Re: [PATCH v2 2/7] PCI: Set "untrusted" flag for truly external devices only

[Bjorn Helgaas <helgaas@kernel.org>]

On Mon, Jun 29, 2020 at 09:49:38PM -0700, Rajat Jain wrote:
> The "ExternalFacing" devices (root ports) are still internal devices that
> sit on the internal system fabric and thus trusted. Currently they were
> being marked untrusted.
> 
> This patch uses the platform flag to identify the external facing devices
> and then use it to mark any downstream devices as "untrusted". The
> external-facing devices themselves are left as "trusted". This was
> discussed here: https://lkml.org/lkml/2020/6/10/1049

Use the imperative mood in the commit log, as you did for 1/7.  E.g.,
instead of "This patch uses ...", say "Use the platform flag ...".
That helps all the commit logs read nicely together.

I think this patch makes two changes that should be separated:

  - Treat "external-facing" devices as internal.

  - Look for the "external-facing" or "ExternalFacing" property on
    Switch Downstream Ports as well as Root Ports.

> Signed-off-by: Rajat Jain <rajatja@google.com>
> ---
> v2: cosmetic changes in commit log
> 
>  drivers/iommu/intel/iommu.c |  2 +-
>  drivers/pci/of.c            |  2 +-
>  drivers/pci/pci-acpi.c      | 13 +++++++------
>  drivers/pci/probe.c         |  2 +-
>  include/linux/pci.h         |  8 ++++++++
>  5 files changed, 18 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> index d759e7234e982..1ccb224f82496 100644
> --- a/drivers/iommu/intel/iommu.c
> +++ b/drivers/iommu/intel/iommu.c
> @@ -4743,7 +4743,7 @@ static inline bool has_untrusted_dev(void)
>  	struct pci_dev *pdev = NULL;
>  
>  	for_each_pci_dev(pdev)
> -		if (pdev->untrusted)
> +		if (pdev->untrusted || pdev->external_facing)

I think checking pdev->external_facing is enough for this case,
because it's impossible to have pdev->untrusted unless a parent has
pdev->external_facing.

IIUC, this usage is asking "might we ever have an external device?"
as opposed to the "pdev->untrusted" uses, which are asking "is *this*
device an external device?"

>  			return true;
>  
>  	return false;
> diff --git a/drivers/pci/of.c b/drivers/pci/of.c
> index 27839cd2459f6..22727fc9558df 100644
> --- a/drivers/pci/of.c
> +++ b/drivers/pci/of.c
> @@ -42,7 +42,7 @@ void pci_set_bus_of_node(struct pci_bus *bus)
>  	} else {
>  		node = of_node_get(bus->self->dev.of_node);
>  		if (node && of_property_read_bool(node, "external-facing"))
> -			bus->self->untrusted = true;
> +			bus->self->external_facing = true;
>  	}
>  
>  	bus->dev.of_node = node;
> diff --git a/drivers/pci/pci-acpi.c b/drivers/pci/pci-acpi.c
> index 7224b1e5f2a83..492c07805caf8 100644
> --- a/drivers/pci/pci-acpi.c
> +++ b/drivers/pci/pci-acpi.c
> @@ -1213,22 +1213,23 @@ static void pci_acpi_optimize_delay(struct pci_dev *pdev,
>  	ACPI_FREE(obj);
>  }
>  
> -static void pci_acpi_set_untrusted(struct pci_dev *dev)
> +static void pci_acpi_set_external_facing(struct pci_dev *dev)
>  {
>  	u8 val;
>  
> -	if (pci_pcie_type(dev) != PCI_EXP_TYPE_ROOT_PORT)
> +	if (pci_pcie_type(dev) != PCI_EXP_TYPE_ROOT_PORT &&
> +	    pci_pcie_type(dev) != PCI_EXP_TYPE_DOWNSTREAM)

This looks like a change worthy of its own patch.  We used to look for
"ExternalFacingPort" only on Root Ports; now we'll also do it for
Switch Downstream Ports.

Can you include DT and ACPI spec references if they exist?  I found
this mention:
https://docs.microsoft.com/en-us/windows-hardware/drivers/pci/dsd-for-pcie-root-ports
which actually says it should only be implemented for Root Ports.

It also mentions a "DmaProperty" that looks related.  Maybe Linux
should also pay attention to this?

If we do change this, should we use pcie_downstream_port(), which
includes PCI-to-PCIe bridges as well?

>  		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
> +	 * These root/down ports expose PCIe (including DMA) outside of the
> +	 * system so make sure we treat everything behind them as
>  	 * untrusted.
>  	 */
>  	if (val)
> -		dev->untrusted = 1;
> +		dev->external_facing = 1;
>  }
>  
>  static void pci_acpi_setup(struct device *dev)
> @@ -1240,7 +1241,7 @@ 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_set_external_facing(pci_dev);
>  	pci_acpi_add_edr_notifier(pci_dev);
>  
>  	pci_acpi_add_pm_notifier(adev, pci_dev);
> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> index 6d87066a5ecc5..8c40c00413e74 100644
> --- a/drivers/pci/probe.c
> +++ b/drivers/pci/probe.c
> @@ -1552,7 +1552,7 @@ static void set_pcie_untrusted(struct pci_dev *dev)
>  	 * untrusted as well.
>  	 */
>  	parent = pci_upstream_bridge(dev);
> -	if (parent && parent->untrusted)
> +	if (parent && (parent->untrusted || parent->external_facing))
>  		dev->untrusted = true;
>  }
>  
> diff --git a/include/linux/pci.h b/include/linux/pci.h
> index a26be5332bba6..fe1bc603fda40 100644
> --- a/include/linux/pci.h
> +++ b/include/linux/pci.h
> @@ -432,6 +432,14 @@ struct pci_dev {
>  	 * mappings to make sure they cannot access arbitrary memory.
>  	 */
>  	unsigned int	untrusted:1;
> +	/*
> +	 * Devices are marked as external-facing using info from platform
> +	 * (ACPI / devicetree). An external-facing device is still an internal
> +	 * trusted device, but it faces external untrusted devices. Thus any
> +	 * devices enumerated downstream an external-facing device is marked
> +	 * as untrusted.

This comment has a subject/verb agreement problem.

> +	 */
> +	unsigned int	external_facing:1;
>  	unsigned int	broken_intx_masking:1;	/* INTx masking can't be used */
>  	unsigned int	io_window_1k:1;		/* Intel bridge 1K I/O windows */
>  	unsigned int	irq_managed:1;
> -- 
> 2.27.0.212.ge8ba1cc988-goog
>