Re: [GIT PULL] Kernel lockdown for secure boot

From: Pavel Machek <pavel@ucw.cz>
To: David Howells <dhowells@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Andy Lutomirski <luto@kernel.org>,
	Matthew Garrett <mjg59@google.com>,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	James Morris <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>,
	Justin Forbes <jforbes@redhat.com>,
	linux-man <linux-man@vger.kernel.org>, joeyli <jlee@suse.com>,
	LSM List <linux-security-module@vger.kernel.org>,
	Linux API <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
Date: Sun, 8 Apr 2018 10:23:06 +0200	[thread overview]
Message-ID: <20180408082306.GB4965@amd> (raw)
In-Reply-To: <10718.1522798745@warthog.procyon.org.uk>

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On Wed 2018-04-04 00:39:05, David Howells wrote:
> Linus Torvalds <torvalds@linux-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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