Re: [PATCH v3] powerpc: mm: support ARCH_MMAP_RND_BITS

[Bhupesh Sharma]

Hi Aneesh,

On Thu, Apr 13, 2017 at 12:06 PM, Aneesh Kumar K.V
<aneesh.kumar@xxxxxxxxxxxxxxxxxx> wrote:
> Bhupesh Sharma <bhsharma@xxxxxxxxxx> writes:
>
>> powerpc arch_mmap_rnd() currently uses hard-coded values - (23-PAGE_SHIFT) for
>> 32-bit and (30-PAGE_SHIFT) for 64-bit, to generate the random offset
>> for the mmap base address for a ASLR ELF.
>>
>> This patch makes sure that powerpc mmap arch_mmap_rnd() implementation
>> is similar to other ARCHs (like x86, arm64) and uses mmap_rnd_bits
>> and helpers to generate the mmap address randomization.
>>
>> The maximum and minimum randomization range values represent
>> a compromise between increased ASLR effectiveness and avoiding
>> address-space fragmentation.
>>
>> Using the Kconfig option and suitable /proc tunable, platform
>> developers may choose where to place this compromise.
>>
>> Also this patch keeps the default values as new minimums.
>>
>> Signed-off-by: Bhupesh Sharma <bhsharma@xxxxxxxxxx>
>> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
>> ---
>> * Changes since v2:
>> v2 can be seen here (https://patchwork.kernel.org/patch/9551509/)
>>     - Changed a few minimum and maximum randomization ranges as per Michael's suggestion.
>>     - Corrected Kees's email address in the Reviewed-by line.
>>     - Added further comments in kconfig to explain how the address ranges were worked out.
>>
>> * Changes since v1:
>> v1 can be seen here (https://lists.ozlabs.org/pipermail/linuxppc-dev/2017-February/153594.html)
>>     - No functional change in this patch.
>>     - Dropped PATCH 2/2 from v1 as recommended by Kees Cook.
>>
>>  arch/powerpc/Kconfig   | 44 ++++++++++++++++++++++++++++++++++++++++++++
>>  arch/powerpc/mm/mmap.c |  7 ++++---
>>  2 files changed, 48 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
>> index 97a8bc8..84aae67 100644
>> --- a/arch/powerpc/Kconfig
>> +++ b/arch/powerpc/Kconfig
>> @@ -22,6 +22,48 @@ config MMU
>>       bool
>>       default y
>>
>> +# min bits determined by the following formula:
>> +# VA_BITS - PAGE_SHIFT - CONSTANT
>> +# where,
>> +#    VA_BITS = 46 bits for 64BIT and 4GB - 1 Page = 31 bits for 32BIT
>
>
> Where did we derive that 46 bits from ? is that based on TASK_SIZE ?

Yes. It was derived from TASK_SIZE :
http://lxr.free-electrons.com/source/arch/powerpc/include/asm/processor.h#L105

Regards,
Bhupesh

>
>> +#    CONSTANT = 16 for 64BIT and 8 for 32BIT
>> +config ARCH_MMAP_RND_BITS_MIN
>> +       default 5 if PPC_256K_PAGES && 32BIT  # 31 - 18 - 8 = 5
>> +       default 7 if PPC_64K_PAGES && 32BIT   # 31 - 16 - 8 = 7
>> +       default 9 if PPC_16K_PAGES && 32BIT   # 31 - 14 - 8 = 9
>> +       default 11 if PPC_4K_PAGES && 32BIT   # 31 - 12 - 8 = 11
>> +       default 12 if PPC_256K_PAGES && 64BIT # 46 - 18 - 16 = 12
>> +       default 14 if PPC_64K_PAGES && 64BIT  # 46 - 16 - 16 = 14
>> +       default 16 if PPC_16K_PAGES && 64BIT  # 46 - 14 - 16 = 16
>> +       default 18 if PPC_4K_PAGES && 64BIT   # 46 - 12 - 16 = 18
>> +
>> +# max bits determined by the following formula:
>
>
> -aneesh
>