linux-kernel - Re: [PATCH 03/27] Enforce module signatures if the kernel is locked down

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Date:   Thu, 02 Nov 2017 18:18:43 -0400
From:   Mimi Zohar <zohar@...ux.vnet.ibm.com>
To:     David Howells <dhowells@...hat.com>
Cc:     linux-security-module@...r.kernel.org, gnomes@...rguk.ukuu.org.uk,
        linux-efi@...r.kernel.org, matthew.garrett@...ula.com,
        gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org,
        jforbes@...hat.com
Subject: Re: [PATCH 03/27] Enforce module signatures if the kernel is locked
 down

On Thu, 2017-11-02 at 22:01 +0000, David Howells wrote:
> Mimi Zohar <zohar@...ux.vnet.ibm.com> wrote:
> 
> > Right, it would never get here if the IMA signature verification
> > fails.  If sig_enforce is not enabled, then it will also work.  So the
> > only case is if sig_enforced is enabled and there is no key.
> > 
> > eg.
> >          else if (can_do_ima_check && is_ima_appraise_enabled())
> >                 err = 0;
> 
> I'm not sure where you want to put that, but I can't just do this:
> 
> 	/* Not having a signature is only an error if we're strict. */
> 	if (err == -ENOKEY && !sig_enforce &&
> 	    (!can_do_ima_check || !is_ima_appraise_enabled()) &&

The above IMA checks aren't needed here.

> 	    !kernel_is_locked_down("Loading of unsigned modules"))
> 		err = 0;
> 	else if (can_do_ima_check && is_ima_appraise_enabled())
> 		err = 0;
> 
> because that'll print out a message in lockdown mode saying that you're not
> allowed to do that and then maybe do it anyway.

Then at least for now, document that even though kernel modules might
be signed and verified by IMA-appraisal, that in lockdown mode they
also require an appended signature.

Mimi