Re: Linux DRTM on UEFI platforms

[Matthew Garrett <mjg59@srcf.ucam.org>]

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.