linux-kernel - RE: [PATCH v2 2/2] thunderbolt: Make iommu_dma

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Message-ID: <BL1PR12MB515731B8F62C45156037CF8CE2169@BL1PR12MB5157.namprd12.prod.outlook.com>
Date:   Mon, 21 Mar 2022 13:21:12 +0000
From:   "Limonciello, Mario" <Mario.Limonciello@....com>
To:     Robin Murphy <robin.murphy@....com>,
        "mika.westerberg@...ux.intel.com" <mika.westerberg@...ux.intel.com>
CC:     "joro@...tes.org" <joro@...tes.org>,
        "baolu.lu@...ux.intel.com" <baolu.lu@...ux.intel.com>,
        "andreas.noever@...il.com" <andreas.noever@...il.com>,
        "michael.jamet@...el.com" <michael.jamet@...el.com>,
        "YehezkelShB@...il.com" <YehezkelShB@...il.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "hch@....de" <hch@....de>
Subject: RE: [PATCH v2 2/2] thunderbolt: Make iommu_dma_protection more
 accurate

[Public]



> -----Original Message-----
> From: Robin Murphy <robin.murphy@....com>
> Sent: Monday, March 21, 2022 06:12
> To: mika.westerberg@...ux.intel.com; Limonciello, Mario
> <Mario.Limonciello@....com>
> Cc: joro@...tes.org; baolu.lu@...ux.intel.com; andreas.noever@...il.com;
> michael.jamet@...el.com; YehezkelShB@...il.com; iommu@...ts.linux-
> foundation.org; linux-usb@...r.kernel.org; linux-kernel@...r.kernel.org;
> hch@....de
> Subject: Re: [PATCH v2 2/2] thunderbolt: Make iommu_dma_protection
> more accurate
> 
> On 2022-03-21 10:58, mika.westerberg@...ux.intel.com wrote:
> > Hi Mario,
> >
> > On Fri, Mar 18, 2022 at 10:29:59PM +0000, Limonciello, Mario wrote:
> >> [Public]
> >>
> >>> Between me trying to get rid of iommu_present() and Mario wanting to
> >>> support the AMD equivalent of DMAR_PLATFORM_OPT_IN, scrutiny has
> >>> shown
> >>> that the iommu_dma_protection attribute is being far too optimistic.
> >>> Even if an IOMMU might be present for some PCI segment in the
> system,
> >>> that doesn't necessarily mean it provides translation for the device(s)
> >>> we care about. Furthermore, all that DMAR_PLATFORM_OPT_IN really
> does
> >>> is tell us that memory was protected before the kernel was loaded, and
> >>> prevent the user from disabling the intel-iommu driver entirely. While
> >>> that lets us assume kernel integrity, what matters for actual runtime
> >>> DMA protection is whether we trust individual devices, based on the
> >>> "external facing" property that we expect firmware to describe for
> >>> Thunderbolt ports.
> >>>
> >>> It's proven challenging to determine the appropriate ports accurately
> >>> given the variety of possible topologies, so while still not getting a
> >>> perfect answer, by putting enough faith in firmware we can at least get
> >>> a good bit closer. If we can see that any device near a Thunderbolt NHI
> >>> has all the requisites for Kernel DMA Protection, chances are that it
> >>> *is* a relevant port, but moreover that implies that firmware is playing
> >>> the game overall, so we'll use that to assume that all Thunderbolt ports
> >>> should be correctly marked and thus will end up fully protected.
> >>>
> >>
> >> This approach looks generally good to me.  I do worry a little bit about
> older
> >> systems that didn't set ExternalFacingPort in the FW but were previously
> setting
> >> iommu_dma_protection, but I think that those could be treated on a
> quirk
> >> basis to set PCI IDs for those root ports as external facing if/when they
> come
> >> up.
> >
> > There are no such systems out there AFAICT.
> 
> And even if there are, as above they've never actually been fully
> protected and still won't be, so it's arguably a good thing for them to
> stop thinking so.

I was meaning that if this situation comes up that we could tag the PCI IDs for
those root ports as ExternalFacing in drivers/pci/quirks.c so that the protection
"is" enacted for them even if it was missing from the firmware.

> 
> >> I'll send up a follow up patch that adds the AMD ACPI table check.
> >> If it looks good can roll it into your series for v3, or if this series goes
> >> as is for v2 it can come on its own.
> >>
> >>> CC: Mario Limonciello <mario.limonciello@....com>
> >>> Signed-off-by: Robin Murphy <robin.murphy@....com>
> >>> ---
> >>>
> >>> v2: Give up trying to look for specific devices, just look for evidence
> >>>      that firmware cares at all.
> >>
> >> I still do think you could know exactly which devices to use if you're in
> >> SW CM mode, but I guess the consensus is to not bifurcate the way this
> >> can be checked.
> >
> > Indeed.
> >
> > The patch looks good to me now. I will give it a try on a couple of
> > systems later today or tomorrow and let you guys know how it went. I
> > don't expect any problems but let's see.
> >
> > Thanks a lot Robin for working on this :)
> 
> Heh, let's just hope the other half-dozen or so subsystems I need to
> touch for this IOMMU cleanup aren't all quite as involved as this turned
> out to be :)

Thanks a lot for this effort!

> 
> Cheers,
> Robin.