Re: [PATCH Part2 v5 00/45] Add AMD Secure Nested Paging (SEV-SNP) Hypervisor Support

On Fri, Nov 12, 2021 at 1:38 PM Dave Hansen [off-list ref] wrote:

On 11/12/21 1:30 PM, Marc Orr wrote:

quoted

In this proposal, consider a guest driver instructing a device to DMA
write a 1 GB memory buffer. A well-behaved guest driver will ensure
that the entire 1 GB is marked shared. But what about a malicious or
buggy guest? Let's assume a bad guest driver instructs the device to
write guest private memory.

So now, the virtual device, which might be implemented as some host
side process, needs to (1) check and lock all 4k constituent RMP
entries (so they're not converted to private while the DMA write is
taking palce), (2) write the 1 GB buffer, and (3) unlock all 4 k
constituent RMP entries? If I'm understanding this correctly, then the
synchronization will be prohibitively expensive.

Are you taking about a 1GB *mapping* here?  As in, something us using a
1GB page table entry to map the 1GB memory buffer?  That was the only
case where I knew we needed coordination between neighbor RMP entries
and host memory accesses.

That 1GB problem _should_ be impossible.  I thought we settled on
disabling hugetlbfs and fracturing the whole of the direct map down to 4k.

No. I was trying to give an example where a host-side process is
virtualizing a DMA write over a large buffer that consists of a lot of
4k or 2MB RMP entries. I picked 1 GB as an arbitrary example.