On Wed, Feb 09, 2022 at 02:00:54PM -0800, Rajat Jain wrote: > Hello, > > On Wed, Feb 9, 2022 at 10:49 AM Rafael J. Wysocki <rafael@xxxxxxxxxx> wrote: > > > > 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. > > Yes, I agree it makes sense to combine with set_pcie_untrusted(). I > can do that in the next iteration of my patch, that I intend to work > on after we reach some sort of conclusion on the other major comments > below. > > > > > > > > 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. > > The pci_dev->untrusted property is currently used by: > > - IOMMU drivers to determine whether bounce buffers should be used, > and whether flush queue should be used for these devices. Then how about naming it "use_iommu" or something like that? "Trust" has nothing to do with this at all. > - PCI subsystem to determine ACS settings (ATS / TB etc) Why is this relevant? > As we can see from the usage above, the current primary use of > untrusted property in the kernel is to flag and protect against > devices that can create a DMA attack on the host physical memory > address space (also documented for these properties in [1][2]). IMHO, > this property belongs to PCI devices because: > * I do not know of any other bus (other than PCI) that can allow DMA > access of the host memory, to a device on that bus. > * There is some use of this property within the PCI (see above), > although I agree it is not much. > * The existing properties are currently documented [1][2] to be part > of PCIe root ports / PCI-PCI bridges (only): > > [1] https://docs.microsoft.com/en-us/windows-hardware/drivers/pci/dsd-for-pcie-root-ports > [2] Documentation/devicetree/bindings/pci/pci.txt Then let us mark these as "able to do DMA" or something like that. "Trust" is a userspace policy decision, not a kernel decision to make. And there are other busses that can do DMA, PCI is not unique here. > One can possibly read the device properties in IOMMU drivers, but > they'd need to keep it in some device structure. That's fine, let's move it there. > I understand moving > the pci_dev->untrusted into struct device has been brought up a couple > of times in the past, and has met with much stronger resistance. Because of the issues I am raising here. It's a bad name and doesn't mean what people think it means. > The > discussion turned into a discussion on security, and the semantics of > this property, and allowing userspace to change this property etc, > requiring major changes, and thus fizzled out of motivation. So I guess no one really cares :) > I'd like to mention that I'm not proposing any changes to the way > (already existing) pci_dev->untrusted is being used, or the semantics > of this flag. I'm only trying to solve a corner case here i.e. > internal devices don't have a way to specify this attribute. Thus > requiring us (Chromeos) to carry hacks like [3]. I believe there are > others who are also looking for this corner case. From [4]: Why does Chromeos care about this flag? What userspace decisions do you make based on it? > ============================== > We have a similar trust issue with the BMC in servers even > though they're internal devices. They're typically network accessible > and infrequently updated so treating them as trustworthy isn't a great > idea. We have been slowly de-privileging the BMC over the last few > years, but the PCIe interface isn't locked down enough for my liking > since the SoCs we use do allow software to set the VDID and perform > arbitrary DMAs (thankfully limited to 32bit). If we're going to add in > infrastructure for handling possibly untrustworthy PCI devices then > I'd like to use that for BMCs too. > ============================= > > [3] https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/3171209 > [4] https://lkml.org/lkml/2020/6/9/1467 > > So from what I see, there is a need to solve this problem for internal > PCI devices. And presently what I have, seemed like the path of least > resistance to me (i.e. without running into big discussions, and major > code changes). It needs those code changes, please do not try to keep adding more to this to avoid the real-work that is needed here. Refer to those other discussions you mention above for what should happen to do this correctly. thanks, greg k-h
