
 {
 	struct pci_dev *pci_dev = to_pci_dev(dev);
@@ -773,6 +791,7 @@  static void pci_acpi_setup(struct device *dev)
 		return;
 
 	pci_acpi_optimize_delay(pci_dev, adev->handle);
+	pci_acpi_set_untrusted(pci_dev);
 
 	pci_acpi_add_pm_notifier(adev, pci_dev);
 	if (!adev->wakeup.flags.valid)
diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
index a1a243ee36bb..83e5b154605b 100644
--- a/drivers/pci/probe.c
+++ b/drivers/pci/probe.c
@@ -1378,6 +1378,19 @@  static void set_pcie_thunderbolt(struct pci_dev *dev)
 	}
 }
 
+static void set_pcie_untrusted(struct pci_dev *dev)
+{
+	struct pci_dev *parent;
+
+	/*
+	 * If the upstream bridge is untrusted we treat this device
+	 * untrusted as well.
+	 */
+	parent = pci_upstream_bridge(dev);
+	if (parent && parent->untrusted)
+		dev->untrusted = true;
+}
+
 /**
  * pci_ext_cfg_is_aliased - Is ext config space just an alias of std config?
  * @dev: PCI device
@@ -1604,6 +1617,8 @@  int pci_setup_device(struct pci_dev *dev)
 	/* Need to have dev->cfg_size ready */
 	set_pcie_thunderbolt(dev);
 
+	set_pcie_untrusted(dev);
+
 	/* "Unknown power state" */
 	dev->current_state = PCI_UNKNOWN;
 
diff --git a/include/linux/pci.h b/include/linux/pci.h
index c133ccfa002e..61ec45343a28 100644
--- a/include/linux/pci.h
+++ b/include/linux/pci.h
@@ -388,6 +388,14 @@  struct pci_dev {
 	unsigned int	reset_fn:1;
 	unsigned int	is_hotplug_bridge:1;
 	unsigned int	is_thunderbolt:1;	/* Thunderbolt controller */
+	/*
+	 * Devices marked being untrusted are the ones that can potentially
+	 * execute DMA attacks and similar. They are typically connected
+	 * through external ports such as Thunderbolt but not limited to
+	 * that. When an IOMMU is enabled they should be getting full
+	 * mappings to make sure they cannot access arbitrary memory.
+	 */
+	unsigned int	untrusted:1;
 	unsigned int	__aer_firmware_first_valid:1;
 	unsigned int	__aer_firmware_first:1;
 	unsigned int	broken_intx_masking:1;	/* INTx masking can't be used */