Re: [PATCH v3 16/19] x86, kaslr: Randomize physical and virtual address of kernel separately

[Baoquan He]

On 03/09/16 at 10:07am, Kees Cook wrote:
> On Wed, Mar 9, 2016 at 5:40 AM, Baoquan He <bhe@xxxxxxxxxx> wrote:
> > On 03/08/16 at 10:24am, Kees Cook wrote:
> >> On Mon, Mar 7, 2016 at 9:34 PM, Baoquan He <bhe@xxxxxxxxxx> wrote:
> >> >> It seems like CONFIG_RANDOMIZE_BASE_MAX_OFFSET should have been
> >> >> eliminated when the on-demand page table code was added. Once that was
> >> >> added, there's no physical max any more. And virtual randomization
> >> >> should have no max at all.
> >> > For physically random, yes, CONFIG_RANDOMIZE_BASE_MAX_OFFSET is not
> >> > needed anymore. But for virtually random,
> >> > CONFIG_RANDOMIZE_BASE_MAX_OFFSET is still mandatory since kernel text
> >> > mapping and kernel module mapping share the 2G virtual address space as
> >> > follows. Though kaslr increase kernel text mapping from 512M to 1G, it's
> >> > still limited, can't exceed CONFIG_RANDOMIZE_BASE_MAX_OFFSET.
> >> >
> >> > [0xffffffff80000000, 0xffffffffffffffff]
> >> >
> >> > But now as you suggested, I would like to change
> >> > CONFIG_RANDOMIZE_BASE_MAX_OFFSET to another name because it's only
> >> > valid for virtual randomization. A more specific name is better.
> >>
> >> Yes, right, the virtual has a 1G max, but I meant that it doesn't need
> >> to be a CONFIG item any more. Physical can use physical memory max as
> >> its max, and virtual max can now be calculated from the existing text
> >> mapping size.
> >
> > Got it, it should be KERNEL_IMAGE_SIZE instead, right?
> 
> Yup, that should be right. And removing
> CONFIG_RANDOMIZE_BASE_MAX_OFFSET will be a nice clean up, actually. It
> can be its own patch separate from the rest of the changes. The config
> was likely never needed (the Kconfigs already default to max values),
> but I had it there out of a desire to make everything configurable
> early on when it was a new feature.

OK, will check code and make a separate patch to fix this as you
suggested. Thanks.

Baoquan