linux-kernel - Re: An actual suggestion (Re: [GIT PULL] Kernel lockdown for secure boot)

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Message-Id: <1522936869.16421.63.camel@linux.vnet.ibm.com>
Date:   Thu, 05 Apr 2018 10:01:09 -0400
From:   Mimi Zohar <zohar@...ux.vnet.ibm.com>
To:     joeyli <jlee@...e.com>, David Howells <dhowells@...hat.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Theodore Y. Ts'o" <tytso@....edu>,
        Matthew Garrett <mjg59@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        James Morris <jmorris@...ei.org>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Justin Forbes <jforbes@...hat.com>,
        linux-man <linux-man@...r.kernel.org>,
        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: An actual suggestion (Re: [GIT PULL] Kernel lockdown for secure
 boot)

On Thu, 2018-04-05 at 10:16 +0800, joeyli wrote:
> Hi David, 
> 
> On Wed, Apr 04, 2018 at 05:17:24PM +0100, David Howells wrote:
> > Andy Lutomirski <luto@...nel.org> wrote:
> > 
> > > Since this thread has devolved horribly, I'm going to propose a solution.
> > > 
> > > 1. Split the "lockdown" state into three levels:  (please don't
> > > bikeshed about the names right now.)
> > > 
> > > LOCKDOWN_NONE: normal behavior
> > > 
> > > LOCKDOWN_PROTECT_INTEGREITY: kernel tries to keep root from writing to
> > > kernel memory
> > > 
> > > LOCKDOWN_PROTECT_INTEGRITY_AND_SECRECY: kernel tries to keep root from
> > > reading or writing kernel memory.
> > 
> > In theory, it's good idea, but in practice it's not as easy to implement as I
> > think you think.
> > 
> > Let me list here the things that currently get restricted by lockdown:
> > 
> [...snip]
> >  (5) Kexec.
> >
> 
> About IMA with kernel module signing and kexec(not on x86_64 yet)...

Only carrying the measurement list across kexec is architecture
specific, but everything else should work.  

> Because IMA can be used to verify the integrity of kernel module or even
> the image for kexec. I think that the
> IMA_KEYRINGS_PERMIT_SIGNED_BY_BUILTIN_OR_SECONDARY must be enabled at runtime
> when kernel is locked-down.

I think we need to understand the problem a bit better.  Is the
problem that you're using the secondary keyring and loading the UEFI
keys onto the secondary keyring?

> Because the root can enroll master key to keyring then IMA trusts the ima key
> derived from master key. It causes that the arbitrary signed module can be loaded
> when the root compromised.

With only the builtin keyring, only keys signed by a builtin key can
be added to the IMA keyring.

Mimi