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From:   Matthew Garrett <mjg59@google.com>
Date:   Tue, 03 Apr 2018 21:08:54 +0000
Message-ID: <CACdnJuvheCbuvTLyDWMQWcUytc4EvuSUaEKywfEX8Kp6qYL9dg@mail.gmail.com>
Subject: Re: [GIT PULL] Kernel lockdown for secure boot
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     luto@kernel.org, David Howells <dhowells@redhat.com>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>, jmorris@namei.org,
        Alan Cox <gnomes@lxorguk.ukuu.org.uk>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        jforbes@redhat.com, linux-man@vger.kernel.org, jlee@suse.com,
        LSM List <linux-security-module@vger.kernel.org>,
        linux-api@vger.kernel.org, Kees Cook <keescook@chromium.org>,
        linux-efi <linux-efi@vger.kernel.org>
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On Tue, Apr 3, 2018 at 2:01 PM Linus Torvalds
<torvalds@linux-foundation.org>
wrote:

> On Tue, Apr 3, 2018 at 1:54 PM, Matthew Garrett <mjg59@google.com> wrote:
> >
> >> .. maybe you don't *want* secure boot, but it's been pushed in your
> >> face by people with an agenda?
> >
> > Then turn it off, or build a self-signed kernel that doesn't do this?

> Umm. So you asked a question, and then when you got an answer you said
> "don't do that then".

> The fact is, some hardware pushes secure boot pretty hard. That has
> *nothing* to do with some "lockdown" mode.

Secure Boot ensures that the firmware will only load signed bootloaders. If
a signed bootloader loads a kernel that's effectively an unsigned
bootloader, there's no point in using Secure Boot - you should just turn it
off instead, because it's not giving you any meaningful security. Andy's
example gives a scenario where by constraining your *userland* sufficiently
you can get close to having the same guarantees, but that involves you
having a read-only filesystem and takes you even further away from having a
general purpose computer.

If you don't want Secure Boot, turn it off. If you want Secure Boot, use a
kernel that behaves in a way that actually increases your security.