Messages in this thread | | | From | Andy Lutomirski <> | Date | Thu, 11 Dec 2014 14:41:54 -0800 | Subject | Re: VDSO randomization not very random |
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On Thu, Dec 11, 2014 at 2:02 PM, Kees Cook <keescook@chromium.org> wrote: > 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. >
As far as I'm concerned, the vdso can go wherever the kernel wants to put it, so long as it doesn't conflict with any real-world non-PIE binaries.
--Andy
> -Kees > > -- > Kees Cook > Chrome OS Security
-- Andy Lutomirski AMA Capital Management, LLC
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