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From:   Matthew Garrett <mjg59@google.com>
Date:   Tue, 03 Apr 2018 23:59:40 +0000
Message-ID: <CACdnJuvqQQ+hidokY5GA7g-yrAs5-358it0cs+m_d4RFDMuB_w@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 4:55 PM Linus Torvalds
<torvalds@linux-foundation.org>
wrote:

> On Tue, Apr 3, 2018 at 4:45 PM, Matthew Garrett <mjg59@google.com> wrote:
> >> Be honest now. It wasn't generally users who clamored for it.
> >
> > If you ask a user whether they want a system that lets an attacker
replace
> > their kernel or one that doesn't, what do you think their answer is
likely
> > to be?

> Goddamnit.

> We both know what the answer will be.

> And it will have *nothing* to do with secure boot.

Right, because they care about outcome rather than mechanism. Secure Boot
is the mechanism we have to make that outcome possible.

> > Again, what is your proposed mechanism for ensuring that off the shelf
> > systems can be configured in a way that makes this possible?

> If you think lockdown is a good idea, and you enabled it, then IT IS
ENABLED.

Ok. So we can build distribution kernels that *always* have this on, and to
turn it off you have to disable Secure Boot and install a different kernel.
Or we can build distribution kernels that only have this on when you're
booting in a context that makes sense, and you can disable it by just
disabling Secure Boot (by running mokutil --disable-validation) and not
have to install a new kernel. Which outcome do you prefer?