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

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Message-ID: <20180408082306.GB4965@amd>
Date:   Sun, 8 Apr 2018 10:23:06 +0200
From:   Pavel Machek <pavel@....cz>
To:     David Howells <dhowells@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Matthew Garrett <mjg59@...gle.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        James Morris <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>,
        Justin Forbes <jforbes@...hat.com>,
        linux-man <linux-man@...r.kernel.org>, joeyli <jlee@...e.com>,
        LSM List <linux-security-module@...r.kernel.org>,
        Linux API <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 Wed 2018-04-04 00:39:05, David Howells wrote:
> Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
> > 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.
> 
> I'm not sure I agree.  Here's my reasoning:
> 
>  (1) Lockdown mode really needs to activated during kernel boot, before
>      userspace has a chance to run, otherwise there's a window of opportunity
>      in which the kernel *isn't* locked down.
> 
>  (2) If the kernel isn't booted in secure boot mode, then there's the
>      opportunity to tamper before the kernel even starts booting.
> 
>  (3) There doesn't seem any point in booting in secure boot mode if you don't
>      protect the running kernel image against tampering.  What does it mean to
>      be in "secure boot mode" in that case?  If the kernel can be tampered
>      with, it would seem to be, by definition, insecure.

This one is not true, either.

If kernel does "printk(KERN_CRIT "loading unsigned module");
mdelay(10000);", it is useful for secure boot and provides way to
owner to play.

Nokia N9 / N950 uses this kind of "security" for example. It is rather
annoying but better than not being able to run custom kernels at all.
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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