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

[Daniel Cashman]

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@xxxxxxxxxx>
>>
>> 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
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