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

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Message-ID: <20201130185045.GA744128@google.com>
Date:   Mon, 30 Nov 2020 10:50:45 -0800
From:   Tom Roeder <tmroeder@...gle.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Keith Busch <kbusch@...nel.org>, Jens Axboe <axboe@...com>,
        Sagi Grimberg <sagi@...mberg.me>,
        Peter Gonda <pgonda@...gle.com>,
        Marios Pomonis <pomonis@...gle.com>,
        linux-nvme@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Thomas.Lendacky@....com, David.Kaplan@....com
Subject: Re: [PATCH v2] nvme: Cache DMA descriptors to prevent corruption.

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?

>
>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?