Re: [PATCH v2] nvme: Cache DMA descriptors to prevent corruption.

[Tom Lendacky]

On 11/30/20 12:50 PM, Tom Roeder wrote:
> On Fri, Nov 20, 2020 at 09:02:43AM +0100, Christoph Hellwig wrote:
>> On Thu, Nov 19, 2020 at 05:27:37PM -0800, Tom Roeder wrote:
>>> This patch changes the NVMe PCI implementation to cache host_mem_descs
>>> in non-DMA memory instead of depending on descriptors stored in DMA
>>> memory. This change is needed under the malicious-hypervisor threat
>>> model assumed by the AMD SEV and Intel TDX architectures, which encrypt
>>> guest memory to make it unreadable. Some versions of these architectures
>>> also make it cryptographically hard to modify guest memory without
>>> detection.
>>
>> I don't think this is a useful threat model, and I've not seen a
>> discussion on lkml where we had any discussion on this kind of threat
>> model either.
> 
> Thanks for the feedback and apologies for the lack of context.
> 
> I was under the impression that support for AMD SEV SNP will start showing 
> up in KVM soon, and my understanding of SNP is that it implies this threat 
> model for the guest. See the patchset for SEV-ES, which is the generation 
> before SNP: 
> https://lkml.org/lkml/2020/9/14/1168.> This doesn't get quite to the SNP threat model, but it starts to assume 
> more maliciousness on the part of the hypervisor.
> 
> You can also see the talk from David Kaplan of AMD from the 2019 Linux 
> Security Summit for info about SNP: 
> https://www.youtube.com/watch?v=yr56SaJ_0QI.
> 
> 
>>
>> Before you start sending patches that regress optimizations in various
>> drivers (and there will be lots with this model) we need to have a
>> broader discussion first.
> 
> I've added Tom Lendacky and David Kaplan from AMD on the thread now, since 
> I don't think I have enough context to say where this discussion should 
> take place or the degree to which they think it has or hasn't.
> 
> Tom, David: can you please comment on this?

Any discussion should certainly take place in the open on the mailing
lists.

Further information on SEV-SNP can be found on the SEV developer web page
at https://developer.amd.com/sev.

There is a white paper specific to SNP:
  https://www.amd.com/system/files/TechDocs/SEV-SNP-strengthening-vm-isolation-with-integrity-protection-and-more.pdf

Also, volume 2 of the AMD APM provides further information on the various
SEV features (sections 15.34 to 15.36):
  https://www.amd.com/system/files/TechDocs/24593.pdf

It is a good idea to go through the various drivers and promote changes
to provide protection from a malicious hypervisor, but, as Christoph
states, it needs to be discussed in order to determine the best approach.

Thanks,
Tom

> 
>>
>> And HMB support, which is for low-end consumer devices that are usually
>> not directly assigned to VMs aren't a good starting point for this.
> 
> I'm glad to hear that this case doesn't apply directly to cases we would 
> care about for assignment to guests. I'm not very familiar with this 
> codebase, unfortunately. Do the same kinds of issues apply for the kinds 
> of devices that would be assigned to guests?