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

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Date:   Mon, 6 Jul 2020 16:40:40 -0700
From:   Rajat Jain <rajatja@...gle.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     David Woodhouse <dwmw2@...radead.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>,
        "open list:AMD IOMMU (AMD-VI)" <iommu@...ts.linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-pci <linux-pci@...r.kernel.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        Raj Ashok <ashok.raj@...el.com>,
        "Krishnakumar, Lalithambika" <lalithambika.krishnakumar@...el.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Prashant Malani <pmalani@...gle.com>,
        Benson Leung <bleung@...gle.com>,
        Todd Broch <tbroch@...gle.com>,
        Alex Levin <levinale@...gle.com>,
        Mattias Nissler <mnissler@...gle.com>,
        Rajat Jain <rajatxjain@...il.com>,
        Bernie Keany <bernie.keany@...el.com>,
        Aaron Durbin <adurbin@...gle.com>,
        Diego Rivas <diegorivas@...gle.com>,
        Duncan Laurie <dlaurie@...gle.com>,
        Furquan Shaikh <furquan@...gle.com>,
        Jesse Barnes <jsbarnes@...gle.com>,
        Christian Kellner <christian@...lner.me>,
        Alex Williamson <alex.williamson@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Oliver O'Halloran" <oohall@...il.com>,
        Saravana Kannan <saravanak@...gle.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Arnd Bergmann <arnd@...db.de>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>
Subject: Re: [PATCH v2 2/7] PCI: Set "untrusted" flag for truly external
 devices only

Hello Bjorn,

On Mon, Jul 6, 2020 at 4:30 PM Bjorn Helgaas <helgaas@...nel.org> wrote:
>
> On Mon, Jul 06, 2020 at 03:31:47PM -0700, Rajat Jain wrote:
> > On Mon, Jul 6, 2020 at 9:38 AM Bjorn Helgaas <helgaas@...nel.org> wrote:
> > > On Mon, Jun 29, 2020 at 09:49:38PM -0700, Rajat Jain wrote:
>
> > > > -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 do. (please see below)
> >
> > > 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.
> >
> > I actually have no references. It seems to me that the microsoft spec
> > assumes that all external ports must be implemented on root ports, but
> > I think it would be equally fair for systems with PCIe switches to
> > implement one on one of their switch downstream ports. I don't have an
> > immediate use of this anyway, so if you think this should rather wait
> > unless someone really has this case, this can wait. Let me know.
>
> I agree that it "makes sense" to pay attention to this property no
> matter where it appears, but since that Microsoft doc went to the
> trouble to restrict it to Root Ports, I think we should leave this
> as-is and only look for it in the Root Port.  Otherwise Linux will
> accept something Windows will reject, and that seems like a needless
> difference.
>
> We can at least include the above link to the Microsoft doc in the
> commit log.

Will do.

>
> > > It also mentions a "DmaProperty" that looks related.  Maybe Linux
> > > should also pay attention to this?
> >
> > Interesting. Since this is not in use currently by the kernel as well
> > as not exposed by (our) BIOS, I don't have an immediate use case for
> > this. I'd like to defer this for later (as-the-need-arises).
>
> I agree, you can defer this until you see a need for it.  I just
> pointed it out in case it would be useful to you.
>
> > > > +     /*
> > > > +      * 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.
> >
> > I assume you meant s/is/are/ in last sentence. Will do.
>
> Right.  There's also something wrong with "enumerated downstream an".

I'm apparently really bad at English :-). This is what I have in my
latest patch I am about to send out:

"Thus any device enumerated downstream an external-facing device, is
marked as untrusted."

Are you suggesting s/an/a/ ? Please let me know what you would like to
see and I'd copy it as-is :-)

Thanks!

Rajat