linux-kernel - Re: [PATCH v3] powerpc: mm: support ARCH_MMAP_RND

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Date:   Thu, 13 Apr 2017 12:28:42 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:     Bhupesh Sharma <bhsharma@...hat.com>
Cc:     linuxppc-dev@...ts.ozlabs.org, kernel-hardening@...ts.openwall.com,
        linux-kernel@...r.kernel.org,
        Alistair Popple <alistair@...ple.id.au>,
        Anatolij Gustschin <agust@...x.de>,
        Kees Cook <keescook@...omium.org>,
        Daniel Cashman <dcashman@...roid.com>,
        Scott Wood <oss@...error.net>,
        Paul Mackerras <paulus@...ba.org>,
        Daniel Cashman <dcashman@...gle.com>,
        Bhupesh SHARMA <bhupesh.linux@...il.com>,
        Alexander Graf <agraf@...e.com>
Subject: Re: [PATCH v3] powerpc: mm: support ARCH_MMAP_RND_BITS



On Thursday 13 April 2017 12:22 PM, Bhupesh Sharma wrote:
> Hi Aneesh,
>
> On Thu, Apr 13, 2017 at 12:06 PM, Aneesh Kumar K.V
> <aneesh.kumar@...ux.vnet.ibm.com> wrote:
>> Bhupesh Sharma <bhsharma@...hat.com> 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@...hat.com>
>>> Reviewed-by: Kees Cook <keescook@...omium.org>
>>> ---
>>> * 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
>

That is getting update to 128TB by default and conditionally to 512TB

-aneesh