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

jlee
2018-04-05T16:11:24Z

Hi Mimi,

On Thu, Apr 05, 2018 at 10:01:09AM -0400, Mimi Zohar wrote:
> 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@kernel.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?
>

Sorry for my mistake. I want to write INTEGRITY_TRUSTED_KEYRING in
above but not IMA_KEYRINGS_PERMIT_SIGNED_BY_BUILTIN_OR_SECONDARY.

My brain is not clear when writing the mail.

> > 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.
>

Thanks for your description. I saw that the IMA_LOAD_X509 already depends
on IMA_TRUSTED_KEYRING (INTEGRITY_TRUSTED_KEYRING). Please ignore my concern.

Thanks a lot!
Joey Lee