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SubjectRe: [PATCH v3 1/4] PCI / ACPI: Identify untrusted PCI devices
On Thu, Nov 29, 2018 at 06:51:50PM +0300, Mika Westerberg wrote:
> 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.
>
> Recent BIOSes with Thunderbolt ports mark these externally facing root
> ports with this ACPI _DSD [1]:

I'm not 100% comfortable with the "Recent BIOSes" wording because that
suggests that we can rely on the fact that *all* BIOSes newer than
some date X mark these ports.

Since this _DSD usage is Microsoft-specific and not required by either
PCIe or ACPI specs, we can't rely on it. A BIOS that doesn't
implement it may not be Windows-certified, but it's perfectly
spec-compliant otherwise and we have to keep in mind the possibility
that ports without this _DSD may still be externally visible and may
still be attack vectors.

> 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: Bjorn Helgaas <bhelgaas@google.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(+)
>
> diff --git a/drivers/acpi/property.c b/drivers/acpi/property.c
> index 8c7c4583b52d..77abe0ec4043 100644
> --- a/drivers/acpi/property.c
> +++ b/drivers/acpi/property.c
> @@ -24,6 +24,14 @@ 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,
> @@ -31,6 +39,9 @@ static const guid_t prp_guids[] = {
> /* Hotplug in D3 GUID: 6211e2c0-58a3-4af3-90e1-927a4e0c55a4 */
> GUID_INIT(0x6211e2c0, 0x58a3, 0x4af3,
> 0x90, 0xe1, 0x92, 0x7a, 0x4e, 0x0c, 0x55, 0xa4),
> + /* 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 921db6f80340..e1949f7efd9c 100644
> --- a/drivers/pci/pci-acpi.c
> +++ b/drivers/pci/pci-acpi.c
> @@ -789,6 +789,24 @@ 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);
> @@ -798,6 +816,7 @@ 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 b1c05b5054a0..257b9f6f2ebb 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
> @@ -1638,6 +1651,8 @@ 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 11c71c4ecf75..c786a2f27bee 100644
> --- a/include/linux/pci.h
> +++ b/include/linux/pci.h
> @@ -396,6 +396,14 @@ struct pci_dev {
> unsigned int is_hotplug_bridge:1;
> unsigned int shpc_managed:1; /* SHPC owned by shpchp */
> 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 */
> --
> 2.19.2
>

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