VDSO randomization not very random

[Hanno BÃck]

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

cu,
-- 
Hanno BÃck
http://hboeck.de/

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