[PATCH 0/2] RFC: Adjust powerpc ASLR elf randomness

[Kees Cook]

On Wed, Feb 1, 2017 at 9:42 PM, Bhupesh Sharma <bhsharma at redhat.com> wrote:
> This RFC patchset tries to make the powerpc ASLR elf randomness
> implementation similar to other ARCHs (like x86).
>
> The 1st patch introduces the support of ARCH_MMAP_RND_BITS in powerpc
> mmap implementation to allow a sane balance between increased randomness
> in the mmap address of ASLR elfs and increased address space
> fragmentation.
>
> The 2nd patch increases the ELF_ET_DYN_BASE value from the current
> hardcoded value of 0x2000_0000 to something more practical,
> i.e. TASK_SIZE - PAGE_SHIFT (which makes sense especially for
> 64-bit platforms which would like to utilize more randomization
> in the load address of a PIE elf).

I don't think you want this second patch. Moving ELF_ET_DYN_BASE to
the top of TASK_SIZE means you'll be constantly colliding with stack
and mmap randomization. 0x20000000 is way better since it randomizes
up from there towards the mmap area.

Is there a reason to avoid the 32-bit memory range for the ELF addresses?

-Kees

-- 
Kees Cook
Pixel Security