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

[Andy Lutomirski]

On Fri, Dec 19, 2014 at 2:04 PM, Hector Marco <hecmargi@xxxxxx> wrote:
>
>
> El 12/12/14 a las 18:17, Andy Lutomirski escribiÃ:
>
>> On Dec 12, 2014 8:33 AM, "Hector Marco" <hecmargi@xxxxxx> 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
>

Hmm.  So apparently we even switch which side of the stack the vdso is
on depending on the randomization setting.

>
> 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.

Wrt the vdso itself, though, there is an extra consideration: CRIU.  I
*think* that the CRIU vdso proxying scheme will work even if the vdso
changes sizes and is adjacent to other mappings.  Cyrill and/or Pavel,
am I right?

I'm not fundamentally opposed to mapping the vdso just like any other
shared library.  I still think that we should have an extra-strong
randomization mode in which all the libraries are randomized wrt each
other, though.  For many applications, the extra page table cost will
be negligible.

--Andy

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



-- 
Andy Lutomirski
AMA Capital Management, LLC
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