linux-kernel - Re: Linux DRTM on UEFI platforms

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Message-ID: <20220330072755.GA1169@srcf.ucam.org>
Date:   Wed, 30 Mar 2022 08:27:55 +0100
From:   Matthew Garrett <mjg59@...f.ucam.org>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Daniel Kiper <daniel.kiper@...cle.com>,
        Alec Brown <alec.r.brown@...cle.com>,
        Kanth Ghatraju <kanth.ghatraju@...cle.com>,
        Ross Philipson <ross.philipson@...cle.com>,
        "dpsmith@...rtussolutions.com" <dpsmith@...rtussolutions.com>,
        "piotr.krol@...eb.com" <piotr.krol@...eb.com>,
        "krystian.hebel@...eb.com" <krystian.hebel@...eb.com>,
        "persaur@...il.com" <persaur@...il.com>,
        "Yoder, Stuart" <stuart.yoder@....com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        "michal.zygowski@...eb.com" <michal.zygowski@...eb.com>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        "lukasz@...rylko.pl" <lukasz@...rylko.pl>,
        linux-efi <linux-efi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        The development of GNU GRUB <grub-devel@....org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: Linux DRTM on UEFI platforms

On Wed, Mar 30, 2022 at 09:23:17AM +0200, Ard Biesheuvel wrote:
> On Wed, 30 Mar 2022 at 09:19, Matthew Garrett <mjg59@...f.ucam.org> wrote:
> > From a conceptual perspective we've thought of the EFI stub as being
> > logically part of the bootloader rather than the early kernel, and the
> > bootloader is a point where the line is drawn. My guy feeling is that
> > jumping into the secure kernel environment before EBS has been called is
> > likely to end badly.
> 
> If you jump back into the system firmware, sure.
> 
> But the point I was trying to make is that you can replace that with
> your own minimal implementation of EFI that just exposes a memory map
> and some protocols and nothing else, and then the secure launch kernel
> would be entirely in charge of the execution environment.

We can't just replace system firmware with an imitation of the same - 
for instance, configuring the cold boot prevention memory overwrite 
requires us to pass a variable through to the real firmware, and that's 
something that we do in the stub.