Re: [GIT PULL] Kernel lockdown for secure boot

From: Matthew Garrett
Date: Wed Apr 04 2018 - 12:27:03 EST

On Tue, Apr 3, 2018 at 11:56 PM Peter Dolding <oiaohm@xxxxxxxxx> wrote:
> On Wed, Apr 4, 2018 at 11:13 AM, Matthew Garrett <mjg59@xxxxxxxxxx> wrote:

> > There are four cases:
> >
> > Verified Boot off, lockdown off: Status quo in distro and mainline
> > Verified Boot off, lockdown on: Perception of security improvement
> > trivially circumvented (and so bad)
> > Verified Boot on, lockdown off: Perception of security improvement
> > trivially circumvented (and so bad), status quo in mainline kernels
> > Verified Boot on, lockdown on: Security improvement, status quo in
> > kernels
> >
> > Of these four options, only two make sense. The most common
> > of Verified Boot on x86 platforms is UEFI Secure Boot,

> Stop right there. Verified boot does not have to be UEFI secureboot.
> You could be using a uboot verified boot or
> google vboot.
> Neither of these provide flags to kernel to say they have been
> performed.

They can be modified to set the appropriate bit in the bootparams - the
reason we can't do that in the UEFI case is that Linux can be built as a
UEFI binary that the firmware execute directly, and so the firmware has no
way to set that flag.

> Now Verified Boot on, lockdown off. Insanely this can be required in
> diagnostic on some embedded platform because EFI secureboot does not
> have a off switch. These are platforms where they don't boot if
> they don't have a PK and KEK set installed. Yes some of these is jtag
> the PK and KEK set in.

> The fact that this Verified Boot on, lockdown off causes trouble
> points to a clear problem. User owns the hardware they should have
> the right to defeat secureboot if they wish to.

Which is why Shim allows you to disable validation if you prove physical
user presence.