Re: [PATCH] ASLRv3: randomize_va_space=3 preventing offset2lib attack

[Hector Marco <hecmargi@upv.es>]

El 12/12/14 a las 18:17, Andy Lutomirski escribió:
> On Dec 12, 2014 8:33 AM, "Hector Marco" <hecmargi@upv.es> wrote:
>>
>> Hello,
>>
>> I agree. I don't think a new randomization mode will be needed, just fix
>> the current randomize_va_space=2. Said other way: fixing the offset2lib
>> will not break any current program and so, no need to add additional
>> configuration options. May be we shall wait for some inputs
>> from the list (may be we are missing something).
>>
>>
>> Regarding to VDSO, definitively, is not randomized enough in 64bits.
>> Brute force attacks would be pretty fast even from the network.
>> I have identified the bug and seems quite easy to fix it.
>>
>> On 32bit systems, this is not a issue because it is mapped in the
>> mmap area. In order to fix the VDSO on 64bit, the following
>> considerations shall
>> be discussed:
>>
>>
>> Performance:
>>      It seems (reading the kernel comments) that the random allocation
>>      algorithm tries to place the VDSO in the same PTE than the stack.
>
> The comment is wrong.  It means PTE table.
>
>>      But since the permissions of the stack and the VDSO are different
>>      it seems that are getting right the opposite.
>
> Permissions have page granularity, so this isn't a problem.
>
>>
>>      Effectively VDSO shall be correctly randomized because it contains
>>      enough useful exploitable stuff.
>>
>>      I think that the possible solution is follow the x86_32 approach
>>      which consist on map the VDSO in the mmap area.
>>
>>      It would be better fix VDSO in a different patch ? I can send a
>>      patch which fixes the VDSO on 64 bit.
>>
>
> What are the considerations for 64-bit memory layout?  I haven't
> touched it because I don't want to break userspace, but I don't know
> what to be careful about.
>
> --Andy

I don't think that mapping the VDSO in the mmap area breaks the
userspace. Actually, this is already happening with the current
implementation. You can see it by running:

setarch x86_64 -R cat /proc/self/maps


Do this break the userspace in some way ?


Regarding the solution to the offset2lib it seems that placing the
executable in a different memory region area could increase the
number of pages for the pages table (because it is more spread).
We should consider this before fixing the current implementation
(randomize_va_space=2).

I guess that the current implementation places the PIE executable in
the mmap base area jointly with the libraries in an attempt to reduce
the size of the page table.

Therefore, I can fix the current implementation (maintaining the
randomize_va_space=2) by moving the PIE executable from the mmap base
area to another one for x86*, ARM* and MIPS (as s390 and PowerPC do).
But we shall agree that this increment in the page table is not a
issue. Otherwise, the randomize_va_space=3 shall be considered.


Hector Marco.

>
>>
>>
>> Regards,
>> Hector Marco.