[kernel-hardening] Re: [PATCH 1/2] powerpc: mm: support ARCH_MMAP_RND_BITS

[Michael Ellerman]

Balbir Singh <bsingharora at gmail.com> writes:

> On Thu, Feb 02, 2017 at 09:23:33PM +1100, Michael Ellerman wrote:
>> +config ARCH_MMAP_RND_BITS_MIN
>> +	# On 64-bit up to 1G of address space (2^30)
>> +	default 12 if 64BIT && PPC_256K_PAGES 	# 256K (2^18), = 30 - 18 = 12
>> +	default 14 if 64BIT && PPC_64K_PAGES  	# 64K  (2^16), = 30 - 16 = 14
>> +	default 16 if 64BIT && PPC_16K_PAGES  	# 16K  (2^14), = 30 - 14 = 16
>> +	default 18 if 64BIT			# 4K   (2^12), = 30 - 12 = 18
>> +	default ARCH_MMAP_RND_COMPAT_BITS_MIN
>> +
>> +config ARCH_MMAP_RND_BITS_MAX
>> +	# On 64-bit up to 32T of address space (2^45)
>
> I thought it was 64T, TASK_SIZE_USER64 is 2^46?

The virtual address space is 64T. The comment is talking about how much
can be taking up by the randomisation factor.

cheers