Re: [PATCH v11 04/20] x86/cpu: Detect TDX partial write machine check erratum

[Dave Hansen]

On 6/19/23 05:21, David Hildenbrand wrote:
> So, ordinary writes to TD private memory are not a problem? I thought
> one motivation for the unmapped-guest-memory discussion was to prevent
> host (userspace) writes to such memory because it would trigger a MC and
> eventually crash the host.

Those are two different problems.

Problem #1 (this patch): The host encounters poison when going about its
normal business accessing normal memory.  This happens when something in
the host accidentally clobbers some TDX memory and *then* reads it.
Only occurs with partial writes.

Problem #2 (addressed with unmapping): Host *userspace* intentionally
and maliciously clobbers some TDX memory and then the TDX module or a
TDX guest can't run because the memory integrity checks (checksum or TD
bit) fail.  This can also take the system down because #MC's are nasty.

Host userspace unmapping doesn't prevent problem #1 because it's the
kernel who screwed up with the _kernel_ mapping.