Date:   Tue, 26 Apr 2022 10:13:17 -0700
From:   Rajat Jain <rajatja@...gle.com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <lenb@...nel.org>, linux-pci@...r.kernel.org,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Rajat Jain <rajatxjain@...il.com>,
        Dmitry Torokhov <dtor@...gle.com>,
        Jesse Barnes <jsbarnes@...gle.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Pavel Machek <pavel@...x.de>,
        "Oliver O'Halloran" <oohall@...il.com>,
        Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        iommu@...ts.linux-foundation.org,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [PATCH v6 1/2] PCI/ACPI: Support Microsoft's "DmaProperty"

On Tue, Apr 26, 2022 at 4:15 AM Robin Murphy <robin.murphy@....com> wrote:
>
> On 2022-04-26 01:06, Rajat Jain via iommu wrote:
> > The "DmaProperty" is supported and currently documented and used by
> > Microsoft [link 1 below], to flag internal PCIe root ports that need
> > DMA protection [link 2 below]. We have discussed with them and reached
> > a common understanding that they shall change their MSDN documentation
> > to say that the same property can be used to protect any PCI device,
> > and not just internal PCIe root ports (since there is no point
> > introducing yet another property for arbitrary PCI devices). This helps
> > with security from internal devices that offer an attack surface for
> > DMA attacks (e.g. internal network devices).
> >
> > Support DmaProperty to mark DMA from a PCI device as untrusted.
> >
> > Link: [1] https://docs.microsoft.com/en-us/windows-hardware/drivers/pci/dsd-for-pcie-root-ports#identifying-internal-pcie-ports-accessible-to-users-and-requiring-dma-protection
> > Link: [2] https://docs.microsoft.com/en-us/windows/security/information-protection/kernel-dma-protection-for-thunderbolt
> > Signed-off-by: Rajat Jain <rajatja@...gle.com>
> > Reviewed-by: Mika Westerberg <mika.westerberg@...ux.intel.com>
> > Acked-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > ---
> > v6: * Take care of Bjorn's comments:
> >         - Update the commit log
> >         - Rename to pci_dev_has_dma_property()
> >         - Use acpi_dev_get_property()
> > v5: * Reorder the patches in the series
> > v4: * Add the GUID.
> >      * Update the comment and commitlog.
> > v3: * Use Microsoft's documented property "DmaProperty"
> >      * Resctrict to ACPI only
> >
> >   drivers/acpi/property.c |  3 +++
> >   drivers/pci/pci-acpi.c  | 21 +++++++++++++++++++++
> >   2 files changed, 24 insertions(+)
> >
> > diff --git a/drivers/acpi/property.c b/drivers/acpi/property.c
> > index 12bbfe833609..bafe35c301ac 100644
> > --- a/drivers/acpi/property.c
> > +++ b/drivers/acpi/property.c
> > @@ -48,6 +48,9 @@ static const guid_t prp_guids[] = {
> >       /* Storage device needs D3 GUID: 5025030f-842f-4ab4-a561-99a5189762d0 */
> >       GUID_INIT(0x5025030f, 0x842f, 0x4ab4,
> >                 0xa5, 0x61, 0x99, 0xa5, 0x18, 0x97, 0x62, 0xd0),
> > +     /* DmaProperty for PCI devices GUID: 70d24161-6dd5-4c9e-8070-705531292865 */
> > +     GUID_INIT(0x70d24161, 0x6dd5, 0x4c9e,
> > +               0x80, 0x70, 0x70, 0x55, 0x31, 0x29, 0x28, 0x65),
> >   };
> >
> >   /* ACPI _DSD data subnodes GUID: dbb8e3e6-5886-4ba6-8795-1319f52a966b */
> > diff --git a/drivers/pci/pci-acpi.c b/drivers/pci/pci-acpi.c
> > index 3ae435beaf0a..d7c6ba48486f 100644
> > --- a/drivers/pci/pci-acpi.c
> > +++ b/drivers/pci/pci-acpi.c
> > @@ -1369,12 +1369,33 @@ static void pci_acpi_set_external_facing(struct pci_dev *dev)
> >               dev->external_facing = 1;
> >   }
> >
> > +static int pci_dev_has_dma_property(struct pci_dev *dev)
> > +{
> > +     struct acpi_device *adev;
> > +     const union acpi_object *obj;
> > +
> > +     adev = ACPI_COMPANION(&dev->dev);
> > +     if (!adev)
> > +             return 0;
> > +
> > +     /*
> > +      * Property also used by Microsoft Windows for same purpose,
> > +      * (to implement DMA protection from a device, using the IOMMU).
>
> Nit: there is no context for "same purpose" here, so this comment is
> more confusing than helpful. Might I suggest a rewording like:
>
>         /*
>          * Property used by Microsoft Windows to enforce IOMMU DMA
>          * protection for any device that the system might not fully
>          * trust; we'll honour it the same way.
>          */
>
> ?

Sure, will do.

>
> Personally I think it would have been more logical to handle this in
> pci_set_dma_untrusted(), but I see some of those implementation aspects
> have already been discussed, and Bjorn's preference definitely wins over
> mine here :)

Yes, this was discussed. The primary reason is that ACPI properties
for PCI devices are not available at the time pci_set_untrusted_dma()
is called.

Thanks & Best Regards,

Rajat

>
> Thanks,
> Robin.
>
> > +      */
> > +     if (!acpi_dev_get_property(adev, "DmaProperty", ACPI_TYPE_INTEGER,
> > +                                &obj) && obj->integer.value == 1)
> > +             return 1;
> > +
> > +     return 0;
> > +}
> > +
> >   void pci_acpi_setup(struct device *dev, struct acpi_device *adev)
> >   {
> >       struct pci_dev *pci_dev = to_pci_dev(dev);
> >
> >       pci_acpi_optimize_delay(pci_dev, adev->handle);
> >       pci_acpi_set_external_facing(pci_dev);
> > +     pci_dev->untrusted |= pci_dev_has_dma_property(pci_dev); >      pci_acpi_add_edr_notifier(pci_dev);
> >
> >       pci_acpi_add_pm_notifier(adev, pci_dev);

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