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

[Bhupesh Sharma]

On Fri, Feb 10, 2017 at 4:31 PM, Michael Ellerman <mpe at ellerman.id.au> wrote:
> Bhupesh Sharma <bhsharma at redhat.com> writes:
>
>> HI Michael,
>>
>> On Thu, Feb 2, 2017 at 3:53 PM, Michael Ellerman <mpe at ellerman.id.au> wrote:
>>> Bhupesh Sharma <bhsharma at redhat.com> writes:
>>>
>>>> powerpc: arch_mmap_rnd() 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.
>>>>
>>>> This value represents a compromise between increased
>>>> ASLR effectiveness and avoiding address-space fragmentation.
>>>> Replace it with a Kconfig option, which is sensibly bounded, so that
>>>> platform developers may choose where to place this compromise.
>>>> Keep default values as new minimums.
>>>>
>>>> This patch makes sure that now powerpc mmap arch_mmap_rnd() approach
>>>> is similar to other ARCHs like x86, arm64 and arm.
>>>
>>> Thanks for looking at this, it's been on my TODO for a while.
>>>
>>> I have a half completed version locally, but never got around to testing
>>> it thoroughly.
>>
>> Sure :)
>>
>>>> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
>>>> index a8ee573fe610..b4a843f68705 100644
>>>> --- a/arch/powerpc/Kconfig
>>>> +++ b/arch/powerpc/Kconfig
>>>> @@ -22,6 +22,38 @@ config MMU
>>>>       bool
>>>>       default y
>>>>
>>>> +config ARCH_MMAP_RND_BITS_MIN
>>>> +       default 5 if PPC_256K_PAGES && 32BIT
>>>> +       default 12 if PPC_256K_PAGES && 64BIT
>>>> +       default 7 if PPC_64K_PAGES && 32BIT
>>>> +       default 14 if PPC_64K_PAGES && 64BIT
>>>> +       default 9 if PPC_16K_PAGES && 32BIT
>>>> +       default 16 if PPC_16K_PAGES && 64BIT
>>>> +       default 11 if PPC_4K_PAGES && 32BIT
>>>> +       default 18 if PPC_4K_PAGES && 64BIT
>>>> +
>>>> +# max bits determined by the following formula:
>>>> +#  VA_BITS - PAGE_SHIFT - 4
>>>> +#  for e.g for 64K page and 64BIT = 48 - 16 - 4 = 28
>>>> +config ARCH_MMAP_RND_BITS_MAX
>>>> +       default 10 if PPC_256K_PAGES && 32BIT
>>>> +       default 26 if PPC_256K_PAGES && 64BIT
>>>> +       default 12 if PPC_64K_PAGES && 32BIT
>>>> +       default 28 if PPC_64K_PAGES && 64BIT
>>>> +       default 14 if PPC_16K_PAGES && 32BIT
>>>> +       default 30 if PPC_16K_PAGES && 64BIT
>>>> +       default 16 if PPC_4K_PAGES && 32BIT
>>>> +       default 32 if PPC_4K_PAGES && 64BIT
>>>> +
>>>> +config ARCH_MMAP_RND_COMPAT_BITS_MIN
>>>> +       default 5 if PPC_256K_PAGES
>>>> +       default 7 if PPC_64K_PAGES
>>>> +       default 9 if PPC_16K_PAGES
>>>> +       default 11
>>>> +
>>>> +config ARCH_MMAP_RND_COMPAT_BITS_MAX
>>>> +       default 16
>>>> +
>>>
>>> This is what I have below, which is a bit neater I think because each
>>> value is only there once (by defaulting to the COMPAT value).
>>>
>>> My max values are different to yours, I don't really remember why I
>>> chose those values, so we can argue about which is right.
>>
>> I am not sure how you derived these values, but I am not sure there
>> should be differences between 64-BIT x86/ARM64 and PPC values for the
>> MAX values.
>
> But your values *are* different to x86 and arm64.
>
> And why would they be the same anyway? x86 has a 47 bit address space,
> 64-bit powerpc is 46 bits, and arm64 is configurable from 36 to 48 bits.
>
> So your calculations above using VA_BITS = 48 should be using 46 bits.
>
> But if you fixed that, your formula basically gives 1/16th of the
> address space as the maximum range. Why is that the right amount?
>
> x86 uses 1/8th, and arm64 uses a mixture of 1/8th and 1/32nd (though
> those might be bugs).
>
> My values were more liberal, giving up to half the address space for 32
> & 64-bit. Maybe that's too generous, but my rationale was it's up to the
> sysadmin to tweak the values and they get to keep the pieces if it
> breaks.

I am not sure why would one want to use more than the practical limits
of 1/8th used by x86 - this causes additional burden of address space
fragmentation.

So we need to balance between the randomness increase and the address
space fragmentation.

>>> +config ARCH_MMAP_RND_BITS_MAX
>>> +       # On 64-bit up to 32T of address space (2^45)
>>> +       default 27 if 64BIT && PPC_256K_PAGES   # 256K (2^18), = 45 - 18 = 27
>>> +       default 29 if 64BIT && PPC_64K_PAGES    # 64K  (2^16), = 45 - 16 = 29
>>> +       default 31 if 64BIT && PPC_16K_PAGES    # 16K  (2^14), = 45 - 14 = 31
>>> +       default 33 if 64BIT                     # 4K   (2^12), = 45 - 12 = 33
>>> +       default ARCH_MMAP_RND_COMPAT_BITS_MAX
>
> I played with my values a bit and allowing 32T is a little bit nuts. It
> means you can actually end up with the adjusted ET_DYN_BASE *above* 32T,
> followed by the heap growing up, and the mmap base *below* 32T, growing
> down. Which is kinda fun, but definitely breaks a lot of assumptions.
>
> So limiting it to a max of 16T is probably more sensible.
>
> Anyway late here, will think about it some more over the weekend.

A user is always free to tweak the maximum values via specific Kconfig
+ defconfig combinations for their platforms, but why have such large
max values as default for say a embedded PPC64 board which only
supports say 16GB of DDR.

A default max of 33bits for such platforms might be an overkill, while
it might be fine for servers which might have greater DDR
availability.

Regards,
Bhupesh