Triaging Bugs with Dynamic Dataflow Analysis

Julio Auto [julio {funny a} julioauto com]

Agenda

The Problem The Solution Demo Solution Details What’s Next? Greetings & References

Preface

We will be talking about analyzing closed-source software here

Absolutely no debugging information needed

However... Depending on the complexity of the

bug, even people with the source might opt for this analysis too E.g. Vendors receiving crash reports

The Problem

Sometimes people just have to analyze bugs in closed-source software

These bugs may come from: A fuzzing session Contributor-sent Proof-of-Concept codes In-the-wild exploit code Etc...

As varying as the sources of bugs are the reasons why one wants to analyze them, but this is irrelevant. The fact is...

The Problem (2)

ANALYZING BUGS CAN BE HARD! A seasoned reverse engineer may

take weeks to get somewhere If the target software is too big If the data consumed is in a very

complex and/or undisclosed format If bugs in this target are so rare that

your reversing team has no previous experience with it

But which bugs do we mostly care for?

The Problem (3)

“Analyzing bugs” is very broad No ./write-me-a-very-detailed-advisory

We will concentrate in answering one question: what exact part of my data made the program crash?

Understanding that and how such data is transformed is primordial

The Solution

Dynamic Dataflow Analysis Watching data and its ramifications as

the doomed program executes What we do really is Taint Analysis

We start with a subset of the program’s data: the attacker’s input – assume it’s evil

Its ‘ramifications’ are tainted memory, tainted registers

... but we do it backwards.

Is any of these from the Evil Input?

This is of interest

Is any of these of interest?

This is the Evil Input

The Solution (2)

TAINT ANALYSIS BACKWARDS TAINT ANALYSIS

The Solution (3)

So we really don’t care about every tainted piece of data in the process space Most of it is legitimate, anyway

Thus, we avoid the explosion of watched data

Plus we can do stuff like: Bug: mov eax, [esi] (where esi =

DEADBEEFh) Analysis runs... ... and reports: esi = user[4] +

var_unk * 8

The Solution (4)

This is all done in two steps: tracing and analysis

First we trace the program from a “good” point until it crashes The trace is incrementally dumped to a

file Not just the disassembly, but also some

extra info▪ E.g.: In the past slide’s example, effective

address ([esi]) == DEADBEEFh Then the trace file goes under

analysis

The Solution (5)

Target starts

Evil Input enters (and

we start tracing)

Target crashes!

The “good” starting point

The Solution (6)

So we feed the trace file to the analyzer and tell it: “Address ranges from ABCDh to ACCDh

and from DCBAh to DCCAh held Evil Input”

“I wanna know if ‘esi’ was tainted by Evil Input”

And magic happens!

The Solution (7)

Considerations Tracing is very time-consuming▪ For the bug I’ll analyze as an example, it takes

about 2 hours to dump the 650,000+ instructions it executes

The analysis... not so much▪ 1 to 2 minutes

May sound like much, but how long would take to do it manually?▪ Plus, you can always use this time to do something

else while the computer is working for you

Demo

Introducing... Visual Data Tracer!

Solution Details

The VDT Tracer is implemented as a WinDbg extension Because WinDbg is free and it’s a great

debugger The VDT Analyzer is a stand-alone

C++ app The tracer needs to understand

some simple instruction “semantics” E.g.: The source and destination

operands Currently only the basic x86 subset is

implemented (no x87, MMX, etc)

Solution Details (2)

The semantic rules are simplified to avoid dumping useless info to the trace file E.g.: a ‘push’ does not meaninfgully

change ‘esp’ (same for ‘inc’, ‘dec’, and their destination ops)

They are also written to fit the very simplistic format of the trace file entries

All of this makes the analysis easier, thus faster, and yet useful

Solution Details (3)

Trace file entry: Mnemonic Destination operand Source operand Up to three source operand “dependences”

Dependences are, for example, the elements of an indirectly addressed memory operand

This effectively exposes the dataflow relations as a Tree (rooted at the crash instruction) Performing the backwards taint analysis becomes

then a matter of searching the tree, which VDT does with a BFS algorithm

Solution Details (4)

Putting it together so far

mov edi, 0x1234 ; dst=edi, src=0x1234

mov eax, [0xABCD] ; dst=eax, src=ptr 0xABCD ; Note 0xABCD is evil addrlea ebx, [eax+ecx*8] ; dst=ebx, src=eax, srcdep1=ecx

mov [edi], ebx ; dst=ptr 0x1234, src=ebx

mov esi, [edi] ; dst=esi, src=ptr 0x1234, srcdep1=edi

mov edx, [esi] ; Crash!!!

Solution Details (5)

Simplifying semantic rules to fit that format is not always easy CMPXCHG r/m32, r32▪ “Compare EAX with r/m32. If equal, ZF is set and r32

is loaded into r/m32. Else, clear ZF and load r/m32 into EAX.”

The aftermath: the need for “conditional taints”▪ ▪ i.e. One of the possibilities of controlling ‘r/m32’ is

controlling ‘r32’ AND ‘eax’ Note that “alternative taints” is also existant,

implemented in the form of srcdep{1,2,3}

32/3232/ mrrEAXmr

Solution Details (6)

Other subtleties to watch for AH defines EAX EAX defines AL AL does not define AH

Similar problem for 1-byte and 2-byte memory accesses

EAX (32)

AX (16)

AL (8) AH (8)

Unnamed (16)

What’s Next?

Extending the coverage of x86 Enhancing speed

God knows how... Heuristically detecting user input

e.g. By making the tracer understand CreateFile()

Automatic exploit generation What else?

Any ideas, let me know...

References

SpiderPig Project - http://piotrbania.com/all/spiderpig/ Very similar ideas, different approach

!exploitable - http://www.codeplex.com/msecdbg A more superficial (but much faster) tool for

bug triaging If you have many bugs to triage, you can

first run !exploitable on them and, then, use VDT on those that seem really interesting

Greetings

Julien Vanegue For all the lecturing, motivating and supporting

Piotr Bania For discussing DDF analysis and much more

People from PSV (http://www.unprotectedhex.com/psv) For letting me idle on IRC, leeching their

knowledge Everyone else who talks to me about

security and similarly cool stuff

Triaging Bugs with Dynamic Dataflow Analysis

Julio Auto [julio {funny a} julioauto com]