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    SubjectRe: Linux DRTM on UEFI platforms
    On Thu, Aug 11, 2022 at 07:25:58PM +0930, Brendan Trotter wrote:
    > Hi,
    >
    > On Thu, Aug 11, 2022 at 3:16 AM Matthew Garrett <mjg59@srcf.ucam.org> wrote:
    > > The kernel has no way to know this - *any* code you've run before
    > > performing a measurement could tamper with the kernel such that it
    > > believes it's fine. This is just as true in DRTM as it is in SRTM. But
    > > you know what the expected measurements should be, so you're able to
    > > either seal secrets to those PCR values or rely on remote attestation.
    >
    > In this scenario the kernel has no idea what the measurement should
    > be, it only knows the measurement that a potentially malicious boot
    > loader felt like giving the kernel previously (e.g. when the kernel
    > was installed).

    Even if the kernel has an idea of what the measurement should be, it has
    no way to verify that what it believes to be true is true - any
    malicious code could simply have modified the kernel to believe that
    anything it asks the TPM returns the "correct" answer.

    > > Measurements are not opaque objects. If you're not able to reconstruct
    > > the expected measurement then you're doing it wrong.
    >
    > OK; so to detect if boot loader has always given kernel a bad/forged
    > measurement; the kernel repeats all of the steps involved in creating
    > the measurement itself exactly the same as the boot loader should have
    > (but might not have) so that kernel can compare a "known
    > good/trustworthy" measurement with the useless measurement that the
    > boot loader created for no sane reason whatsoever?

    No, some external agent does. Code running on the local machine can
    never determine whether the machine is trustworthy.

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