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Date: Tue, 09 Dec 2014 11:18:53 -0500
From: Steve Grubb <>
Cc: Hanno Böck <>
Subject: Re: Offset2lib: bypassing full ASLR on 64bit Linux

On Friday, December 05, 2014 01:54:35 PM Hanno Böck wrote:
> On Thu, 04 Dec 2014 21:19:04 +0100
> Hector Marco <> wrote:
> > This is a disclosure of a weakness of the ASLR Linux implementation.
> > The problem appears when the executable is PIE compiled and it has an
> > address leak belonging to the executable. We named this weakness:
> > offset2lib.
> Thanks for that.
> Two things on that:
> Cynics might say "most linux distros aren't vulnerable to ASLR bypass
> because they don't use ASLR at all".
> Can we please take this as an opportunity to discuss the state of ASLR
> on Linux in general?

I studied this area 2 years ago for a gray hat talk and in preparation to help 
set the policy going forward for Fedora and RHEL. The general reason I've 
heard mentioned about why its not used as fully as possible is that it adds 
memory pages that can't be coalesced or consolidated because they are not the 

For Fedora and RHEL 7, the intended policy is PIE for all daemons, privileged 
apps, network facing, and parsers of untrusted media. Enforcement is not so 
easy. How do you identify in an automated way parsers of untrusted media? I 
have a script that can grade an installed system that uses rpms:

It has options to grade the system or an individual rpm for compliance with 
the intended policy.

During my research, I found a couple interesting things. These ASLR related 
tidbits below are pulled from the speech I gave about when open source 
security mechanisms don't work as intended (all measured on a 64 bit system):

1) On non-PIE applications, the heap doesn't get much randomization. Just 14 

2)  Also non-PIE applications seems to be some bias in the numbers chosen. 
This could be an effect of 14 bits of randomization. It did follow a bell curve 
such that guessing some addresses was much luckier than others. (I did not get 
a sample size large enough on PIE apps to see if the same bias could be 

3) When using PIE, you pretty much got 29 bits of randomness everywhere. That 
lead to the question of why the heap on non-PIE is so limited in address 
scope. As I remember, there was some coupling with sbrk() that caused 
this....which might need revisiting.

4) Then I started wondering about the heap when you use other memory manager 
libraries such as jemalloc. This turned out to be interesting. You get about 
19 bits of randomness using it. Its not as bad as non-PIE glibc but not as 
good as PIE glibc. You also got the same amount of randomness whether the app 
was PIE or not. This is an area ripe for more experimenting, exploiting, and 
patching. Supposedly some of these heap managers use mmap as the underlying 
allocator. So, why aren't they getting 29 bits, too? :-)

Here's the current numbers from 3.17.4 kernel + glibc-2.18:

$ ./all-bits 
heap       14 bits
exec       No randomization
mmap       29 bits
so         No randomization
stack      28 bits
pie-exec   29 bits
pie-heap   29 bits
pie-so     29 bits

$ ./all-mask 
heap       0x0000000003FFF000
exec       0x0000000000000000
mmap       0x000001FFFFFFF000
so         0x0000000000000000
stack      0x00000000FFFFFFF0
pie-exec   0x000001FFFFFFF000
pie-heap   0x000001FFFFFFF000
pie-so     0x000001FFFFFFF000


> It's pretty sad, afaik Linux was one of the first
> to have ASLR (in the form of pax) back in 2001. Today everyone uses
> ASLR by default except Linux.
> Most distros don't ship pic/pie executables by default. Why? I haven't
> done benchmarks, the saying is that this has a notable performance hit
> on 32 bit but almost none on 64 bit. If this is true then could we at
> least have all major distros enable it on 64 bit?
> Second:
> I wrote a small test .c to print out offset diffs. As expected
> printf-main offset is static on normal Linux with pic/pie and random
> on a pax-enabled system.
> What i found notable: diff-ing two function offsets from different
> libraries (I use printf-sin) is alway static, even on Pax. Is this by
> design? Can't different libraries be loaded at different offsets in ram?

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