Re: [resend Patch v3 1/2] kaslr: check if kernel location is changed

[Vivek Goyal]

On Mon, Oct 13, 2014 at 08:43:00AM -0700, H. Peter Anvin wrote:
> On 10/13/2014 08:19 AM, Vivek Goyal wrote:
> >>>
> >>> This really shouldn't have happened this way on x86-64.  It has to happen
> >>> this way on i386, but I worry that this may be a serious misdesign in kaslr
> >>> on x86-64.  I'm also wondering if there is any other fallout of this?
> >>
> >> I agree. On x86_64, we should stick to previous design and this new
> >> logic of performing relocations does not sound very clean and makes
> >> things very confusing.
> >>
> >> I am wondering that why couldn't we simply adjust page tables in case of
> >> kaslr on x86_64, instead of performing relocations.
> > 
> > Well, IIUC, if virtual addresses are shifted w.r.t what virtual address
> > kernel was compiled for, then relocation will have to be done.
> > 
> > So question will be if physical address shift is enough for kaslr or
> > virtual address shift is necessary.
> > 
> 
> I would assume that without a virtual address shift kaslr is pretty darn
> pointless.  Without the physical address shift the 1:1 map can be used,
> and again, kaslr becomes pointless.  However, there is absolutely no
> reason why they should be coupled.  They can, in fact, be independently
> randomized.

Agreed. On x86_64, we should be able to randomize virtual address space
and physical address space independently. And in that case whole of
the physical memory should be available for a possible location for
kernel. (As opposed to a small limit (I guess 1GB) now)

Thanks
Vivek
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