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

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Message-ID: <20180408081050.GA4965@amd>
Date:   Sun, 8 Apr 2018 10:10:50 +0200
From:   Pavel Machek <pavel@....cz>
To:     Matthew Garrett <mjg59@...gle.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>, 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 2018-04-03 21:08:54, Matthew Garrett wrote:
> On Tue, Apr 3, 2018 at 2:01 PM Linus Torvalds
> <torvalds@...ux-foundation.org>
> wrote:
> 
> > On Tue, Apr 3, 2018 at 1:54 PM, Matthew Garrett <mjg59@...gle.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

Not true.

I have kernel with printk() enabled. Yes, once userland is started,
you can boot another kernel, maybe.

Maybe my kernel is locked down with exception of kexec, and it does
printk(KERN_CRIT "kexecing") followed by mdelay(5000). That's pretty
good security.

									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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