Re: [PATCH v3 1/4] mm: mmap: Add new /proc tunable for mmap_base ASLR.

Daniel Cashman
2015-11-25T19:36:55Z

On 11/24/2015 08:40 PM, Michael Ellerman wrote:
> On Wed, 2015-11-18 at 15:20 -0800, Daniel Cashman wrote:
>
>> From: dcashman <dcashman@google.com>
>>
>> ASLR currently only uses 8 bits to generate the random offset for the
>> mmap base address on 32 bit architectures. This value was chosen to
>> prevent a poorly chosen value from dividing the address space in such
>> a way as to prevent large allocations. This may not be an issue on all
>> platforms. Allow the specification of a minimum number of bits so that
>> platforms desiring greater ASLR protection may determine where to place
>> the trade-off.
>
> ...
>
>> diff --git a/arch/Kconfig b/arch/Kconfig
>> index 4e949e5..141823f 100644
>> --- a/arch/Kconfig
>> +++ b/arch/Kconfig
>> @@ -511,6 +511,70 @@ config ARCH_HAS_ELF_RANDOMIZE
>> - arch_mmap_rnd()
>> - arch_randomize_brk()
>>
>> +config HAVE_ARCH_MMAP_RND_BITS
>> + bool
>> + help
>> + An arch should select this symbol if it supports setting a variable
>> + number of bits for use in establishing the base address for mmap
>> + allocations and provides values for both:
>> + - ARCH_MMAP_RND_BITS_MIN
>> + - ARCH_MMAP_RND_BITS_MAX
>> +
>> +config ARCH_MMAP_RND_BITS_MIN
>> + int
>> +
>> +config ARCH_MMAP_RND_BITS_MAX
>> + int
>> +
>> +config ARCH_MMAP_RND_BITS_DEFAULT
>> + int
>> +
>> +config ARCH_MMAP_RND_BITS
>> + int "Number of bits to use for ASLR of mmap base address" if EXPERT
>> + range ARCH_MMAP_RND_BITS_MIN ARCH_MMAP_RND_BITS_MAX
>> + default ARCH_MMAP_RND_BITS_DEFAULT if ARCH_MMAP_RND_BITS_DEFAULT
>
> Here you support a default which is separate from the minimum.
>
>> + default ARCH_MMAP_RND_BITS_MIN
>> + depends on HAVE_ARCH_MMAP_RND_BITS
>
> ...
>> +
>> +config ARCH_MMAP_RND_COMPAT_BITS
>> + int "Number of bits to use for ASLR of mmap base address for compatible applications" if EXPERT
>> + range ARCH_MMAP_RND_COMPAT_BITS_MIN ARCH_MMAP_RND_COMPAT_BITS_MAX
>> + default ARCH_MMAP_RND_COMPAT_BITS_MIN
>
> But here you don't.
>
> Just forgot?

Yes. Good catch.

> I'd like to have a default which is separate from the minimum. That way we can
> have a default which is reasonably large, but allow it to be lowered easily if
> anything breaks.

Will add it, along w/the documentation cleanup and other changes.

Thank You,
Dan