linux-kernel - Re: [PATCH v3 1/4] PCI / ACPI: Identify untrusted PCI devices

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Date:   Tue, 4 Dec 2018 10:57:35 +0200
From:   Mika Westerberg <mika.westerberg@...ux.intel.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     iommu@...ts.linux-foundation.org, Joerg Roedel <joro@...tes.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Ashok Raj <ashok.raj@...el.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Jacob jun Pan <jacob.jun.pan@...el.com>,
        Andreas Noever <andreas.noever@...il.com>,
        Michael Jamet <michael.jamet@...el.com>,
        Yehezkel Bernat <YehezkelShB@...il.com>,
        Lukas Wunner <lukas@...ner.de>,
        Christian Kellner <ckellner@...hat.com>,
        Mario.Limonciello@...l.com,
        Anthony Wong <anthony.wong@...onical.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Christoph Hellwig <hch@...radead.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        linux-acpi@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/4] PCI / ACPI: Identify untrusted PCI devices

On Mon, Dec 03, 2018 at 06:28:00PM -0600, Bjorn Helgaas wrote:
> On Thu, Nov 29, 2018 at 06:51:50PM +0300, Mika Westerberg wrote:
> > A malicious PCI device may use DMA to attack the system. An external
> > Thunderbolt port is a convenient point to attach such a device. The OS
> > may use IOMMU to defend against DMA attacks.
> > 
> > Recent BIOSes with Thunderbolt ports mark these externally facing root
> > ports with this ACPI _DSD [1]:
> 
> I'm not 100% comfortable with the "Recent BIOSes" wording because that
> suggests that we can rely on the fact that *all* BIOSes newer than
> some date X mark these ports.
> 
> Since this _DSD usage is Microsoft-specific and not required by either
> PCIe or ACPI specs, we can't rely on it.  A BIOS that doesn't
> implement it may not be Windows-certified, but it's perfectly
> spec-compliant otherwise and we have to keep in mind the possibility
> that ports without this _DSD may still be externally visible and may
> still be attack vectors.

OK.

I will change it to "Some BIOSes .." following what you suggested
earlier. That should make it clear not all BIOSes are required to
implement this.

> >   Name (_DSD, Package () {
> >       ToUUID ("efcc06cc-73ac-4bc3-bff0-76143807c389"),
> >       Package () {
> >           Package () {"ExternalFacingPort", 1},
> > 	  Package () {"UID", 0 }
> >       }
> >   })
> > 
> > If we find such a root port, mark it and all its children as untrusted.
> > The rest of the OS may use this information to enable DMA protection
> > against malicious devices. For instance the device may be put behind an
> > IOMMU to keep it from accessing memory outside of what the driver has
> > allocated for it.
> > 
> > While at it, add a comment on top of prp_guids array explaining the
> > possible caveat resulting when these GUIDs are treated equivalent.
> > 
> > [1] https://docs.microsoft.com/en-us/windows-hardware/drivers/pci/dsd-for-pcie-root-ports#identifying-externally-exposed-pcie-root-ports
> > 
> > Signed-off-by: Mika Westerberg <mika.westerberg@...ux.intel.com>
> 
> Acked-by: Bjorn Helgaas <bhelgaas@...gle.com>

Thanks!