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Date: Fri, 26 Apr 2024 14:06:16 -0600
From: Hank Leininger <hlein@...elogic.com>
To: oss-security@...ts.openwall.com
Subject: Update on the distro-backdoor-scanner effort
tl;dr: We've pursued a number of avenues, with plans for more; so far
no "smoking gun" of other backdoors of a similar vein; help wanted.
So: what is this, where are we, what's next, what do we need, credits.
What is this?
- Ongoing work to look for backdoors similar to those found in
xz-utils, or using vectors that were discussed in its aftermath,
that have made their way into Linux distributions' build pipelines;
see https://marc.info/?l=oss-security&m=171208242904550&w=4
- All the below is also covered in code, READMEs, or GH issues at
https://github.com/hlein/distro-backdoor-scanner; the tools are
intended to be bog-standard and work on any of the supported
distros, and to document what they need so our work should be
repeatable by anybody.
Where we are: main things investigated:
- Similar exploitation toolkits / operator-behavior in other packages?
- Unpack and scan all packages in multiple distribution families
looking for the "fist" of the operator: similar stage0 / stage1 /
stage2 loaders, characteristic command-line switch combinations,
etc.; can we find earlier generations of any of the widgets used
(and now burned) in other packages previously backdoored? See the
patterns in bin/package_scan_all.sh
- Unpacked and scanned (generally post-distro-patches, to focus on
as-used-by-distros, not just rescan the same upstreams 4 times):
- ~11k EndeavourOS/Arch packages
- ~40k Debian packages
- ~19k Gentoo packages
- ~9k Rocky/RPM packages
- Output is manageable; able to rule out all hits not part of the
actual xz-utils backdoors as false positives.
- Examine the provenance of every .m4 in every package unpacked above
- What m4 macro files unique/introduced by a project? Which are
recognizable, but updated/modified from any revision ever in an
upstreams (GNU autotools, etc.) in some way? m4 files have serial
numbers, and the xz-utils backdoor used a big jump in its backdoor
.m4; maybe an attempt to keep from getting clobbered after
upstream upgrades?
- Turns out serial numbers are made up and the points don't matter.
But still, this author appears to have _thought_ they were
important. So if they'd done similar somewhere else, should stand
out there too.
- Analyzing about 50k m4 files found about 5k that didn't match an
upstream. around 1k of those had a near-match so we can diff them.
That's still too many to digest manually. 3 had big serial jumps.
2 of those seem benign; the third is of course, the trojan in
xz-utils.
- Big TODOs here are to implement fuzzy hashing when we don't have
a perfect match, so that we can pick the best knowngood candidate
to offer a diff against and to group the unknowns amongst
themselves, and something to facilitate tracking of diff-review
(CSV or another sqlite DB that tracks review status?), and then
to actually read all the diffs (currently only spot-checked).
- Compare decompression of xz-utils vs other compatible tools
- Just to check for some obvious Thompsonesque weird machine where
xz injects malicious .c code into a tarball it unpacks, etc. Very
unlikely to find anything.
- Found nothing except some minor bugs in other decompressors (will
submit upstream bugs, but low priority).
- Still plan to add more different decompressors for completeness.
What's next: rough notions only, not yet implemented:
- Analyze IFUNC real-world use. They're dodgy and weird and useful for
backdoors like this one. Removing IFUNC support from glibc has been
floated: https://marc.info/?l=glibc-alpha&m=171389592724184&w=4
But that'll get hung up on "but what if users". AFAWK nobody knows.
So let's find out: survey sources & binaries from major distros and
get some actual numbers. Also thegrugq made an interesting
observation: it'd be telling which projects recently _added_ IFUNC
use, if any. See
https://github.com/hlein/distro-backdoor-scanner/issues/16
- Check for irregular contents in .pc files, inspired by Vegard
Nossum's oss-security post
https://marc.info/?l=oss-security&m=171335763115933&w=4
This seems it'd be pretty easy to look for known bads. Starting
notes: https://github.com/hlein/distro-backdoor-scanner/issues/7
- Systematically compare git-tagged versions of software to release
artifacts for that same version. What differs, and why? There's
often minor differences for what seem like good releng reasons. But
in the xz-utils case, the backdoor author was able to get access to
post Release assets even w/o commit/merge access; their backdoor was
injected in files/contents that didn't match the Git repo contents.
See https://github.com/hlein/distro-backdoor-scanner/issues/17
What do we need:
- Testers, especially on other distros in a family we support but
only tested on one so far.
- Reproducers to rerun our analysis yourself and make sure you concur
with our conclusions.
- Contributors to the currently outstanding issues/tools.
- Analysis help on the m4 diffs (once we have fuzzy-matching to choose
best-fit diff comparison targets).
- Brainstorming to come up with the next big items to put on the list.
Credits:
Most of this work has been done by Sam James of the Gentoo team and
Hank Leininger (me), partially sponsored by KoreLogic. Thanks also
to folks who helped us get a handle on a lot of different distros'
ecosystems, especially Solar Designer (Rocky/RPM family), brocellous
(Arch family).
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