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

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Message-ID: <CALCETrWDtRQ1M=JGuB-6E6aBgO9S3+43AFKCq2AyftDutYcuNQ@mail.gmail.com>
Date:   Tue, 3 Apr 2018 16:48:31 -0700
From:   Andy Lutomirski <luto@...nel.org>
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 Tue, Apr 3, 2018 at 4:39 PM, David Howells <dhowells@...hat.com> 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.

That's simply not true.  A sensible verified boot chain (a la Chrome
OS) is likely to load, as one verified chunk, a kernel and initramfs.
Then initramfs can flip on lockdown all by itself before it enables
networking or any other attack vectors.