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

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Date:   Wed, 15 Aug 2018 14:57:02 -0700
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     Vivek Goyal <vgoyal@...hat.com>
Cc:     Yannik Sembritzki <yannik@...britzki.me>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        David Howells <dhowells@...hat.com>,
        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 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

On Wed, 2018-08-15 at 17:52 -0400, Vivek Goyal wrote:
> On Wed, Aug 15, 2018 at 02:13:17PM -0700, James Bottomley wrote:
> > On Wed, 2018-08-15 at 23:08 +0200, Yannik Sembritzki wrote:
> > > On 15.08.2018 22:47, Linus Torvalds wrote:
> > > > It basically says: we don't allow modules that weren't built
> > > > with
> > > > the kernel. Adding a new key later and signing a module with it
> > > > violates that premise.
> > > 
> > > Considering the following scenario:
> > > A user is running a distro kernel, which is built by the distro,
> > > and has the distro signing key builtin (i.e. fedora). Now, the
> > > user has taken ownership of their system and provisioned their
> > > own platform key. Accordingly, the user signs the distro kernel
> > > with their own key.
> > > 
> > > If I understand you correctly, modules signed by the users own
> > > key, but not signed with the distro key, will stop working in
> > > this case?
> > 
> > They never actually would have worked, but yes.
> > 
> > > IMO, this is not okay. The layer of trust should extend from the
> > > bottom (user-provisioned platform key) up. Only trusting the
> > > kernel builtin key later on (wrt. kernel modules) contradicts
> > > this principal.
> > 
> > The kernel can't tell whether the UEFI user has taken ownership or
> > not so it has no basis on which to make a decision to trust the
> > UEFI keys or not, so we should *always* not trust them.
> > 
> > Consider a UEFI system for which a user has taken ownership, but
> > which has some signed ROMs which are UEFI secure boot
> > verified.  Simply to get their system to boot the user will be
> > forced to add the ODM key to the UEFI db ... and I'm sure in that
> > situation the user wouldn't want to trust the ODM key further than
> > booting.
> 
> IIUC, it is fine to trust these ODM keys, User keys and "foo" keys
> for loading kernel but not for modules?

It's fine to trust the secure boot keys for the boot environment.  If
you argue kexec is linux booting linux then yes, that's a supported
use.

>  If yes, then atleast we can enable trusting keys in
> .secondary_trusted_keys keyring for kernel signature verificaton and
> that will solve the kexec/kdump issue on distribution kernels.

I think it's OK ... I can't think of any reason you'd want a signed
kernel to boot but not to be able to kexec to a kernel with the same
signer.

James