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Date:   Tue, 3 Apr 2018 22:21:02 +0100
From:   Al Viro <viro@ZenIV.linux.org.uk>
To:     Matthew Garrett <mjg59@google.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>, luto@kernel.org,
        David Howells <dhowells@redhat.com>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>, jmorris@namei.org,
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        linux-api@vger.kernel.org, Kees Cook <keescook@chromium.org>,
        linux-efi <linux-efi@vger.kernel.org>
Subject: Re: [GIT PULL] Kernel lockdown for secure boot
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On Tue, Apr 03, 2018 at 09:08:54PM +0000, Matthew Garrett wrote:

> > 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.

That assumes you *can* turn that shit off.  On the hardware where manufacturer
has installed firmware that doesn't allow that SB is a misfeature that has
to be worked around.  Making that harder might improve the value of SB to
said manufacturers, but what's the benefit for everybody else?