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Date: Tue, 09 Dec 2014 17:23:15 +0100
From: Martino Dell'Ambrogio <>
Subject: Re: PIE bypass using VDSO ASLR weakness

On 12/09/2014 04:33 PM, Reno Robert wrote:
> Hi Daniel, COMPAT_VDSO is not enabled. Just that randomization is 20 bits
> and same values are generated on repeated execution.
I don't have the same behavior on a default linux-image-3.2.0-31-generic 
3.2.0-31.50 (COMPAT_VDSO not set).

I generated some values and, while distribution is not perfect, it feels 
fairly unpredictable :

$ wc -l VDSO-ASLR.lst; cat VDSO-ASLR.lst |sort |uniq -c |sort -n -r 
|head; sort -u VDSO-ASLR.lst |wc -l
15798 VDSO-ASLR.lst
      12 (0x00007fff4cdff000)
      11 (0x00007fffd09ff000)
      11 (0x00007fffc2bff000)
      11 (0x00007fffa0dff000)
      11 (0x00007fff4d7ff000)
      11 (0x00007fff1d3ff000)
      11 (0x00007fff1a1ff000)
      11 (0x00007fff14dff000)
      11 (0x00007fff0e9ff000)
      10 (0x00007fffe1dff000)

This is 9869 different results over 15798 samples, with a very light 
shift toward the shown addresses.

What's weird, though, is that a run of the same set through Burp Suite 
analysis tells me that there are at best 8 estimated bits of effective 
This suggests, if my interpretation is correct, that there may be some 
weakness (by calculating adjacent and/or subsequent values) allowing to 
find a valid combination under 256 tries.

I'm not a cryptologist so I may be missing a point here, but I think 
it's worth discussing.

Martino Dell'Ambrogio
Security Auditor

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