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Date: Wed, 25 Sep 2013 18:55:48 +0400
From: Solar Designer <solar@...nwall.com>
To: oss-security@...ts.openwall.com
Subject: Re: Reproducible Builds for Fedora
Dhiru, all -
I did not review the code, so my reply is based on Sebastian's message only:
On Wed, Sep 25, 2013 at 10:08:01AM +0200, Sebastian Krahmer wrote:
> base=`basename $f`
> objdump -d rpm1/$f | grep -v $base > dump1
> objdump -d rpm2/$f | grep -v $base > dump2
> diff -u dump1 dump2 > /dev/null
> if [ $? -ne 0 ] ; then
> echo "File disassembly differs $f"
> cnt=`expr $cnt + 1`
> fi
> [...]
>
> for ELF files and doing a sha256sum for other file types. My concern is
> that attackers could construct a package that contains function-names that
> match the basename of the binary that you are checking. The "grep -v"
> will remove that, leaving a clean compare for injected code like
> 'call $base' etc. That would leave a wrong feeling about equal binaries.
Ensuring that "objdump -d" has stayed the same between a known-good and
another build of a binary is not sufficient to tell that the new build
is not trojaned. Changes to other sections (e.g., to embedded data that
the program uses or/and to relocations) or/and to the ELF header may be
sufficient to introduce meaningful backdoors.
Recent research:
https://www.usenix.org/conference/woot13/weird-machines-elf-spotlight-underappreciated-metadata
"Our proof-of-concept toolkit highlights how important it is that
defenders expand their focus beyond the code and data sections of
untrusted binaries"
[ Dhiru, weren't you there in person? ;-) ]
December 2006 paper saying that a related technique has "been used in
the virus world many years prior to this paper":
http://uninformed.org/?v=6&a=3&t=sumry
Besides ELF being Turing-complete on its own, the ELF header may contain
native executable code too:
http://www.muppetlabs.com/~breadbox/software/tiny/teensy.html
Alexander
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