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Date: Sat, 16 Jun 2018 21:34:22 +0200
From: Lionel Debroux <>
Subject: Fun with DBM-type databases...


Many crashes upon offline data corruption in multiple database systems.
Only two of these DB systems patched after 4+ months. Much work needed
to backport fixes and move away from insecure & unmaintained DB systems.

In January, February and May 2018, I spent some of my free time fuzzing
a set of DBs more or less loosely from the DBM family, which are
packaged by Debian: LMDB, GDBM, freecdb, QDBM, Tokyocabinet,
Kyotocabinet, TDB, NTDB. Offline data corruption is infrequent, but it
can produce all kinds of trouble when it occurs.
The results aren't good: only freecdb escaped unscathed from 4 wall
clock days x 1 Sandy Bridge core of afl-fuzz in deterministic mode,
spanning more than 1.1G execs. However, I expected no less from a code
base made by DJB :)
The other libraries display a variety of crashes of the usual kinds,
even in standard, non-sanitizer builds, upon offline data corruption:
some or all of SIG{FPE,SEGV,BUS,ABRT}, at various ratios ranging from <<
1% to ~15% (!!), with excessive memory consumption and memory leaks.
Sadly, the most popular library was the one which crashed the most.

Only two of those libraries received a full set of patches for the
crashing issues I reported: in chronological order, TDB and GDBM. Put
another way: there's lots of unfixed DoS in the aforementioned
databases, and/or the CLI programs for dealing with them...

I have not tried to request dozens of CVE IDs, too much work for a free
time project. The sets of crashing inputs are available on request,
though some of them are easy to duplicate (and probably exceed) anyway.

The motivation behind this work
It's our long-time collective problem with the Berkeley DB libraries:
* many programs depend on libdb, including core distro components (in
both the RPM and DEB ecosystems), which makes libdb an essential package
of the main (and most) Linux distros;
* libdb's reliability and security are horrible. What got me interested
in libdb in late 2014 was an infinite loop (well, 1h of CPU time on 1
core before I killed the process) parsing a BDB DB corrupted by a forced
computer power down; libdb was shown to be able to corrupt a DB on its
own, in such a way that salvaging it yields an infinite loop [1]. And of
course, fuzzers can crash libdb in seconds;
* the old 5.x versions of libdb, which everyone uses, have been
unmaintained for years;
* the newer versions are still maintained, and the latest one at the
time of this writing contains fixes for more than 44 vulnerabilities
with CVE numbers (Oracle CPU bulletins from July 2015, April 2016, April
2017), nearly all of which are full DoS. The brand-new 18.1.25 release's
changelog [2] mentions more fixes, e.g. #25920 and #26270, which
correlate with reports I sent in April 2017, several days after 6.2.32
was published. I guess a future CPU bulletin will tell us how many CVE
IDs were fixed this time. However, the newer versions use the AGPLv3,
which makes them unsuitable for most projects, and therefore, the newer
versions are simply unpackaged by most distros...

The Oracle security people who exchanged e-mails with me over the past
few years are nice, but the sheer amount of issues and glacial pace of
the fixes on Berkeley DB (like on other software more or less supposedly
maintained by Oracle, anyway) aren't... At least as far as BDB is
concerned, Oracle is purely reactive to vulnerability reports.

If client programs are to replace the usage of BDB by the usage of
another similar library, more or less close to the original DBM spirit -
which they should do even for reliability reasons, per the above - then
the replacement libraries had better be (relatively) robust against
offline data corruption, as trading an unfixable security pig with an
unfixed security pig is a weak improvement.

Detailed description and timeline
* LMDB (2017/01/27-28, reported on the 28th)
Description of database and issues
LMDB is BDB's closest derivative, with a somewhat similar API. Its
designers took years of usage feedback on BDB into account, to build a
DB with high emphasis on reliability under normal conditions and
performance. They did a great job at those, no doubt about that... but
they didn't do a good job at robustness against offline corruption.

Under Debian sid amd64:
# apt install lmdb-utils zzuf
$ rm -f input*
$ echo -e "key\nvalue" | mdb_load -T -n input
$ zzuf -qcs 0:1000 -C 10 -U 3 mdb_dump -n -l input
$ zzuf -qcs 0:1000 -C 10 -U 3 mdb_dump -n -a input
$ zzuf -qcs 0:1000 -C 10 -U 3 mdb_stat -n -e -fff -r -a input

yields an assortment of SIGFPE, SIGABRT and SIGSEGV in seconds.
afl-clang-fast + afl-fuzz produce more of them, as well as SIGBUS.
The segfaults are of the read variety, but include reads to wild
addresses (-4, 0x44, some RO mmap-ed areas).

Communication with maintainer and partial fixes
The maintainer, Howard Chu, replied very quickly and fixed several
issues (but not all of them) on his side. He published these fixes on a
separate "fuzz" branch, after I pinged him. He doesn't consider
malicious edits of on-disk data a valid threat. He wants to be shown
"cases where in regular usage, and in the face of either application or
OS crash, that LMDB has gotten corrupted". Maybe I could have
communicated different / better points.

* GDBM (early 2018/02, reported 2018/02/03)
Description of database and issues
GDBM's the most popular implementation of DBM on Linux distros (and
elsewhere ?), its many and essential reverse dependencies make it an
essential package.

zzuf and afl produced SIGABRT, SIGSEGV and SIGFPE in a standard build of
libgdbm + gdbm_dump 1.14.1 in less than a second. Excessive memory
consumption (thrashing, OOM kill) was displayed by one of the samples
produced by afl.
The segfaults included allocations of 0-sized blocks followed by OOB
writes to these 0-sized blocks, and OOB writes to unallocated parts of
arenas. The crash rate was consistently ~15%, both over the first
million execs and the ~100M total execs of the first round, which wasn't
built using AFL_USE_ASAN=1 or even AFL_HARDEN=1...

Communication with maintainer and fixes + improvements
GDBM's maintainer Sergey Poznyakoff is busy, but after a ping sent 92
days after the initial report, he has been able to spend time fixing a
significant number of issues found by multiple fuzzing rounds, in such a
way that asan-enabled builds of libgdbm + gdbmtool now usually survive
several sequences of commands mixing reads, writes, reorganize
operations, with a sprinkling of recover on other instances.
He also implemented command sequences passed through argc/argv to
gdbmtool; I sent that feature request after seeing that tdbtool supports
them and that they're more convenient than passing commands through
stdin or an external file.
GDBM 1.15 was published today; a number of issues are present in the
still popular 1.8.x versions, and fixes will need to be backported

* QDBM, Tokyo Cabinet, Kyoto Cabinet (late 2018/01 and early 2018/02,
reported 2018/02/03 and 2018/02/10):
Description of database and issues
In the early 2000s, QDBM was originally made to be faster than DBM;
Tokyo Cabinet and Kyoto Cabinet are its successors, which offer a
variety of internal data structures. Some of the CLI front-ends for
these libraries can't be fuzzed using afl (directly, I suppose I could
have made a preprocessor or something along those lines) because they
take a folder name as destination argument, and create / expect multiple
files in these folders.

Sample fuzzing invocations for TC and KC:
$ kchashmgr create -ts -bnum 65536 input_hash/hash
$ afl-fuzz -i input_hash -o output -S hash -m 1024 -- kchashmgr list -pv @@
$ kctreemgr create input_tree/tree
$ afl-fuzz -i input_tree -o output -S tree -m 1024 -- kctreemgr list -pv @@
The afl-fuzz invocations for tcbmgr, tcfmgr, tchmgr, tctmgr are similar.

All six invocations on standard (not even AFL_HARDEN=1 builds) quickly
yield an assortment of SIGABRT, SIGSEGV, SIGBUS and/or SIGFPE.

Likewise for QDBM, `vlmgr list` and `vlmgr repair` crash easily with
SIGSEGV: OOB + wild memory reads.

Communication with maintainer (or lack thereof)
I received no reply to either the original messages or two subsequent

* TDB (a couple weeks in February 2018 and early March 2018, reported
Description of database and issues
TDB is currently the database underlying Samba.

I used afl-fuzz and a bit of honggfuzz on tdbdump + tdbtool for a couple
wall clock weeks and billions of execs. They produced OOB reads, wild
reads, bus errors and excessive memory consumption. Valgrind reported a
bit of UMR as well. Still, the crash rate was pretty low, so TDB's in a
reasonable shape overall :)

Communication with maintainers and fixes
Huge hat tip to Volker Lendecke, who managed to produce a full set of
patches fixing all of the issues displayed by the batch of files I sent
to Samba security on a late Saturday evening... in less than 12 hours,
i.e. on a Sunday. I'm still impressed :)
Without physical or network access to the fuzzing computer, I didn't get
to restart the new fuzzing round on the patched version from the
previous output directory until the next day. Over two wall clock weeks
x 9 jobs on the fixed version produced zero crash.

* NTDB (same original fuzzing period as TDB, reported 2018/03/03 at the
same time as TDB)
Description of database and issues
At some point, NTDB was designed to replace TDB in Samba; this never

I used afl-fuzz on ntdbdump + ntdbtool.
Despite a nullptr deref I accidentally found in the CLI front-end when
invoking it the wrong way, afl-fuzz flagged only controlled asserts, so
this database is in a decent shape as well.

Communication with maintainers and fixes
Volker Lendecke was clear about it: Samba does no longer use NTDB, and
probably doesn't have the capacity to fix the code.

Several takeaways
* in a couple dozen projects, it's the first time I deal with a
maintainer who has time to, but is - so far at least - unwilling to fix
issues found by fuzzers. I do understand the point that high reliability
under normal conditions, and high performance, are paramount to LMDB's
design and implementation, and that they did a (very) good job about it,
which is a good thing.
However, IMO, reliability against offline data corruption, be it
malicious or not, should be considered higher priority. Even though it
is infrequent, silent data corruption sometimes occurs in the real
world, for instance because of faulty drivers / firmware / hardware (and
ZFS is considered one of the technical solutions, if nothing else
because it can detect them, unlike most traditional filesystems).
Backup tapes are less popular nowadays, but I work at a place where
they're used on a daily basis, which means that saving LMDBs to tapes is
either already being done, or will soon be done. In the current state of
things, salvaging LMDBs which became corrupted on backup tapes (if we're
a bit unlucky) in case we have to restore them might prove less than fun
for our sysadmins / customer sysadmins down the road.

* that said: let's state explicitly that even in its current form, LMDB
isn't just the most obvious replacement for BDB, it's also a security
improvement over it, despite being far from perfect. LMDB is arguably
what most BDB client programs should be switching to, all the more...

* ... I'm told Samba's interested in (optionally ?) switching from TDB
to LMDB for scalability and reliability reasons, which, again, makes
complete sense. At the time of this writing, this move shall be a
security regression (well, the bar is set quite high by TDB), though.

* distros could well want to unpackage the obsolete, unmaintained NTDB
from future releases - even if it's not in a terrible shape. The project
is unpopular outside Samba: in Debian, the NTDB packages form an island.

* QDBM, Tokyo Cabinet and Kyoto Cabinet don't seem to be actively
maintained (no releases for a while, nobody responding at the specified
e-mail addresses), and the code bases aren't in good shape. From a
security point of view (I haven't examined functional requirements), it
looks like it's not just Berkeley DB that projects could find wise to
move away from... and that's work, since QDBM, TC and KC have more
reverse dependencies than e.g. TDB.

General notes
Well, pretty much the same ones as in my earlier work on module / track
handling libraries ( ): the usual
FLOSS maintainership sustainability problem (noticeable on GDBM and
probably QDBM / TC / KC, though Oracle does it more slowly), the need to
backport fixes but the packager resources the process consumes, the
still limited (though growing) amount of fuzzing resources spent on core
FLOSS by upstream maintainers, professional security researchers and
amateurs like me. The usefulness of sandboxing is probably less obvious
on database systems than on media files, though the checking / recovery
programs, especially, could benefit.

* Sam Hocevar and others for zzuf, Michal Zalewski for afl, Robert
Święcki for honggfuzz;
* the upstream maintainers of the libraries and programs I fuzzed;
* security teams;
* other persons who were aware of this journey :)

Lionel Debroux.


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