[langsec-discuss] LZO, subtle bugs, theorem provers

Thomas Dullien thomasdullien at google.com
Mon Jul 7 21:17:21 UTC 2014


My 2c, but my view at the moment is that none of our static analyzers /
theorem provers can
be usefully applied to legacy code.

The successes of formal verification are usually when the checkers & the
code are co-developed;
e.g. the code is checked on each build while it is written.

I haven't had time to look at the LZ bug in detail yet, realistically I may
get around to it in August;
once I get around I'll write down how hard I think it would be to detect
this with an automated checker.

Cheers,
Halvar


On Mon, Jul 7, 2014 at 2:04 PM, Meredith L. Patterson <clonearmy at gmail.com>
wrote:

> No promises, but the samples look like ones that Escher (
> http://www.eschertech.com/index.php) could find. I'm evaluating Escher
> for verifying parts of Hammer (since there are sound principles for what
> "correct" means in regard to the parsing algorithms it implements), and can
> tell you more after I've had some more experience with it.
>
> Cheers,
> --mlp
>
>
> On Mon, Jul 7, 2014 at 1:56 PM, Dan Kaminsky <dan at doxpara.com> wrote:
>
>> Do we have a theorem prover / static analyzer that finds this bug?
>>
>> We sure have a lot that miss it.
>>
>>
>> On Monday, July 7, 2014, <dan at geer.org> wrote:
>>
>>>
>>> This is a verbatim transcript of what appeared on Peter Neumann's
>>> RISKS Digest; note Henry Baker's leadoff comment.
>>>
>>> -----------------8<------------cut-here------------8<-----------------
>>>
>>> Date: Fri, 27 Jun 2014 06:11:41 -0700
>>> From: Henry Baker <hbaker1 at pipeline.com>
>>> Subject: Buffer overflows in 20-year-old LZ decompression code
>>>   (Don A. Bailey)
>>>
>>> FYI -- It's time to start using theorem provers on *all* code; if you
>>> can't
>>> convince the theorem prover re buffer overflows, you'll have to insert
>>> executable code to explicitly check.  HB
>>>
>>> Thursday, June 26, 2014
>>> Raising Lazarus - The 20 Year Old Bug that Went to Mars
>>>
>>> http://blog.securitymouse.com/2014/06/raising-lazarus-20-year-old-bug-that.html
>>>
>>> It's rare that you come across a bug so subtle that it can last for two
>>> decades.  But, that's exactly what has happened with the
>>> Lempel-Ziv-Oberhumer (LZO) algorithm.  Initially written in 1994, Markus
>>> Oberhumer designed a sophisticated and extremely efficient compression
>>> algorithm so elegant and well architected that it outperforms zlib and
>>> bzip
>>> by four or five times their decompression speed.
>>>
>>> As a result, Markus has made a successful and well deserved career out of
>>> optimizing code for various platforms.  I was impressed to find out that
>>> his
>>> LZO algorithm has gone to the planet Mars on NASA devices multiple times!
>>> Most recently, LZO has touched down on the red planet within the Mars
>>> Curiosity Rover, which just celebrated its first martian anniversary on
>>> Tuesday.
>>>
>>> Because of the speed and efficiency of the algorithm, LZO has made its
>>> way
>>> into both proprietary and open source projects world-wide.  It's has
>>> lived
>>> in automotive systems, airplanes, and other embedded systems for over a
>>> decade.  The algorithm has even made its way into projects we use on a
>>> daily
>>> basis, such as OpenVPN, MPlayer2, Libav, FFmpeg, the Linux kernel,
>>> Juniper
>>> Junos, and much, much, more.
>>>
>>> In the past few years, LZO has gained traction in file systems as well.
>>>  LZO
>>> can be used in the Linux kernel within btrfs, squashfs, jffs2, and
>>> ubifs.  A
>>> recent variant of the algorithm, LZ4, is used for compression in ZFS for
>>> Solaris, Illumos, and FreeBSD.
>>>
>>> LZO is even enabled in kernels for Samsung Android devices to increase
>>> kernel loading speed and improve the user experience, as noted in the
>>> Android Hacker's Handbook.
>>>
>>> With its popularity increasing, Lempel-Ziv-Oberhumer has been rewritten
>>> by
>>> many engineering firms for both closed and open systems.  These rewrites,
>>> however, have always been based on Oberhumer's core open source
>>> implementation.  As a result, they all inherited a subtle integer
>>> overflow.
>>> Even LZ4 has the same exact bug, but changed very slightly.
>>>
>>> Engineered Genetics
>>>
>>> Code reuse is a normal part of engineering, and is something we do every
>>> day.  But, it can be dangerous.  By reusing code that is known to work
>>> well,
>>> especially in highly optimized algorithms, projects can become subject to
>>> vulnerabilities in what is perceived as trusted code.  Auditing highly
>>> optimized algorithms is a fragile endeavor.  It is very easy to break
>>> these
>>> types of algorithms.  Therefore, reused code that is highly specialized
>>> is
>>> often presumed safe because of its age, its proven efficiency, and its
>>> fragility.
>>>
>>> This creates a sort of digital DNA, a digital genetic footprint that can
>>> be
>>> traced over time.  Though there are certainly many instances of
>>> proprietary
>>> variants of LZO and LZ4, the following six implementations are available
>>> in
>>> open source software
>>>
>>>     Oberhumer LZO (core/reference open source implementation)
>>>     Linux kernel's LZO implementation
>>>     Libav's LZO implementation
>>>     FFmpeg's LZO implementation
>>>     Linux kernel's LZ4 implementation
>>>     LZ4 core/reference implementation
>>>
>>> Despite each implementation of the algorithm being noticeably different,
>>> each variant is vulnerable in the exact same way.  Let's take a look at a
>>> version of the algorithm that is easy to read online, the Linux kernel
>>> implementation found here.
>>>
>>> In all variants of LZ[O4], the vulnerability occurs when processing a
>>> Literal Run.  This is a chunk of compressed data that isn't compressed at
>>> all.  Literals are uncompressed bytes that the user decided, for whatever
>>> reason, should not be compressed.  A Literal Run is signaled by a state
>>> machine in LZO, and by a Mask in LZ4.
>>>
>>>  56                         if (likely(state == 0)) {
>>>  57                                 if (unlikely(t == 0)) {
>>>  58                                         while (unlikely(*ip == 0)) {
>>>  59                                                 t += 255;
>>>  60                                                 ip++;
>>>  61                                                 NEED_IP(1);
>>>  62                                         }
>>>  63                                         t += 15 + *ip++;
>>>  64                                 }
>>>  65                                 t += 3;
>>>
>>> In the above sample, the integer overflow is evident.  The variable 't'
>>> is
>>> incremented by 255 every time the compression payload contains a nil byte
>>> (0x00) when a Literal Run is detected.  Regardless of whether the
>>> variable
>>> 't' is signed or unsigned, 255 will be added to it.  The only check is to
>>> ensure that the input buffer contains another byte.  This means that 't'
>>> can
>>> accumulate until it is a very large unsigned integer.  If 't' is a 32bit
>>> integer, it only takes approximately sixteen (16) megabytes of zeroes to
>>> generate a sufficiently large value for 't'.  Though 't' can overflow
>>> here,
>>> this is not where the attack occurs.  There is another more important
>>> overflow just below this chunk of code.
>>>
>>> 66 copy_literal_run:
>>> 67 #if defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)
>>> 68                                 if (likely(HAVE_IP(t + 15) &&
>>> HAVE_OP(t + 15))) {
>>> 69                                         const unsigned char *ie = ip
>>> + t;
>>> 70                                         unsigned char *oe = op + t;
>>> 71                                         do {
>>> 72                                                 COPY8(op, ip);
>>> 73                                                 op += 8;
>>> 74                                                 ip += 8;
>>> 75                                                 COPY8(op, ip);
>>> 76                                                 op += 8;
>>> 77                                                 ip += 8;
>>> 78                                         } while (ip < ie);
>>> 79                                         ip = ie;
>>> 80                                         op = oe;
>>> 81                                 } else
>>> 82 #endif
>>>
>>> Above, we see the "copy_literal_run" chunk of code.  This is the section
>>> of
>>> the LZO algorithm that uses the variable 't' as a size parameter.  On
>>> line
>>> 68, the code ensures that the input buffer (IP) and output buffer (OP)
>>> are
>>> large enough to contain 't' bytes.  However, in the Linux kernel
>>> implementation, they pad by 15 bytes to ensure the 16 byte copy does not
>>> overflow either buffer.  This is where things fail.
>>>
>>> The macros HAVE_IP and HAVE_OP validate that 't' bytes are available in
>>> the
>>> respective buffer.  But, before the macro is called, the expression (t +
>>> 15)
>>> is evaluated.  If the value of 't' is large enough, this expression will
>>> cause an integer overflow.  The attacker can make this expression result
>>> in
>>> a value of zero (0) through fourteen (14) by forcing 't' to equal the
>>> values
>>> -15 to -1, respectively.  This means that the HAVE macros will always
>>> believe that enough space is available in both input and output buffers.
>>>
>>> On line 70, the pointer 'oe' will now point to before the 'op' buffer,
>>> potentially pointing to memory prior to the start of the output buffer.
>>>  The
>>> subsequent code will copy sixteen (16) bytes from the input pointer to
>>> the
>>> output pointer, which does nothing as these pointers should point to a
>>> "safe" location in memory.  However, there are two side effects here that
>>> the attacker must abuse: lines 78 and 80.
>>>
>>> Because 'ie' will always have an address lower in memory than 'ip', the
>>> loop
>>> is immediately broken after the first sixteen (16) byte copy.  This means
>>> that the value 't' did not cause a crash in the copy loop, making this
>>> copy
>>> essentially a no-op from the attacker's point of view.  Most
>>> importantly, on
>>> line 80 (and 79), the buffer pointer is set to the overflown pointer.
>>>  This
>>> means that now, the output pointer points to memory outside of the
>>> bounds of
>>> the output buffer.  The attacker now has the capability to corrupt
>>> memory,
>>> or at least cause a Denial of Service (DoS) by writing to an invalid
>>> memory
>>> page.
>>>
>>> The Impact of Raising Dead Code
>>>
>>> Each variant of the LZO and LZ4 implementation is vulnerable in slightly
>>> different ways.  The attacker must construct a malicious payload to fit
>>> each
>>> particular implementation.  One payload cannot be used to trigger more
>>> than
>>> a DoS on each implementation.  Because of the slightly different overflow
>>> requirements, state machine subtleties, and overflow checks that must be
>>> bypassed, even a worldwide DoS is not a simple task.
>>>
>>> This results in completely different threats depending on the
>>> implementation
>>> of the algorithm, the underlying architecture, and the memory layout of
>>> the
>>> target application.  Remote Code Execution (RCE) is possible on multiple
>>> architectures and platforms, but absolutely not all.  Denial of Service
>>> is
>>> possible on most implementations, but not all.  Adjacent Object
>>> Over-Write
>>> (OOW) is possible on many architectures.
>>>
>>> Lazarus raised from the dead
>>>
>>> Because the LZO algorithm is considered a library function, each specific
>>> implementation must be evaluated for risk, regardless of whether the
>>> algorithm used has been patched.  Why?  We are talking about code that
>>> has
>>> existed in the wild for two decades.  The scope of this algorithm touches
>>> everything from embedded microcontrollers on the Mars Rover, mainframe
>>> operating systems, modern day desktops, and mobile phones.  Engineers
>>> that
>>> have used LZO must evaluate the use case to identify whether or not the
>>> implementation is vulnerable, and in what format.
>>>
>>> Here is a list of impact based on each library. Implementations, or use
>>> cases of each library may change the threat model enough to warrant
>>> reclassification.  So, please have a variant audited by a skilled third
>>> party, such as <shameless plug>.
>>>
>>>     Oberhumer LZO
>>>         RCE: Impractical
>>>         DoS: Practical
>>>         OOW: Practical
>>>         NOTE: 64bit platforms are impractical for all attacks
>>>     Linux kernel LZO
>>>         RCE: Impractical
>>>         DoS: Practical
>>>         OOW: Practical
>>>         NOTE: Only i386/PowerPC are impacted at this time
>>>     Libav LZO
>>>         RCE: Practical
>>>         DoS: Practical
>>>         OOW: Practical
>>>         NOTE: ALL ARCHITECTURES/PLATFORMS are RCE capable
>>>     FFmpeg LZO
>>>         RCE: Practical
>>>         DoS: Practical
>>>         OOW: Practical
>>>         NOTE: ALL ARCHITECTURES/PLATFORMS are RCE capable
>>>     Linux kernel LZ4
>>>         RCE: Practical
>>>         DoS: Practical
>>>         OOW: Practical
>>>         NOTE: 64bit architectures are NOT considered practical
>>>     LZ4
>>>         RCE: Practical
>>>         DoS: Practical
>>>         OOW: Practical
>>>         NOTE: 64bit architectures are NOT considered practical
>>>
>>> For a bug report on each implementation, please visit the Lab Mouse
>>> Security's vulnerability site.
>>>
>>> How Do You Know If You're Vulnerable
>>>
>>> Projects Using LZO/LZ4
>>>
>>> The easiest way to identify whether your specific implementation is
>>> vulnerable is to determine the maximum chunk size that is passed to the
>>> decompress routine.  If buffers of sixteen (16) megabytes or more can be
>>> passed to the LZO or LZ4 decompress routine in one call, then
>>> exploitation
>>> of the integer overflow is possible.  For example, ZFS constrains buffer
>>> sizes to 128k.  So, even though they use a vulnerable implementation of
>>> LZ4,
>>> an attack is not possible without a second bug to bypass the buffer size
>>> constraint.
>>>
>>> The second easiest way is to identify the bit size of the count variable.
>>> If the count variable (for example, named 't' in the Linux kernel code
>>> shown
>>> above) is 64bit, it would take such a massive amount of data to trigger
>>> the
>>> overflow that the attack would likely be infeasible, regardless of how
>>> much
>>> data can be passed to the vulnerable function in one call.  This is due
>>> to
>>> the fact that even modern computers do not have enough RAM available to
>>> store the data required to implement such an attack.
>>>
>>> However, there is a specific issue with the previous check.  Validate
>>> that
>>> even if the count variable is 64bit in size, the value used is still
>>> 64bit
>>> when a length value is checked.  If the actual length value is truncated
>>> to
>>> 32bits, the attack will still work with only sixteen (16) megabytes of
>>> data.
>>>
>>> Users
>>>
>>> All users of FFmpeg, Libav, and projects that depend on them, should
>>> consider themselves at risk to remote code execution.  Period.  Please
>>> update your software from the FFmpeg and Libav websites, or refrain from
>>> using these applications until your distribution has an adequate patch.
>>>
>>> It should be noted that certain Linux distributions package Mplayer2 with
>>> the base system by default.  MPlayer2 is vulnerable to RCE "out of the
>>> box".
>>> If your distribution packages MPlayer2 by default, be sure to disable the
>>> embedded media player plugin (gecko-mediaplayer) for your browser.
>>> Firefox/Iceweasel, Chromium, Opera, Konqueror, and other Linux-based
>>> browsers are vulnerable to RCE regardless of the platform/architecture
>>> when
>>> an MPlayer2 plugin is enabled.
>>>
>>> Vendor Status
>>>
>>> Lab Mouse has reached out to and worked with each vendor of the
>>> vulnerable
>>> algorithm.  As of today, June 26th, 2014, all LZO vendors have patches
>>> either available online, or will later today.  Please update as soon as
>>> possible to minimize the existing threat surface.
>>>
>>> In the near future, Lab Mouse will publish a more technical blog on why
>>> and
>>> how RCE is possible using this bug.  We consider that information to be
>>> imperative for both auditors and engineers, as it assists in identifying,
>>> classifying, and prioritizing a threat.  However, that report will be
>>> released once the patches have been widely distributed for a sufficient
>>> amount of time.
>>>
>>> For more information, please visit our contact page.  We are more than
>>> happy
>>> to help your team with their use case, or implementation of these
>>> algorithms.
>>>
>>> Summary
>>>
>>> Overall, this is how this bug release breaks down.
>>>
>>>   Vendors have patches ready or released
>>>   Distributions have been notified
>>>   Vendors of proprietary variants have been notified (where they could
>>> be found)
>>>   All bug reports can be found here
>>>   RCE is not only possible but practical on all Libav/FFmpeg based
>>> projects
>>>   All others are likely impractical to RCE, but still possible given a
>>>     sufficiently skilled attacker
>>>
>>> It is always exciting to uncover a vulnerability as subtle as this issue,
>>> especially one that has persisted and propagated for two decades.  But,
>>> it
>>> makes me pause and consider the way we look at engineering as a model.
>>>
>>> Speed and efficiency are imperatives for modern projects.  We're building
>>> technology that touches our lives like never before.  I know that most
>>> engineers strive to build not only elegant, but safe code.  But, we still
>>> see security as a disparate discipline from engineering.  Security and
>>> engineering could not be more tightly bound.  Without engineering, you
>>> can't
>>> provide security to users.  Without security, engineering cannot provide
>>> a
>>> stable and provable platform.
>>>
>>> Neil deGrasse Tyson famously claimed, God is in the gaps.  There is a
>>> similar issue in engineering.  The individual often sees stability where
>>> the
>>> individual doesn't have expertise.  Our God is the algorithm.  We "bless"
>>> certain pieces of code because we don't have the time or knowledge to
>>> evaluate it.  When we, as engineers and analysts, take that perspective,
>>> we
>>> are doing a disservice to the people that use our projects and services.
>>>
>>> Often the best eyes are fresh or untrained eyes.  The more we stop
>>> telling
>>> ourselves to step over the gaps in our code bases, the more holes we'll
>>> be
>>> able to fill.  All it takes is one set of eyes to find a vulnerability,
>>> there is no level of expertise required to look and ask questions.  Just
>>> look.  Maybe you'll find the next 20 year old vulnerability.
>>>
>>> Thanks
>>>
>>> I'd like to thank the following people for their great assistance
>>> patching,
>>> coordinating, and advising on this issue:
>>>
>>>     Greg Kroah-Hartman (Linux)
>>>     Linus Torvalds (Linux)
>>>     Kees Cook (Google)
>>>     Xin LI (FreeBSD)
>>>     Michael Niedermayer (FFmpeg)
>>>     Luca Barbato (Libav/Gentoo)
>>>     Markus Oberhumer
>>>     Christopher J. Dorros (NASA MSL)
>>>     Dan McDonald (Omniti)
>>>     Yves-Alexis Perez (Debian)
>>>     Kurt Seifried (Red Hat)
>>>     Willy Tarreau (Linux)
>>>     Solar Designer (Openwall)
>>>     The US-CERT team
>>>     The Oracle security team
>>>     The GE security team
>>>     Kelly Jackson Higgins (UBM)
>>>     Steve Ragan (IDG Enterprise)
>>>     Elinor Mills
>>>
>>> Feeling Guilty?
>>>
>>> Are you reading this post, thinking about all the administrators and
>>> engineers that are going to have to patch the LZO/LZ4 issue in your
>>> team's
>>> systems?  Take some time to tell them how you feel with our hand crafted
>>> Lab
>>> Mouse Security custom Sympathy Card!
>>>
>>> Hand crafted with the finest bits and bytes, our Sympathy Card shows your
>>> engineer what they mean to you and your team.  This is a limited run of
>>> cards, and will proudly display the Linux kernel LZO exploit written by
>>> Lab
>>> Mouse on the card.
>>>
>>> Best wishes,
>>> Don A. Bailey, Founder / CEO, @InfoSecMouse, Lab Mouse Security, 26 Jun
>>> 2014
>>>
>>> _______________________________________________
>>> langsec-discuss mailing list
>>> langsec-discuss at mail.langsec.org
>>> https://mail.langsec.org/cgi-bin/mailman/listinfo/langsec-discuss
>>>
>>
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