Re: [PATCH v3] PCI: ACPI: Support Microsoft's "DmaProperty"

[Rajat Jain <rajatja@google.com>]

Hello,

On Wed, Feb 16, 2022 at 10:16 PM Greg Kroah-Hartman
<gregkh@linuxfoundation.org> wrote:
>
> On Wed, Feb 16, 2022 at 02:05:41PM -0800, Rajat Jain wrote:
> > 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@google.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;
>
> Why not continue to only do this for PCIe devices like it is actually
> being used for?  Why expand it?

Because devices hanging off of PCIe root ports are not the only ones
that may need DMA protection. There may be internal PCI devices (that
don't hang off a PCIe root port) that may need DMA protection.
Examples include internal network controllers that may offer an attack
surface by handling network data or running vendor firmware.

>
> And what driver/device is going to use this?

This is already used by PCI subsystem to enforce stricter ACS
settings, and IOMMU drivers to enforce stricter IOMMU settings.

Thanks & Best Regards,

Rajat

>
> thanks,
>
> greg k-h