linux-kernel - Re: [PATCH] Fix kexec forbidding kernels signed with custom platform keys to boot

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Message-ID: <25236.1534430630@warthog.procyon.org.uk>
Date:   Thu, 16 Aug 2018 15:43:50 +0100
From:   David Howells <dhowells@...hat.com>
To:     James Bottomley <James.Bottomley@...senPartnership.com>
Cc:     dhowells@...hat.com, Vivek Goyal <vgoyal@...hat.com>,
        Yannik Sembritzki <yannik@...britzki.me>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Peter Anvin <hpa@...or.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Dave Young <dyoung@...hat.com>, Baoquan He <bhe@...hat.com>,
        "Justin M. Forbes" <jforbes@...hat.com>,
        Peter Jones <pjones@...hat.com>,
        Matthew Garrett <mjg59@...gle.com>
Subject: Re: [PATCH] Fix kexec forbidding kernels signed with custom platform keys to boot

James Bottomley <James.Bottomley@...senPartnership.com> wrote:

> I've told you several times you can't use the secure boot keys for any form
> of trust beyond boot,

Yes - and you've been told several times that you're wrong.

As far as I can tell, you seem to think that whilst keys from the UEFI storage
could be used to verify a hacked module, they couldn't be used to verify a
hacked boot-time component (shim, grub, kernel, etc.).

However, if you can load a hacked module, you can very likely replace the
shim, say, with a hacked one.  In fact, replacing the shim may be easier
because modules are tied to their parent kernel in other ways besides the
signing key, whereas a shim must be standalone.

I will grant, however, that it I can understand a desire to reduce the attack
surface by not trusting the UEFI keys beyond booting - but then you shouldn't
use them for kexec *either*.

> Personally, I don't see any use for the UEFI keys in the kernel beyond
> kexec

Allowing you to load the NVidia module, say, into the kernel without the
distribution having to build it in with the kernel.

David