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


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