Re: [PATCH] efi_loader: Get rid of kaslr-seed

[Mark Kettenis <mark.kettenis@xs4all.nl>]

> From: Ard Biesheuvel <ardb@kernel.org>
> Date: Thu, 16 Dec 2021 16:28:06 +0100
> 
> On Thu, 16 Dec 2021 at 16:25, Mark Kettenis <mark.kettenis@xs4all.nl> wrote:
> >
> > > From: Ilias Apalodimas <ilias.apalodimas@linaro.org>
> > > Date: Thu, 16 Dec 2021 16:52:08 +0200
> > >
> > > Right now we unconditionally pass a 'kaslr-seed' property to the kernel
> > > if the DTB we ended up in EFI includes the entry.  However the kernel
> > > EFI stub completely ignores it and only relies on EFI_RNG_PROTOCOL.
> > > So let's get rid of it unconditionally since it would mess up the
> > > (upcoming) DTB TPM measuring as well.
> >
> > NAK
> >
> > OpenBSD uses the kaslr-seed property in the bootloader to mix in some
> > additional entropy.  (It will also use EFI_RNG_PROTOCOL if it is
> > avilable, but most U-Boot boards don't provide that, or at least not
> > yet).
> >
> 
> What is the point of using both the DT property and the protocol if
> both are available?

Unless kaslr-seed is coming from a different entropy source, there
probably isn't a point.  But it doesn't hurt and it made the
bootloader code simpler.

It does mean there is some room for compromise though.  If U-Boot
would only remove kaslr-seed if it implements EFI_RNG_PROTOCOL it
wouldn't be a problem.

> > Even on Linux the EFI stub isn't the only way to load a Linux kernel.
> > You can use a conventional EFI bootloader like grub.
> >
> 
> No, you cannot, at least not on architectures other than x86. GRUB on
> ARM always boots via the EFI stub.

Ok.  It isn't immediately clear from the documentation that this is
the case.  It would still be possible to write such a bootloader, but
if it isn't a thing, it isn't a thing.  But not all the world is
Linux.