Re: [PATCH v2 1/4] PCI / ACPI: Identify untrusted PCI devices

[Mika Westerberg]

On Mon, Nov 26, 2018 at 06:17:11PM -0600, Bjorn Helgaas wrote:
> Hi Mika,

Hi,

> On Mon, Nov 26, 2018 at 02:15:23PM +0300, Mika Westerberg wrote:
> > Recent systems with Thunderbolt ports may support IOMMU natively.
> 
> This sentence doesn't make sense to me.  There's no logical connection
> between having an IOMMU and having a Thunderbolt port.
> 
> > This means that the platform utilizes IOMMU to prevent DMA attacks
> > over externally exposed PCIe root ports (typically Thunderbolt
> > ports)
> 
> Nor this one.  The platform only uses the IOMMU to prevent DMA attacks
> if the OS chooses to do that.

I guess I'm trying to say here that the recent changes add such support
to the platform BIOS that allows the OS to enable IOMMU without being
compromised by a malicious device that is already connected. The BIOS
sets the new ACPI DMAR bit in that case.

> > The system BIOS marks these PCIe root ports as being externally facing
> > ports by implementing following ACPI _DSD [1] under the root port in
> > question:
> 
> There's no standard that requires this, so the best we can say is that
> a system BIOS *may* mark externally facing ports with this mechanism.

There is no standard but I'm quite sure this is something that will be
required to be implemented properly by the OEM by Microsoft hardware
compatibility suite.

> I think it would make sense to say something like:
> 
>   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 an IOMMU to defend against DMA attacks.
> 
>   Some BIOSes mark externally facing Root Ports with the this ACPI
>   _DSD:
> 
>     ...
> 
>   If we find such a Root Port, mark it and all its children as
>   "is_untrusted".  The rest of the OS may use this to prevent use of
>   the device unless we have an IOMMU to prevent malicious DMA [I don't
>   know your actual policy yet].

OK, I will update the commit message accordingly.

> >   Name (_DSD, Package () {
> >       ToUUID ("efcc06cc-73ac-4bc3-bff0-76143807c389"),
> >       Package () {
> >           Package () {"ExternalFacingPort", 1},
> > 	  Package () {"UID", 0 }
> >       }
> >   })
> > 
> > To make it possible for IOMMU code to identify these devices, we look up
> > for this property and mark all children devices (including the root port
> > itself) with a new flag (->is_untrusted). This flag is meant to be used
> > with all PCIe devices (not just Thunderbolt) that cannot be trusted in
> > the same level than integrated devices and may need to put behind full
> > IOMMU protection.
> > 
> > [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@xxxxxxxxxxxxxxx>
> > ---
> >  drivers/acpi/property.c |  3 +++
> >  drivers/pci/pci-acpi.c  | 18 ++++++++++++++++++
> >  drivers/pci/probe.c     | 22 ++++++++++++++++++++++
> >  include/linux/pci.h     |  8 ++++++++
> >  4 files changed, 51 insertions(+)
> > 
> > diff --git a/drivers/acpi/property.c b/drivers/acpi/property.c
> > index 8c7c4583b52d..4bdad32f62c8 100644
> > --- a/drivers/acpi/property.c
> > +++ b/drivers/acpi/property.c
> > @@ -31,6 +31,9 @@ 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..84233cf46fc2 100644
> > --- a/drivers/pci/pci-acpi.c
> > +++ b/drivers/pci/pci-acpi.c
> > @@ -789,6 +789,23 @@ 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.
> > +	 */
> > +	dev->is_untrusted = val == 1;
> 
> Simpler to parse:
> 
>   if (val)
>     dev->is_untrusted = 1;

OK.

> > +}
> > +
> >  static void pci_acpi_setup(struct device *dev)
> >  {
> >  	struct pci_dev *pci_dev = to_pci_dev(dev);
> > @@ -798,6 +815,7 @@ 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..144623ae2e68 100644
> > --- a/drivers/pci/probe.c
> > +++ b/drivers/pci/probe.c
> > @@ -1378,6 +1378,26 @@ static void set_pcie_thunderbolt(struct pci_dev *dev)
> >  	}
> >  }
> >  
> > +static void set_pcie_untrusted(struct pci_dev *dev)
> > +{
> > +	struct pci_dev *parent;
> > +
> > +	/*
> > +	 * Walk up the device hierarchy and check for any upstream bridge
> > +	 * that has is_untrusted set to true. In that case treat this one
> > +	 * untrusted as well.
> > +	 */
> > +	parent = pci_upstream_bridge(dev);
> > +	while (parent) {
> > +		if (parent->is_untrusted) {
> > +			dev->is_untrusted = true;
> > +			break;
> > +		}
> > +
> > +		parent = pci_upstream_bridge(parent);
> > +	}
> 
> In what circumstance is this loop needed?  I expected this would be
> sufficient:
> 
>   bridge = pci_upstream_bridge(dev);
>   if (bridge && bridge->is_untrusted)
>     dev->is_untrusted = true;

I agree the loop is not necessary. I'll change it in the next version.

> > +}
> > +
> >  /**
> >   * pci_ext_cfg_is_aliased - Is ext config space just an alias of std config?
> >   * @dev: PCI device
> > @@ -1638,6 +1658,8 @@ 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..3fa73cc6cf68 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	is_untrusted:1;
> 
> We have a mix of "is_X" and "X", but I think "untrusted" is sufficient
> since "untrusted" is a perfectly good predicate all by itself, i.e.,
> 
>   if (dev->untrusted)
> 
> reads easily and makes good sense.

Works for me :)