[U-Boot] [PATCH 2/2] arm64: booti: allow to place kernel image anywhere in physical memory

[Mark Rutland <mark.rutland@arm.com>]

On Tue, Mar 07, 2017 at 07:16:56AM -0500, Tom Rini wrote:
> On Tue, Mar 07, 2017 at 11:43:52AM +0000, Mark Rutland wrote:
> > On Tue, Feb 28, 2017 at 12:15:09PM -0500, Tom Rini wrote:
> > > On Wed, Mar 01, 2017 at 02:03:58AM +0900, Masahiro Yamada wrote:
> > > > 2017-02-27 7:41 GMT+09:00 Tom Rini <trini@konsulko.com>:
> > > > If we put the image at 2MiB aligned base, the relocation would
> > > > always happen.
> > > 
> > > Correct.  But I honestly don't know if non-randomized text offset is the
> > > common case people will optimize for or randomized for added security will be
> > > the more common case.  
> > 
> > FWIW, the randomized text_offset is a bootloader debugging/testing
> > feature, and there's no security aspect to it.
> > 
> > It was added [1] as an additional to hint to bootloader authors that
> > they must respect the text_offset field.
> 
> Right, and we do this today.  But since this doubles as a kind of cheap
> KASLR I would also expect to see it used, even if not intended, in this
> way.

I can certainly imagine people loading the kernel at a random physical
base address (i.e. a random 2M base + text_offset), and doing that's
perfectly fine for kernels happy to be loaded at arbitrary bases. That
may help to frustrate some DMA attacks.

I take it that's what you meant?

Given text_offset itself is fixed at compile time, randomizing it
provides absolutely no security benefit, and we should be careful not to
give the impression that it does.

Thanks,
Mark.