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    SubjectRe: VDSO randomization not very random
    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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