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@xxxxxxxxxxxxx> 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.
