Re: VDSO randomization not very random

[Kees Cook <keescook@chromium.org>]

Hi Hanno,

On Thu, Dec 11, 2014 at 10:45:01PM +0100, Hanno Böck wrote:
> Hello,
> 
> I already reported this into your bugzilla, however Greg KH told me it
> might be a better idea to post it here:
> 
> With current Linux kernels it seems the address randomization for
> loading the vdso library is not that random and can easily be
> bruteforced.
> 
> This can easily be demonstrated. Get libvdso address from one
> executable:
> $ ldd /usr/bin/less|grep vdso
> 	linux-vdso.so.1 (0x00007fff73bfe000)
> 
> Now run ldd mutliple times and check if the same address appears:
> c=0; while (true); do let c=c+1; ldd /usr/bin/less|grep
> 0x00007fff73bfe000; [ "$?" == 0 ] && echo $c; done
> 
> It usually takes only a few seconds and around 1000-2000 tries until
> the loading address is repeated (note that results may vary, it seems
> the randomization is biased, some values repeat more often than others).
> 
> This information is mostly from this blog entry:
> http://v0ids3curity.blogspot.in/2014/12/return-to-vdso-using-elf-auxiliary.html
> And here's a thread on oss-security discussing the issue:
> http://www.openwall.com/lists/oss-security/2014/12/09/10
> 
> The latest version of paxtest added a check for this that guesses the
> randomness of vdso:
> https://grsecurity.net/~spender/paxtest-0.9.13.tar.gz $ ./randvdso 
> VDSO randomisation test                  : 11 quality bits (guessed)
> 
> Bugzilla entry:
> https://bugzilla.kernel.org/show_bug.cgi?id=89591

I'm hoping this will get addressed as part of the discussion around
the "ASLRv3" patches. PIE (as well as VDSO) randomization has been a
per-arch implementation, and it would be best to unify this in a common
high-entropy solution.

I think the problem with VDSO right now is that it is randomized in
relationship to the stack, rather than being randomized on its own.

-Kees

-- 
Kees Cook
Chrome OS Security