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uninformed 01 04
Loop Detection
Peter Silberman
peter.silberman@gmail.com
1) Foreword
Abstract: During the course of this paper the reader will gain new knowledge
about previous and new research on the subject of loop detection. The topic of
loop detection will be applied to the field of binary analysis and a case study
will given to illustrate its uses. All of the implementations provided in this
document have been written in C/C++ using Interactive Disassembler (IDA)
plug-ins.
Thanks: The author would like to thank Pedram Amini, thief, Halvar Flake,
skape, trew, Johnny Cache and everyone else at nologin who help with ideas, and
kept those creative juices flowing.
2) Introduction
The goal of this paper is to educate the reader both about why loop detection
is important and how it can be used. When a security researcher thinks of
insecure coding practices, things like calls to strcpy and sprintf are some of
the first things to come to mind. These function calls are considered low
hanging fruit. Some security researchers think of integer overflows or
off-by-one copy errors as types of vulnerabilities. However, not many people
consider, or think to consider, the mis-usage of loops as a security problem.
With that said, loops have been around since the beginning of time (e.g. first
coding languages). The need for a language to iterate over data to analyze
each object or character has always been there. Still, not everyone thinks to
look at a loop for security problems. What if a loop doesn't terminate
correctly? Depending on the operation the loop is performing, it's possible
that it could corrupt surrounding memory regions if not properly managed. If
the loop frees memory that no longer exists or is not memory, a double-free bug
could've been found. These are all things that could, and do, happen in a
loop.
As the low hanging fruit is eliminated in software by security researchers and
companies doing decent to moderate QA testing, the security researchers have to
look elsewhere to find vulnerabilities in software. One area that has only
been touched on briefly in the public relm, is how loops operate when
translated to binaries BugScan is an example of a company that has implemented
"buffer iteration" detection but hasn't talked publically about it.
http://www.logiclibrary.com. The reader may ask: why would one want to look at
loops? Well, a lot of companies implement their own custom string routines,
like strcpy and strcat, which tend to be just as dangerous as the standard
string routines. These functions tend to go un-analyzed because there is no
quick way to say that they are copying a buffer. Due to this reason, loop
detection can help the security research identify areas of interest. During
the course of this article the reader will learn of the different ways to
detect loops using graph analysis, how to implement loop detection, see a new
loop detection IDA plug-in, and a case study that will tie it all together.
3) Algorithms Used to Detect Loops
A lot of research has been done on the subject of loop detection. The
research, however, was not done for the purpose of finding and exploiting
vulnerabilities that exist inside of loops. Most research has been done with
an interest in recognizing and optimizing loops A good article about loop
optimization and compiler optimization is
http://www.cs.princeton.edu/courses/archive/spring03/cs320/notes/loops.pdf .
Research on the optimization of loops has led scientists to classify various
types of loops. There are two distinct categories to which any loop will
belong. Either the loop will be an irreducible loop Irreducible loops are
defined as "loops with multiple entry [points]"
(http://portal.acm.org/citation.cfm?id=236114.236115) or a reducible loop
Reducible loops are defined as "loops with one entry [point]"
(http://portal.acm.org/citation.cfm?id=236114.236115). Given that there are
two different distinct categories, it stands to reason that the two types of
loops are detected in different fashions. Two popular papers on loop detection
are Interval Finding Algorithm and Identifying Loops Using DJ Graphs. This
document will cover the most widely accepted theory on loop detection.
3.1) Natural Loop Detection
One of the most well known algorithms for loop detection is demonstrated in the
book Compilers Principles, Techniques, and Tools by Alfred V. Aho, Ravi Sethi
and Jeffrey D. Ullman. In this algorithm, the authors use a technique that
consists of two components to find natural loops A natural loop "Has a single
entry point. The header dominates all nodes in the loop."
(http://www-2.cs.cmu.edu/afs/cs.cmu.edu/academic/class/15745-s03/public/lectures/L7_handouts.pdf
all loops are not natural loops.
The first component of natural loop detection is to build a dominator tree out
of the control flow graph (CFG). A dominator can be found when all paths to a
given node have to go through another node. A control flow graph is essentially
a map of code execution with directional information. The algorithm in the
book calls for the finding of all the dominators in a CFG. Let's look at the
actual algorithm.
Starting from the entry node, the algorithm needs to check if there is a path
to the slave from the entry node. This path has to avoid the master node. If
it is possible to get to the slave node without touching the master node, it
can be determined that the master node does not dominate the slave node. If it
is not possible to get to the slave node, it is determined that the master node
does dominate the slave. To implement this routine the user would call the
is_path_to(ea_t from, ea_t to, ea_t avoid) function included in loopdetection.cpp.
This function will essentially check to see if there is a path from the
parameter from that can get to the parameter to, and will avoid the node
specified in avoid. Figure illustrates this algorithm.
As the reader can see from Figure 1, there is a loop in this CFG. Let B to C
to D be the path of nodes that create a loop, it will be represented as
B->C->D. There is also another loop from nodes B->D. Using the algorithm
described above it is possible to verify which of these nodes is involved in
the natural loop. The first question to ask is if the flow of the program can
get from A to D while avoiding B. As the reader can see, it is impossible in
this case to get to D avoiding B. As such, a call to the is_path_to function
will tell the user that B Dominates D. This can be represented as B Dom D, and
B Dom C. This is due to the fact that there is no way to reach C or D without
going through B. One question that might be asked is how exactly does this
demonstrate a loop? The answer is that, in fact, it doesn't. The second
component of the natural loop detection checks to see if there is a link, or
backedge, from D to B that would allow the flow of the program to return to
node B to complete the loop. In the case of B->D there exists a backedge that
does complete the loop.
3.2) Problems with Natural Loop Detection
There is a very big problem with natural loops. The problem is with the
natural loop definition which is ``a single entry point whose header dominates
all the nodes in the loop''. Natural loop detection does not deal with
irreducible loops, as defined previously. This problem can be demonstrated in
figure
As the reader can see both B and D are entry points into C. Also neither D nor
B dominates C. This throws a huge wrench into the algorithm and makes it only
able to pick up loops that fall under the specification of a natural loop or
reducible loop It is important to note that it is next that it is next to
impossible to reproduce
4) A Different Approach to Loop Detection
The reader has seen how to detect dominators within a CFG and how to use that
as a component to find natural loops. The previous chapter described why
natural loop detection was flawed when trying to detect irreducible loops. For
binary auditing, the tool will need to be able to pick up all loops and then
let the user deduce whether or not the loops are interesting. This chapter
will introduce the loop algorithm used in the IDA plug-in to detect loops.
To come up with an algorithm that was robust enough to detect both loops in the
irreducible and reducible loop categories, the author decided to modify the
previous definition of a natural loop. The new definition reads "a loop can
have multiple entry points and at least one link that creates a cycle." This
definition avoids the use of dominators to detect loops in the CFG.
The way this alternative algorithm works is by first making a call to the
is_reference_to(ea_t to, ea_t ref) function. The function is_reference_to will
determine if there is a reference from the ea_t specified by ref to the
parameter to. This check within the loop detection algorithm determines if
there is a backedge or link that would complete a loop. The reason this check
is done first is for speed. If there is no reference that would complete a
loop then there is no reason to call is_path_to, thus preventing unnecessary
calculations. However, if there is a link or backedge, a call to the
overloaded function is_path_to(ea_t from, ea_t to) is used to determine if the
nodes that are being examined can even reach each other. The is_path_to function
simulates all possible code execution conditions by following all possible
edges to determine if the flow of execution could ever reach parameter to when
starting at parameter from. The function is_path_to(ea_t from, ea_t to) returns
one (true) if there is indeed a path going from from to to. With both of these
functions returning one, it can be deduced that these nodes are involved in the
loop.
4.1) Problems with new approach
In every algorithm there can exists small problems, that make the algorithm far
from optimal. This problem applies to the new approach presented above. The
algorithm presented above has not been optimized for performance. The algorithm
runs in a time of O(N2), which carries quite a load if there are more than 600
or so nodes.
The reason that the algorithm is so time consuming is that instead of
implementing a Breadth First Search (BFS), a Depth First Search (DFS) was
implemented, in the is_path_to function which computes all possible paths to and
from a given node. Depth First Search is much more expensive than Breadth First
Search, and because of that the algorithm may in some rare cases suffer. If
the reader is interested in how to implement a more efficient algorithm for
finding the dominators, the reader should check out Compiler Design
Implementation by Steven S. Muchnick.
It should be noted that in future of this plug-in there will be optimizations
made to the code. The optimizations will specifically deal new implementations
of a Breadth First Search instead of the Depth First Search, as well as other
small optimizations.
5) Loop Detection Using IDA Plug-ins
In every algorithm and theory there exists small problems. It is important to
understand the algorithm presented
The plug-in described in this document uses the Function Analyzer Class
(functionanalyzer) that was developed by Pedram Amini
(http://labs.idefense.com) as the base class. The Loop Detection
(loopdetection) class uses inheritance to glean its attributes from Function
Analyzer. The reason inheritance is used is primarily for ease of development.
Inheritance is also used so that instead of having to re-add functions to a new
version of Function Analyzer, the user only has to replace the old file. The
final reason inheritance is used is for code conformity, which is accomplished
by creating virtual functions. These virtual functions allow the user to
override methods that are implemented in the Function Analyzer. This means
that if a user understands the structure of function analyzer, they should not
have a hard time understanding loop detections structure.
5.1) Plug-in Usage
To best utilize this plug-in the user needs to understand its features and
capabilities. When a user runs the plug-in they will be prompted with a window
that is shown in figure . Each of the options shown in figure are described
individually.
Graph Loop
This feature will visualize the loops, marking the entry of a loop with green
border, the exit of a loop with a red border and a loop node with a yellow
border. Highlight Function Calls This option allows the user to highlight the
background of any function call made within the loop. The highlighting is done
within IDA View.
Output Stack Information
This is a feature that is only enabled with the graph loop option. When this
option is enabled the graph will contain information about the stack of the
function including the variables name, whether or not it is an argument, and
the size of the variable. This option is a great feature for static auditing.
Highlight Code
This option is very similar to Highlight Function except instead of just
highlighting function calls within loops it will highlight all the code that is
executed within the loops. This makes it easier to read the loops in IDA View
Verbose Output
This feature allows the user to see how the program is working and will give
more information about what the plug-in is doing.
Auto Commenting
This option adds comments to loops nodes, such as where the loop begins, where
it exits, and other useful information so that the user doesn't have to
continually look at the graph.
All Loops Highlighting of Functions
This feature will find every loop within the IDA database. It will then
highlight any call to any function within a loop. The highlighting is done
within the IDA View making navigation of code easier.
All Loops Highlighting of Code
This option will find every loop within the database. It will then highlight
all segments of code involved in a loop. The highlighting of code will allow
for easier navigation of code within the IDA View.
Natural Loops
This detection feature allows the user to only see natural loops. It may not
pick up all loops but is an educational implementation of the previously
discussed algorithm.
Recursive Function Calls
This detection option will allow the user to see where recursive function calls
are located.
5.2) Known Issues
There a couple of known issues with this plug-in. It does not deal with rep*
instructions, nor does it deal with mov** instructions that might result in
copied buffers. Future versions will deal with these instructions, but since
it is open-sourced the user can make changes as they see fit. Another issue is
that of ``no-interest''. By this the author means detecting loops that aren't
of interest or don't pose a security risk. These loops, for example, may be
just counting loops that don't write memory. Halvar Flake describes this topic
in his talk that was given at Blackhat Windows 2004. Feel free to read his
paper and make changes accordingly. The author will also update the plug-in
with these options at a later date.
5.3) Case Study: Zone Alarm
For a case study the author chose Zone Alarm's vsdatant.sys driver. This
driver does a lot of the dirty work for Zone Alarm such as packet filtering,
application monitoring, and other kernel level duties. Some may wonder why it
would be worthwhile to find loops in a driver. In Zone Alarm's case, the user
can hope to find miscalculations in lengths where they didn't convert a signed
to unsigned value properly and therefore may cause an overflow when looping.
Anytime an application takes data in remotely that may be type-casted at some
point, there is always a great chance for loops that overflow their bounds.
When analyzing the Zone Alarm driver the user needs to select certain options
to get a better idea of what is going on with loops. First, the user should
select verbose output and All Loops Highlighting of Functions to see if there
are any dangerous function calls within the loop. This is illustrated in
figure .
After running through the loop detection phase, some interesting results are
found that are shown in figure .
Visiting the address 0x00011a21 in IDA shows the loop. To begin, the reader
will need to find the loop's entry point, which is at:
.text:00011A1E jz short loc_11A27
At the loop's entry point, the reader will notice:
.text:00011A27 push 206B6444h ; Tag
.text:00011A2C push edi ; NumberOfBytes
.text:00011A2D push 1 ; PoolType
.text:00011A2F call ebp ;ExAllocatePoolWithTag
At this point, the reader can see that every time the loop passes through its
entry point it will allocate memory. To determine if the attacker can cause a
double free error, further investigation is needed.
.text:00011A31 mov esi, eax
.text:00011A33 test esi, esi
.text:00011A35 jz short loc_11A8F
If the memory allocation within the loop fails, the loop terminates correctly.
The next call in the loop is to ZwQuerySystemInformation which tries to acquire
the SystemProcessAndThreadsInformation.
.text:00011A46 mov eax, [esp+14h+var_4]
.text:00011A4A add edi, edi
.text:00011A4C inc eax
.text:00011A4D cmp eax, 0Fh
.text:00011A50 mov [esp+14h+var_4], eax
.text:00011A54 jl short loc_11A1C
This part of the loop is quite un-interesting. In this segment the code
increments a counter in eax until eax is greater than 15. It is obvious that
it is not possible to cause a double free error in this case because the user
has no control over the loop condition or data within the loop. This ends the
investigation into a possible double free error.
Above is a good example of how to analyze loops that may be of interest. With
all binary analysis it is important to not only identify dangerous function
calls but to also identify if the attacker can control data that might be
manipulated or referenced within a loop.
6) Conclusion
During the course of this paper, the reader has had a chance to learn about the
different types of loops and some of the method of detecting them. The reader
has also gotten an in-depth view of the new IDA plug-in released with this
article. Hopefully now when the reader sees a loop, whether in code or binary,
the reader can explore the loop and determine if it is a security risk or not.
Bibliography
Tarjan, R. E. 1974. Testing flow graph reducibility. J
Comput. Syst. Sci. 9, 355-365.
Sreedhar, Vugranam, Guang Gao, Yong-Fong Lee. Identifying
loops using DJ graphs.
http://portal.acm.org/citation.cfm?id=236114.236115
Flake, Halvar. Automated Reverse Engineering.
http://www.blackhat.com/presentations/win-usa-04/bh-win-04-flake.pdf
