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\begin{document}

\title{Overview Over Attack Vectors and Countermeasures for Buffer Overflows}

\author{\IEEEauthorblockN{Valentin Brandl}
\IEEEauthorblockA{\textit{Faculity of Computer Science and Mathematics} \\
\textit{OTH Regensburg}\\
Regensburg, Germany \\
valentin.brandl@st.oth-regensburg.de\\
MatrNr. 3220018}
}

\maketitle

\begin{abstract}

  This paper tries to explain the details behind buffer overflows, explore the
  problems stemming from those kinds of software vulnerabilities and discus
  possible countermeasures with focus on their effectiveness, performance impact
  and ease of use.

\end{abstract}

\begin{IEEEkeywords}
Buffer Overflow, Software Security
\end{IEEEkeywords}



\section{Motivation}\label{ref:motivation}

When the first programming languages were designed, memory had to be managed
manually to make the best use of slow hardware. This opened the door for many
kinds of programming errors. Memory can be deallocated more than once
(double-free), invalid pointers can be dereferenced (\mintinline{C}{NULL}
pointer dereference; this is still a problem in many modern languages), the
program could read or write out of bounds of a buffer (information leaks,
\acp{bof}). Languages that are affected by this are e.g.\ C, C++ and Fortran.
While most if not all of these problems are solved in modern programming
languages, these languages are still used in critical parts of the worlds
infrastructure, either because they allow to implement really performant
programs, offer deterministic runtime behaviour (e.g.\ no pauses due to garbage
collection), because they power legacy systems or for portability reasons.
Scientists and software engineers have proposed lots of solutions to this
problem over the years and this paper aims to compare and give an overview about
those.

Reading out of bounds can result in an information leak and is less critical
than \acp{bof} in most cases, but there are exceptions, e.g.\ the Heartbleed
bug~\cite{Heardbleed2014} in OpenSSL which allowed dumping secret keys from
memory. Out of bounds writes are almost always critical and result in code
execution vulnerabilities or at least application crashes.

In 2018, 14\% (2368 out of 16556)~\cite{Cve2018} of all software vulnerabilities
that have a CVE assigned, were overflow related. This shows that, even if this
type of bug is very old and well known, it's still relevant today.

\section{Background}\label{ref:background}

\subsection{Technical Details}

Code execution via \ac{bof} vulnerabilities almost always works by overwriting
the return address in the current stack frame (known as \enquote{stack
smashing})~\cite{Smashing2004}, so when the \mintinline{ASM}{RET} instruction is
executed, an attacker controlled address is moved into the instruction pointer
register and the code pointed to by this address is
executed~\cite{Detection2018}. Other ways include overwriting addresses in the
\ac{plt} (the \ac{plt} contains addresses of dynamically linked library
functions) of a binary so that, if a linked function is called, an attacker
controlled function is called instead, or (in C++) overwriting the vtable where
the pointers to an object's methods are stored.

A simple vulnerable C program might look like this:

\begin{figure}[h!]
\begin{minted}{c}
void vuln(char *input) {
  char buf[50];
  size_t len = strlen(input);
  for (size_t i = 0; i < len; i++) {
    buf[i] = input[i];
  }
}
int main(int argc, char **argv) {
  vuln(argv[1]);
  return 0;
}
\end{minted}
\caption{Vulnerable C program}\label{lst:vuln}
\end{figure}

A successful stack \ac{bof} exploit would place the payload in the memory by
supplying it as an argument to the program (or by placing it in an environment
variable, writing it to a file that the program reads, via network packet, ...)
and eventually overwrite the return address by providing an input with $> 50$
bytes and therefore writing out of bounds. When executing the
\mintinline{C}{return} instruction, and the jumps into the payload, the
attacker's code is executed. This works due to the way, how function calls on
CPUs work: The stack frame of the current function lies between the \ac{bp} and
\ac{sp} as shown in~\cref{fig:before}. When a function is called, the value
of the \ac{bp} and \ac{ip} is pushed to the stack (\cref{fig:call}) and the
\ac{ip} is set to the address of the called function. When the function returns,
the old \ac{ip} is restored from the stack and the execution continues from
where the function was called. If an overflow overwrites the old \ac{ip}
(\cref{fig:exploit}), the attacker controls where execution continues.

\begin{figure}[h!]
  \begin{subfigure}[b]{.3\textwidth}
  \includegraphics[width=\textwidth]{./dot/before.pdf}
  \caption{Stack layout before function call}\label{fig:before}
  \end{subfigure}\\

  \begin{subfigure}[b]{.3\textwidth}
  \includegraphics[width=\textwidth]{./dot/call.pdf}
  \caption{Stack layout after function call}\label{fig:call}
  \end{subfigure}\\

  \begin{subfigure}[b]{.3\textwidth}
  \includegraphics[width=\textwidth]{./dot/exploit.pdf}
  \caption{Stack layout after overflow}\label{fig:exploit}
  \end{subfigure}
  \caption{Stack layouts during an \ac{bof} exploit}
\end{figure}%

This is only one of several types and exploitation techniques. Others include

\begin{itemize}

  \item Heap-based \ac{bof}: In this case there is no way of overwriting the
    return address but objects on the heap might contain function pointers
    (e.g.\ for dynamic dispatch) which can be overwritten to execute the
    attackers code, when executed~\cite{Detection2018}.

  \item Integer overflow: Some calculation on fixed sized integers is used to
    allocate memory. The calculation leads to an integer overflow and only a
    small buffer is allocated~\cite{Detection2018}. Later a big integer into the
    buffer is used and reads or writes outside the buffer. This kind of
    vulnerability can also lead to other problems because at least in C, signed
    integer overflow is undefined behaviour.

\end{itemize}

This paper won't explore other kinds of \ac{bof} in detail because the concept
is always the same: Unchecked indexing into memory allows the attacker to
overwrite some kind of return or call address, which allows hijacking of the
execution flow.

The most trivial kinds of payloads is known as a \mintinline{ASM}{NOP} sled.
Here the attacker appends as many \mintinline{ASM}{NOP} instructions before any
shell-code (e.g.\ to invoke \mintinline{shell}{/bin/sh}) and points the
overwritten \ac{ip} or function pointer somewhere inside the
\mintinline{ASM}{NOP}s. The execution \enquote{slides} (hence the name) through
the \mintinline{ASM}{NOP}s until it reaches the shell-code. Most of the
mitigation techniques described in this paper protect against this kind of
exploit but there are different and more complex ways of exploiting \acp{bof}
that are not that easily mitigated.

\section{Concept and Methods}\label{ref:concept}

\subsection{Research Methods}

This paper describes several techniques that have been proposed to mitigate the
problems introduced by \acp{bof} and tries to answer the following questions:

\begin{itemize}

  \item What is the performance impact?

  \item How effective is the technique? Did it actually prevent exploitation of
    \acp{bof}?

  \item How realistic is it for developers to use the technique in real-world
    code? Can it be introduced incrementally?

\end{itemize}

The paper focuses on solutions for the C language, since it is still the second
most used language as of December 2019~\cite{Tiobe2019}. Some of the described
techniques are language agnostic but this is not a focus of this paper. In the
end, there is a discussion about the current state.

For the literature research, the paper~\citetitle{Detection2018} served as a
base. From there a snowball system search with combinations of the keywords
\enquote{buffer}, \enquote{overflow}, \enquote{detection}, \enquote{prevention}
and \enquote{dependent typing} was performed using
\url{https://scholar.google.com/}.

Results are evaluated and prioritized using the following criteria:

\begin{itemize}

  \item Type of publication in the following order:

    \begin{enumerate}
      \item conference paper
      \item unreleased paper
      \item books
      \item online sources
    \end{enumerate}

  \item Number of citations

  \item Publisher

  \item Author's reputation and institute

  \item Overall quality (first by checking structure and abstract, then by
    the actual content)

\end{itemize}


\subsection{\Ac{rbc}}

The easiest and maybe single most effective method to prevent \acp{bof} is to
check, if a write or read operation is out of bounds. This requires storing the
size of a buffer together with the pointer to the buffer (so called fat
pointers) and check for each read or write in the buffer, if it is in bounds at
runtime. Still almost any language that comes with a runtime, uses runtime
checking. For this technique to be effective effective in general, writes to a
raw pointer must be disallowed.  Otherwise the security checks can be
circumvented. \Ac{rbc} introduces a runtime overhead for every indexed read or
write operation. This is a problem if a program runs on limited hardware or
might impact real-time properties.

Introducing \ac{rbc} into an existing codebase is not easy. Using fat pointers
in a few functions does not prevent other parts of the code to use raw pointers
into the same buffer. So for this to be effective, the whole codebase needs to
be changed to disallow raw pointers, which, depending on the size, might not be
feasible. Still, if done correctly and consequently, it is simply impossible to
exploit \acp{bof} for code execution. \Ac{dos} is still possible because the
program terminates gracefully when a out of bounds index is used.

\subsection{Prevent/Detect Overwriting Return Address}

Since most traditional \ac{bof} exploits work by overwriting the return address
in the current stack frame, preventing or at least detecting this, can be quite
effective without much overhead at runtime. \citeauthor{Rad2001} describe a
technique that stores a redundant copy of the return address in a secure memory
area that is guarded by read-only memory, so it cannot be overwritten by
overflows. When returning, the copy of the return address is compared to the one
in the current stack frame and only, if it matches, the \mintinline{ASM}{RET}
instruction is actually executed~\cite{Rad2001}. While this is effective against
stack based \acp{bof}, in the described form, it does not protect against vtable
overwrites. An extension could be made to also protect the \ac{plt} and vtables
but custom constructs using function pointers would still be vulnerable. Since
this technique is a compiler extension, no modification of the codebase is
required to enable it, and while it does not prevent all kinds of \ac{bof},
mitigates all stack based \acp{bof} with only minimal overhead when calling and
returning from a function.

An older technique from 1998 proposes to put a canary word (named after the
canaries that were used in mines to detect low oxygen levels) between the data
of a stack frame and the return address~\cite{Stackguard1998}\cite{AtkDef2016}.
When returning, the canary is checked, if it is still intact and if not, a
\ac{bof} occurred.  This technique is implemented by major
compilers~\cite{Gcc2003} but can be defeated, if there is an information leak
that leaks the canary to the attacker.  The attacker is then able to construct a
payload, that keeps the canary intact.  This mitigation has a minimal
performance impact~\cite{Gcc2003} and offers a good level of protection. It is a
compiler extension so no modification of the code base is needed.

% \subsection{Restricting Language Features to a Secure Subset}
% \subsection{Static Analysis}
\subsection{Type System Solutions}

\citeauthor{Dep2007} propose an extension to the C type system that extends it
with dependent types. These types have an associated value, e.g.\ a pointer type
can have the buffer size associated to it~\cite{Dep2007}. This prevents indexing
into a buffer with out-of-bounds values. This extension is a superset of C so
any valid C code can be compiled using the extension and the codebase is
improved incrementally.  If the type extension is advanced enough, the
additional information might form the base for a formal verification. In some
cases, the type extensions can even be inferred~\cite{Dep2007}.

This technique prevents all kinds of overflows, if used, but requires changes to
the codebase and is only effective where these changes are applied. Since it is
a compile-time solution, it does affect the compile-time but has no negative
effect on the runtime.

\subsection{Address Space Layout Randomization}

\Ac{aslr} aims to prevent exploitation of \acp{bof} by placing code at random
locations in memory~\cite{AtkDef2016}. That way, it is not trivial to set the
return address to point to the payload in memory. This is effective against
every kind of \ac{bof} vulnerability but it is still possible to exploit
\ac{bof} vulnerabilities in combination with information leaks or other
techniques like heap spraying. Also on 32 bit systems, the address space is
small enough to try a brute-force attempt until the payload in memory is
hit~\cite{Effectiveness2014}.

This is another technique that works without modification of the code base. Also
there is no runtime overhead because nothing changed except the location of the
program.

\subsection{w\^{}x Memory}

w\^{}x (also known as \ac{nx} or \ac{dep}) makes memory either writable or
executable~\cite{AtkDef2016}. That way, an attacker cannot place arbitrary
payloads in memory.  There are still techniques to exploit this by reusing
existing executable code.  The ret-to-libc exploiting technique uses existing
calls to the libc with attacker controlled parameters, e.g.\ if the program uses
the \mintinline{shell}{system} command, the attacker can plant
\mintinline{shell}{/bin/sh} as parameter on the stack, followed by the address
of \mintinline{shell}{system} and get a shell on the system. \ac{rop} (a
superset of ret-to-libc exploits) uses so called \ac{rop} gadgets, combinations
of memory modifying instructions followed by the \mintinline{ASM}{RET}
instruction to build instruction chains, that execute the desired shell-code.
This is done by placing the desired return addresses in the right order on the
stack and reuses the existing code to circumvent the w\^{}x protection. These
combinations of memory modification followed by \mintinline{ASM}{RET}
instructions are called \ac{rop} chains and are Turing complete~\cite{Rop2007},
so in theory it is possible to implement any imaginable payload, as long as the
exploited program contains enough gadgets and the overflowing buffer has enough
space.


\section{Discussion}\label{ref:discussion}

\subsection{Effectiveness}

\subsubsection{\ac{aslr}}

\Ac{aslr} has been proven effective and is wildly used in production. It is
included in most major operating systems~\cite{FBSDaslr}. Some even use kernel
\ac{aslr}~\cite{Linuxaslr}. Since this mechanism is active at runtime, it does
not require any changes in the code itself, the program only has to be compiled
as a \ac{pie}. On 32-bit CPUs, only 16-bit of the address are randomized. These
16-bit can be brute forced in a few minutes or seconds~\cite{AslrEffective2004}.

There is no runtime overhead since the only change is the position of the
program in memory. Since there is no additional work except maybe recompilation,
this technique can and should be used on modern systems.

\subsubsection{w\^{}x}

With the rise of \ac{rop} techniques, w\^{}x protection has been shown to be
ineffective. It makes vulnerabilities harder to exploit by preventing the most
naive types of payloads but it doesn't actually prevent exploits from happening.

\Ac{nx} does not prevent any exploits but makes it harder for an attacker that
does not know the system, the program is running on (e.g.\ a network service).
It has no runtime overhead and is a compile-time option so it does not hurt to
enable \ac{nx}.

\subsubsection{Runtime Bounds Checks}

Checking for overflows at runtime is very effective but can have a huge
performance impact so it is not feasible in every case. It also comes with other
footguns. There might be integer overflows when calculating the bounds which
might introduce other problems.

\subsection{State of the Art}

Operating systems started to compile C code to \ac{pie} by
default~\cite{ArchPie2017} and \ac{aslr} is enabled, too. Same goes for \ac{nx}
and stack canaries~\cite{ArchPie2017}. The combination of these mitigations
makes it hard to write general exploits for modern operating systems.

To check the current state, the latest release of the \ac{gcc} (9.2) and the
latest commit of the LLVM-project (\mintinline[breaklines]{shell}{181ab91efc9})
are compiled using the default configuration. The experiments are performed on a
64-bit Debian 9.11 system running on version 4.19.0 of the Linux kernel. The
following commands are used for compilation:

\begin{figure}[h!]
\begin{subfigure}[b]{.3\textwidth}
\begin{minted}{shell}
  mkdir objdir \
  && cd objdir \
  && ../configure \
    --build=x86_64-linux-gnu \
    --host=x86_64-linux-gnu \
    --target=x86_64-linux-gnu \
    --disable-multilib \
  && make -j8
\end{minted}
\caption{\ac{gcc} compilation script}\label{lst:gcc}
\end{subfigure}
\\
\begin{subfigure}[b]{.3\textwidth}
\begin{minted}{shell}
  mkdir build \
  && cd build \
  && cmake -DLLVM_ENABLE_PROJECTS=clang \
    -DCMAKE_BUILD_TYPE=Release \
    -G "Unix Makefiles" ../llvm \
  && make -j8
\end{minted}
\caption{clang compilation script}
\end{subfigure}
\end{figure}

The \mintinline{shell}{build}, \mintinline{shell}{host} and
\mintinline{shell}{target} parameters in~\ref{lst:gcc} only describe the target
platform for the compiler and \mintinline{shell}{disable-multilib} disables
32-bit support. The \mintinline{sh}{-j8} flag only tells make to use all 8
available cores for compilation.

The fresh builds of \ac{gcc} and clang compile the code from~\cref{lst:vuln} to
check which mitigations are enabled by default. Using
\mintinline[breaklines]{shell}{gcc -o vuln.gcc vuln.c} and
\mintinline[breaklines]{shell}{clang -o vuln.clang vuln.c} to compile the source
code, the \mintinline{shell}{checksec.sh} tool~\cite{Checksec2019} shows which
mitigations are active in the new binary:

\begin{table}[h!]
\begin{center}
\begin{tabular}{lll}
  \toprule
  Mitigation & Active in \ac{gcc}? & Active in clang? \\
  \toprule
  Stack Canary & No & No \\
  \midrule
  \ac{nx} & Yes & Yes \\
  \midrule
  \ac{pie} & No & No \\
  \bottomrule
\end{tabular}
\caption{Enabled mitigations in a default \ac{gcc} and clang
  build}\label{tab:mitigations}
\end{center}
\end{table}

Surprisingly enough, two of the most popular C compilers enable only one of the
described compile-time mitigations by default. Maintainer of operating system
packages of the compiler might choose a more secure configuration for the
compiler as shown in~\cite{ArchPie2017} but still, compiler vendors might want
to choose better defaults, too.

So far, all described mitigations don't change anything about the existence of
\acp{bof} but just try to prevent the exploitation for code execution. The
vulnerable programs will still terminate if the stack canary is overwritten, a
call into \ac{nx} memory occurs or execution continues inside garbage data due
to \ac{aslr}. The underlying problem persists, only the worst results are
mitigated. \Ac{dos} is still a problem in safety critical systems (e.g.\ cars,
planes, medical devices) or in any area with real-time requirements.

Language extensions to fix the problem of \acp{bof} as described
in~\cite{Dep2007} require lots of discipline to use them everywhere. They are
only useful if the whole codebase uses the new features. Introducing them in an
existing codebase is quite unrealistic since it requires lots of modifications.
On the other hand, this actually prevents \acp{bof} from happening and not just
from being exploited, so it looks like an interesting concept for safety
critical software.

\section{Conclusion}\label{ref:conclusion}

While there are many techniques, that protect against different types of
\acp{bof}, none of them is effective in every situation but in combination they
offer good protection against code execution attacks. Maybe we've come to a
point where we have to stop using memory unsafe languages where it is not
inevitable. There are many modern programming languages, that aim for the same
problem space as C, C++ or Fortran but without the issues coming from these
languages. If it is feasible to use a garbage collector, languages like Go, Java
or even scripting languages like Python might work just fine. If real-time
properties are required, Rust could be the way to go, without any language
runtime and with deterministic memory management. For any other problem, almost
any other memory safe language is better than using unsafe C.

% \section{Sources (Dummy Section for Deadline)}

% \begin{itemize}

%   \item RAD:\ A Compile-Time Solution to Buffer Overflow Attacks~\cite{Rad2001}
%     (might not protect against e.g.\ vtable overwrites, \ac{plt} address
%     changes, \dots)

%   \item Dependent types for low-level programming~\cite{Dep2007}

%   \item StackGuard: Automatic Adaptive Detection and Prevention of
%     Buffer-Overflow Attachs~\cite{Stackguard1998} (ineffective in combination
%     with information leaks)

%   \item Type-Assisted Dynamic Buffer Overflow Detection~\cite{TypeAssisted2002}

%   \item On the Effectiveness of NX, SSP, RenewSSP, and \ac{aslr} against Stack
%     Buffer Overflows~\cite{Effectiveness2014}

%   \item What Do We Know About Buffer Overflow Detection?: A Survey on Techniques
%     to Detect A Persistent Vulnerability~\cite{Detection2018}

%   \item Survey of Attacks and Defenses on Stack-based Buffer Overflow
%     Vulnerability~\cite{AtkDef2016}

%   \item Beyond stack smashing: recent advances in exploiting buffer
%     overruns~\cite{Smashing2004}

%   \item Runtime countermeasures for code injection attacks against C and C++
%     programs~\cite{Counter2012}

% \end{itemize}


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