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Date:   Thu, 11 Aug 2022 19:25:02 +0100
From:   Matthew Garrett <mjg59@srcf.ucam.org>
To:     Brendan Trotter <btrotter@gmail.com>
Cc:     The development of GNU GRUB <grub-devel@gnu.org>,
        Ard Biesheuvel <ardb@kernel.org>,
        Daniel Kiper <daniel.kiper@oracle.com>,
        Alec Brown <alec.r.brown@oracle.com>,
        Kanth Ghatraju <kanth.ghatraju@oracle.com>,
        Ross Philipson <ross.philipson@oracle.com>,
        "piotr.krol@3mdeb.com" <piotr.krol@3mdeb.com>,
        "krystian.hebel@3mdeb.com" <krystian.hebel@3mdeb.com>,
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        "Yoder, Stuart" <stuart.yoder@arm.com>,
        Andrew Cooper <andrew.cooper3@citrix.com>,
        "michal.zygowski@3mdeb.com" <michal.zygowski@3mdeb.com>,
        James Bottomley <James.Bottomley@hansenpartnership.com>,
        "lukasz@hawrylko.pl" <lukasz@hawrylko.pl>,
        linux-efi@vger.kernel.org, linux-kernel@vger.kernel.org,
        James Morris <jmorris@namei.org>
Subject: Re: Linux DRTM on UEFI platforms
Message-ID: <20220811182502.GA32433@srcf.ucam.org>
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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.