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

Dave Hansen
2021-11-12T21:37:56Z

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