Re: [PATCH v2 1/2] PCI: Allow internal devices to be marked as untrusted

[Rafael J. Wysocki]

On Wed, Feb 9, 2022 at 7:39 PM Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote:
>
> On Wed, Feb 09, 2022 at 06:46:12AM +0100, Greg Kroah-Hartman wrote:
> > On Tue, Feb 08, 2022 at 04:23:27PM -0800, Rajat Jain wrote:
> > > On Tue, Feb 1, 2022 at 6:01 PM Rajat Jain <rajatja@xxxxxxxxxx> wrote:
> > > >
> > > > Today the pci_dev->untrusted is set for any devices sitting downstream
> > > > an external facing port (determined via "ExternalFacingPort" or the
> > > > "external-facing" properties).
> > > >
> > > > However, currently there is no way for internal devices to be marked as
> > > > untrusted.
> > > >
> > > > There are use-cases though, where a platform would like to treat an
> > > > internal device as untrusted (perhaps because it runs untrusted firmware
> > > > or offers an attack surface by handling untrusted network data etc).
> > > >
> > > > Introduce a new "UntrustedDevice" property that can be used by the
> > > > firmware to mark any device as untrusted.
> > >
> > > Just to unite the threads (from
> > > https://www.spinics.net/lists/linux-pci/msg120221.html). I did reach
> > > out to Microsoft but they haven't acknowledged my email. I also pinged
> > > them again yesterday, but I suspect I may not be able to break the
> > > ice. So this patch may be ready to go in my opinion.
> > >
> > > I don't see any outstanding comments on this patch, but please let me
> > > know if you have any comments.
> > >
> > > > Signed-off-by: Rajat Jain <rajatja@xxxxxxxxxx>
> > > > ---
> > > > v2: * Also use the same property for device tree based systems.
> > > >     * Add documentation (next patch)
> > > >
> > > >  drivers/pci/of.c       | 2 ++
> > > >  drivers/pci/pci-acpi.c | 1 +
> > > >  drivers/pci/pci.c      | 9 +++++++++
> > > >  drivers/pci/pci.h      | 2 ++
> > > >  4 files changed, 14 insertions(+)
> > > >
> > > > diff --git a/drivers/pci/of.c b/drivers/pci/of.c
> > > > index cb2e8351c2cc..e8b804664b69 100644
> > > > --- a/drivers/pci/of.c
> > > > +++ b/drivers/pci/of.c
> > > > @@ -24,6 +24,8 @@ void pci_set_of_node(struct pci_dev *dev)
> > > >                                                     dev->devfn);
> > > >         if (dev->dev.of_node)
> > > >                 dev->dev.fwnode = &dev->dev.of_node->fwnode;
> > > > +
> > > > +       pci_set_untrusted(dev);
> > > >  }
> > > >
> > > >  void pci_release_of_node(struct pci_dev *dev)
> > > > diff --git a/drivers/pci/pci-acpi.c b/drivers/pci/pci-acpi.c
> > > > index a42dbf448860..2bffbd5c6114 100644
> > > > --- a/drivers/pci/pci-acpi.c
> > > > +++ b/drivers/pci/pci-acpi.c
> > > > @@ -1356,6 +1356,7 @@ void pci_acpi_setup(struct device *dev, struct acpi_device *adev)
> > > >
> > > >         pci_acpi_optimize_delay(pci_dev, adev->handle);
> > > >         pci_acpi_set_external_facing(pci_dev);
> > > > +       pci_set_untrusted(pci_dev);
> > > >         pci_acpi_add_edr_notifier(pci_dev);
> > > >
> > > >         pci_acpi_add_pm_notifier(adev, pci_dev);
> > > > diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> > > > index 9ecce435fb3f..41e887c27004 100644
> > > > --- a/drivers/pci/pci.c
> > > > +++ b/drivers/pci/pci.c
> > > > @@ -6869,3 +6869,12 @@ static int __init pci_realloc_setup_params(void)
> > > >         return 0;
> > > >  }
> > > >  pure_initcall(pci_realloc_setup_params);
> > > > +
> > > > +void pci_set_untrusted(struct pci_dev *pdev)
> > > > +{
> > > > +       u8 val;
> > > > +
> > > > +       if (!device_property_read_u8(&pdev->dev, "UntrustedDevice", &val)
>
> If we do this, can we combine it with set_pcie_untrusted(), where we
> already set pdev->untrusted?  Maybe that needs to be renamed; I don't
> see anything PCIe-specific there, and it looks like it works for
> conventional PCI as well.
>
> > Please no, "Untrusted" does not really convey much, if anything here.
> > You are taking an odd in-kernel-value and making it a user api.
> >
> > Where is this "trust" defined?  Who defines it?  What policy does the
> > kernel impose on it?
>
> I'm a bit hesitant about this, too.  It really doesn't have anything
> in particular to do with the PCI core.  It's not part of the PCI
> specs, and it could apply to any kind of device, not just PCI (ACPI,
> platform, USB, etc).
>
> We have:
>
>   dev->removable                # struct device
>   pdev->is_thunderbolt
>   pdev->untrusted
>   pdev->external_facing
>
> and it feels a little hard to keep everything straight.  Most of them
> are "discovered" based on some DT or ACPI firmware property.  None of
> them really has anything specifically to do with *PCI*, and I don't
> think the PCI core depends on any of them.  I think
> pdev->is_thunderbolt is the only one we discover based on a PCI
> feature (the Thunderbolt Capability), and the things we *use* it for
> are actually not things specified by that capability [1].
>
> Could drivers just look for these properties directly instead of
> relying on the PCI core to get in the middle?  Most callers of
> device_property_read_*() are in drivers.  I do see that doing it in
> the PCI core might help enforce standard usage in DT/ACPI, but we
> could probably do that in other ways, too.

FWIW, I agree that looking at these things in drivers would be better.

> [1] https://lore.kernel.org/r/20220204222956.GA220908@bhelgaas