linux-kernel - Re: [GIT PULL] Kernel lockdown for secure boot

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Date:   Tue, 03 Apr 2018 22:51:11 +0000
From:   Matthew Garrett <mjg59@...gle.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     luto@...nel.org, David Howells <dhowells@...hat.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>, jmorris@...ei.org,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        jforbes@...hat.com, linux-man@...r.kernel.org, jlee@...e.com,
        LSM List <linux-security-module@...r.kernel.org>,
        linux-api@...r.kernel.org, Kees Cook <keescook@...omium.org>,
        linux-efi <linux-efi@...r.kernel.org>
Subject: Re: [GIT PULL] Kernel lockdown for secure boot

On Tue, Apr 3, 2018 at 3:46 PM Linus Torvalds
<torvalds@...ux-foundation.org>
wrote:

> For example, I love signed kernel modules. The fact that I love them
> has absolutely zero to do with secure boot, though. There is
> absolutely no linkage between the two issues: I use (self-)signed
> kernel modules simply because I think it's a good thing in general.

> The same thing is true of some lockdown patch. Maybe it's a good thing
> in general. But whether it's a good thing is _entirely_ independent of
> any secure boot issue. I can see using secure boot without it, but I
> can very much also see using lockdown without secure boot.

> The two things are simply entirely orthogonal. They have _zero_
> overlap. I'm not seeing why they'd be linked at all in any way.

Lockdown is clearly useful without Secure Boot (and I intend to deploy it
that way for various things), but I still don't understand why you feel
that the common case of booting a kernel from a boot chain that's widely
trusted derives no benefit from it being harder to subvert that kernel into
subverting that boot chain. For cases where you're self-signing and feel
happy about that, you just set CONFIG_LOCK_DOWN_IN_EFI_SECURE_BOOT to n and
everyone's happy?