Re: [PATCH] thunderbolt: Make iommu_dma_protection more accurate

Robin Murphy
2022-03-18T15:15:28Z

On 2022-03-18 14:47, mika.westerberg@linux.intel.com wrote:
> On Fri, Mar 18, 2022 at 02:08:16PM +0000, Robin Murphy wrote:
>> On 2022-03-18 13:25, mika.westerberg@linux.intel.com wrote:
>>> Hi Robin,
>>>
>>> On Fri, Mar 18, 2022 at 12:01:42PM +0000, Robin Murphy wrote:
>>>>> This adds quite a lot code and complexity, and honestly I would like to
>>>>> keep it as simple as possible (and this is not enough because we need to
>>>>> make sure the DMAR bit is there so that none of the possible connected
>>>>> devices were able to overwrite our memory already).
>>>>
>>>> Shall we forget the standalone sibling check and just make the
>>>> pdev->untrusted check directly in tb_acpi_add_link() then?
>>>
>>> I think we should leave tb_acpi_add_link() untouched if possible ;-)
>>> This is because it is used to add the device links from firmware
>>> description that we need for proper power management of the tunneled
>>> devices. It has little to do with the identification of the external
>>> facing DMA-capable PCIe ports.
>>>
>>> Furthermore these links only exists in USB4 software connection manager
>>> systems so we do not have those in the existing Thunderbolt 3/4 systems
>>> that use firmware based connection manager (pretty much all out there).
>>>
>>>> On reflection I guess the DMAR bit makes iommu_dma_protection
>>>> functionally dependent on ACPI already, so we don't actually lose
>>>> anything (and anyone can come back and revisit firmware-agnostic
>>>> methods later if a need appears).
>>>
>>> I agree.
>>
>> OK, so do we have any realistic options for identifying the correct PCI
>> devices, if USB4 PCIe adapters might be anywhere relative to their
>> associated NHI? Short of maintaining a list of known IDs, the only thought I
>> have left is that if we walk the whole PCI segment looking specifically for
>> hotplug-capable Gen1 ports, any system modern enough to have Thunderbolt is
>> *probably* not going to have any real PCIe Gen1 hotplug slots, so maybe
>> false negatives might be tolerable, but it still feels like a bit of a
>> sketchy heuristic.
>
> Indeed.
>
>> I suppose we could just look to see if any device anywhere is marked as
>> external-facing, and hope that if firmware's done that much then it's done
>> everything right. That's still at least slightly better than what we have
>> today, but AFAICS still carries significant risk of a false positive for an
>> add-in card that firmware didn't recognise.
>
> The port in this case, that is marked as external facing, is the PCIe
> root port that the add-in-card is connected to and that is known for the
> firmware in advance.
>
>> I'm satisfied that we've come round to the right conclusion on the DMAR
>> opt-in - I'm in the middle or writing up patches for that now - but even
>> Microsoft's spec gives that as a separate requirement from the flagging of
>> external ports, with both being necessary for Kernel DMA Protection.
>
> Is the problem that we are here trying to solve the fact that user can
> disable the IOMMU protection from the command line? Or the fact that the
> firmware might not declare all the ports properly so we may end up in a
> situation that some of the ports do not get the full IOMMU protection.

It's about knowing whether or not firmware has declared the ports at
all. If it hasn't then the system is vulnerable to *some* degree of DMA
attacks regardless of anything else (the exact degree depending on
kernel config and user overrides). Complete mitigation is simply too
expensive to apply by default to every device the IOMMU layer is unsure
about. The Thunderbolt driver cannot be confident that protection is in
place unless it can somehow know that the IOMMU layer has seen that
untrusted property on the relevant ports.

> These are Microsoft requirements for the OEMs in order to pass their
> firmware test suite so here I would not expect to have issues. Otherwise
> they simply cannot ship the thing with Windows installed.
>
> IMHO we should just trust the firmare provided information here
> (otherwise we are screwed anyway as there is no way to tell if the
> devices connected prior the OS can still do DMA), and use the external
> facing port indicator to idenfity the ports that need DMA protection.

Indeed that's exactly what I want to do, but it begs the question of how
we *find* the firmware-provided information in the first place!

I seem to have already started writing the dumb version that will walk
the whole PCI segment and assume the presence of any external-facing
port implies that we're good. Let me know if I should stop ;)

Cheers,
Robin.