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

Bhupesh Sharma bhsharma at redhat.com
Fri Feb 3 05:21:49 AEDT 2017

Hi Balbir,

On Thu, Feb 2, 2017 at 12:14 PM, Balbir Singh <bsingharora at gmail.com> wrote:
> On Thu, Feb 02, 2017 at 11:12:46AM +0530, Bhupesh Sharma 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.
> From what I see we get 28 bits of entropy right for 64k pages
> bits as compared to 14 bits earlier?

That's correct. We can go upto 28-bits of entropy for 64BIT platforms
using 64K pages with the current approach. I see arm64 using > 28 bits
of entropy randomness in some cases, but I think 28-bit MAX entropy is
sensible for 64BIT/64K combination on PPC.

>> 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).
> This helps PIE executables as such and leaves other not impacted?

It basically affects all shared object files (as noted in [1]).
However as Kees noted in one of his reviews, I think this 2nd patch
might not be needed for all generic ppc platforms.

[1] http://lxr.free-electrons.com/source/arch/powerpc/include/asm/elf.h#L26.


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