Re: [PATCH 3/4] powerpc: Reduce ELF_ET_DYN_BASE

[Kees Cook]

On Fri, Jun 23, 2017 at 12:01 AM, Michael Ellerman <mpe@xxxxxxxxxxxxxx> wrote:
> Kees Cook <keescook@xxxxxxxxxxxx> writes:
>
>> Now that explicitly executed loaders are loaded in the mmap region,
>> position PIE binaries lower in the address space to avoid possible
>> collisions with mmap or stack regions. For 64-bit, align to 4GB to
>> allow runtimes to use the entire 32-bit address space for 32-bit
>> pointers.
>
> The change log and subject are a bit out of whack with the actual patch
> because previously we used 512MB.
>
> How about?
>
>   powerpc: Move ELF_ET_DYN_BASE to 4GB / 4MB
>
>   Now that explicitly executed loaders are loaded in the mmap region,
>   we have more freedom to decide where we position PIE binaries in the
>   address space to avoid possible collisions with mmap or stack regions.
>
>   For 64-bit, align to 4GB to allow runtimes to use the entire 32-bit
>   address space for 32-bit pointers. On 32-bit use 4MB.

Good idea, thanks. I'll resend the series with the commit logs updated.

> Is there any particular reasoning behind the 4MB value on 32-bit?

So, I've dug around a bit on this, and I *think* the rationale is to
avoid mapping a possible 4MB page table entry when it won't be using
at least a portion near the lower end (NULL address area covered
blocked by mmap_min_addr). It seems to be mainly tradition, though.

> I gave this a quick spin and it booted OK on all my test boxes, which
> covers 64-bit/32-bit kernel and userspace. So seems to work!

Awesome, thanks for the testing!

> Acked-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
>
> cheers

-Kees

-- 
Kees Cook
Pixel Security