Re: [kernel-hardening] Re: [PATCH v2] powerpc/mm: Add support for runtime configuration of ASLR limits

[Michael Ellerman <mpe@ellerman.id.au>]

Balbir Singh <bsingharora@gmail.com> writes:

> On Fri, 2017-04-21 at 00:36 +1000, Michael Ellerman wrote:
>> Add powerpc support for mmap_rnd_bits and mmap_rnd_compat_bits, which are two
>> sysctls that allow a user to configure the number of bits of randomness used for
>> ASLR.
...
>
> Could we please suggest that this should not be sent to stable or back-ported
> as the 128T default is new?

I'm not sure I follow. I didn't tag it for stable. Do you mean we should
explicitly mark it as not-for-stable?

If someone sends it to stable@vger I'll be pinged before it goes in and
nak it, if someone wants to backport it (incorrectly) to their own tree
then good luck to them :)

>> diff --git a/arch/powerpc/mm/mmap.c b/arch/powerpc/mm/mmap.c
>> index a5d9ef59debe..6141cfff634e 100644
>> --- a/arch/powerpc/mm/mmap.c
>> +++ b/arch/powerpc/mm/mmap.c
>> @@ -59,13 +59,14 @@ static inline int mmap_is_legacy(void)
>>  
>>  unsigned long arch_mmap_rnd(void)
>>  {
>> -	unsigned long rnd;
>> +	unsigned long shift, rnd;
>>  
>> -	/* 8MB for 32bit, 1GB for 64bit */
>> +	shift = mmap_rnd_bits;
>> +#ifdef CONFIG_COMPAT
>>  	if (is_32bit_task())
>> -		rnd = get_random_long() % (1<<(23-PAGE_SHIFT));
>> -	else
>> -		rnd = get_random_long() % (1UL<<(30-PAGE_SHIFT));
>> +		shift = mmap_rnd_compat_bits;
>> +#endif
>> +	rnd = get_random_long() % (1 << shift);
>
> Nitpick, 1 should be 1UL?

No, shift can only be 29 at most IIRC?

But it's a bit fragile, so I'll change it.

cheers