linux-kernel - Re: [PATCH 4/9] KEYS: Allow unrestricted boot-time addition of keys to secondary keyring

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Date:   Thu, 17 Nov 2016 11:18:31 +0000
From:   David Howells <dhowells@...hat.com>
To:     Petko Manolov <petkan@...-labs.com>
Cc:     dhowells@...hat.com, keyrings@...r.kernel.org,
        matthew.garrett@...ula.com, linux-security-module@...r.kernel.org,
        linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org,
        Mimi Zohar <zohar@...ux.vnet.ibm.com>
Subject: Re: [PATCH 4/9] KEYS: Allow unrestricted boot-time addition of keys to secondary keyring

Petko Manolov <petkan@...-labs.com> wrote:

> > > Well, I for one do not explicitly trust these keys.  I may even want to 
> > > completely remove or replace them.
> > 
> > Fine be me.  However, if you remove them all I would guess that you cannot 
> > perform a secure boot.
> 
> Maybe not on PC, but there's plenty of other architectures out there.  What
> if i replace all UEFI keys with my own?

Then I would imagine that you can do a secure boot, but that you have to sign
your own shim, grub, kernel, etc..

> > Note that it's to be expected that the keys being loaded from the UEFI
> > database cannot have their signatures checked - which is why they would
> > have to be implicitly trusted.  For the same reason, the kernel does not
> > check the signatures on the keys compiled into the kernel image.
> 
> I build all kernels that matter to me and i _do_ trust myself.
> Unfortunately i can't say the same for any corporation out there.
> 
> Trusting a key because your vendor shipped the HW with it so that you have no 
> way to verify the signature is pretty weak argument IMHO.

I'm not making an argument there.  There is a reason I think that I can't
check them.  Well, possibly I could *if* those keys are actually signed *and*
I have certs built into the kernel by which I can verify all those keys in
UEFI variables.  I don't know whether this is actually practical.

> > You can argue this either way.  There's a config option to allow you to
> > turn this on or off.  Arguably, this should be split in two: one for the
> > whitelist (db, MokListRT) and one for the blacklist (dbx).
> 
> I did not see the config option.  There is one?

See patch 8 where these variables are actually parsed.  CONFIG_LOAD_UEFI_KEYS
is available there.

David