[PATCH] arm: fix integer overflow in ELF_ET_DYN_BASE

[Andrey Ryabinin]

On 03/20/2015 02:31 PM, Yury Gribov wrote:
> On 03/20/2015 02:12 PM, Andrey Ryabinin wrote:
>> Usually ELF_ET_DYN_BASE is 2/3 of TASK_SIZE. With 3G/1G user/kernel
>> split this is not so, because 2*TASK_SIZE overflows 32 bits,
>> so the actual value of ELF_ET_DYN_BASE is:
>>     (2 * TASK_SIZE / 3) = 0x2a000000
> 
> AFAIK on most platforms (e.g. Intel) that's (TASK_SIZE / 3 * 2) so ARM is kind of special here.
> 
>>
>> When ASLR is disabled PIE binaries will load at ELF_ET_DYN_BASE address.
>> On 32bit platforms AddressSanitzer uses addresses [0x20000000 - 0x40000000]
>> for shadow memory [1]. So ASan doesn't work for PIE binaries when ASLR disabled
>> as it fails to map shadow memory.
>> Also after Kees's 'split ET_DYN ASLR from mmap ASLR' patchset PIE binaries
>> has a high chance of loading somewhere in between [0x2a000000 - 0x40000000]
>> even if ASLR enabled. This makes ASan with PIE absolutely incompatible.
>>
>> Fix overflow by dividing TASK_SIZE prior to multiplying.
>> After this patch ELF_ET_DYN_BASE equals to (for CONFIG_VMSPLIT_3G=y):
>>     (TASK_SIZE / 3 * 2) = 0x7f555554
>>
>> [1] https://code.google.com/p/address-sanitizer/wiki/AddressSanitizerAlgorithm#Mapping
> 
> Perhaps we should fix other platforms as well?
> 

Seems only avr32 and cris platforms have the same problem.
All other 32bit platforms where ELF_ET_DYB_BASE = (2 * TASK_SIZE / 3) have TASK_SIZE < 2G, so there is no overflow.