Re: [PATCH] iommu/intel: Exclude devices using RMRRs from IOMMU API domains

[Linda Knippers]

On 5/21/2014 9:20 AM, Alex Williamson wrote:
> On Wed, 2014-05-21 at 11:38 +0100, David Woodhouse wrote:
>> On Wed, 2014-05-14 at 13:27 -0600, Alex Williamson wrote:
>>> The user of the IOMMU API domain expects to have full control of
>>> the IOVA space for the domain.  RMRRs are fundamentally incompatible
>>> with that idea.  We can neither map the RMRR into the IOMMU API
>>> domain, nor can we guarantee that the device won't continue DMA with
>>> the area described by the RMRR as part of the new domain.  Therefore
>>> we must prevent such devices from being used by the IOMMU API.
>>
>> Ick, ick, ick. The more the ramifications of RMRRs become apparent, the
>> more I wish we'd just done the Right Thingâ and declared that firmware
>> SHALL NOT leave any device doing (IOMMU-visible) DMA after the OS takes
>> over. That way, if they wanted this kind of abomination then they'd have
>> to come up with a way of doing it differently. Hell, can't you do PCIe
>> transactions which claim to be already translated, and thus just bypass
>> the IOMMU?
>>
>> OK, rant over... 
>>
>> Why can't we map the RMRR into the IOMMU API domain? If we're setting up
>> a VM guest, that basically means we'd want to poke a hole in its memory
>> map and mark the RMRR-afflicted range as reserved or absent. It's
>> horrible, but *everything* about RMRRs is horrible. It's not impossible,
>> and it would allow us to give these devices away to guests. Don't we
>> sometimes *have* devices that we want to give to guests, that are
>> afflicted with RMRRs?
> 
> You're right, it is possible to assign devices with RMRRs, but in order
> to do so we'd need to expose the RMRR areas for a device beyond the
> inner workings of intel-iommu and mark those ranges as reserved in the
> guest.  That alone makes hotplug of such devices into a guest
> impossible.
> 
> Enabling such a use case also potentially provides an untrusted guest
> with direct access to regions of platform memory that potentially allows
> for untold platform specific exploits.
> 
> We currently have no visibility to RMRRs from the IOMMU API, so we can't
> even attempt to do the above, nor can we guarantee that we have any
> ability to make a device discontinue use of an RMRR area when it is
> assigned to a VM domain.  Therefore the only safe thing to do is prevent
> use of such devices by a VM domain.
> 
>> There are discussions about RMRRs being (ab)used for more than their
>> existing brain-damaged purpose. Where we have a peripheral device that
>> will (mis)interpret certain address ranges as "local" rather than
>> forwarding transactions up towards main memory, we need to ensure that
>> such ranges are never used as virtual addresses. This has largely been
>> an invisible problem until we found a device where the affected range
>> matched the IOVA our DMA API uses by default. Using an RMRR has been
>> proposed as a simple way to achieve that... which means that you end up
>> not being able to assign *those* devices to IOMMU domains either.
>>
>> I do suspect it's going to lead to complaints... but I'm just not sure I
>> can bring myself to care. Sane designs don't require RMRRs. If someone
>> comes to me and complains that their HP storage controller or whatever
>> can't be assigned to a guest, I'm quite prepared to tell them to replace
>> it with something non-broken. Will you back me up when it happens?
> 
> Exactly, I have a hard time bringing myself to care about supporting
> such devices.  Vendors that proliferate RMRR usage need to be aware of
> the implications of RMRRs for all use cases of a device.  First and
> foremost, we need to lock out devices with RMRRs because we have no
> ability to either honor or disable RMRRs when used by the IOMMU API.  If
> vendors that rely on RMRRs want to make such devices assignable by
> providing interfaces to describe and map the area into a VM, or even a
> mechanism to disable the RMRR, more power to them.  The current
> situation is simply unsafe and needs to be prevented.  

I care but I think this patch should go in until there is a better
solution.

-- ljk

> Thanks,
> 
> Alex
> 
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> 

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