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work/01paper.tex
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@ -65,7 +65,14 @@ MatrNr. 3220018}
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.
and ease of use. It discusses compiler based (such as ASLR, NX, stack
canaries) as well as type system based (e.g.\ dependent types) solutions to
this prevalent type of software bugs based on their performance impact and the
effort needed to introduce the mitigations into existing software projects. An
analysis of the current state of the art informs the reader about what to
expect when writing software today. The analysis shows that most techniques
actually tackle the problem of exploiting buffer overflows for code execution
but do nothing to prevent introducing them in the first place.
\end{abstract}
@ -77,27 +84,26 @@ Buffer Overflow, Software Security
\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) or 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.
In the early days of programming, memory as managed manually to make the best
use of slow hardware and low memory. 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) or 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 modern programming
languages solve most if not all of these problems, critical parts of the worlds
infrastructure are still implemented in these old languages, either because they
allow the implementation of 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.
Reading out of bounds can result in an information leak and is one of the less
critical results of \ac{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
@ -115,8 +121,7 @@ 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.
C++) overwriting the \ac{vmt}, which stores the pointers to an object's methods.
A simple vulnerable C program might look like this:
@ -139,18 +144,18 @@ int main(int argc, char **argv) {
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.
variable, writing it to a file that the program reads, via network packet,
\dots) and eventually overwrite the return address by providing an input with
more than 50 bytes and therefore writing out of bounds. When executing the
\mintinline{C}{return} instruction, and the \ac{ip} jumps into the payload, the
attacker's code is executed. This works due to the way, how CPUs perform
function calls: The stack frame of the current function lies between the \ac{bp}
and \ac{sp} as shown in~\cref{fig:before}. When calling a function, the value of
the \ac{bp} and \ac{ip} is pushed to the stack (\cref{fig:call}) and the CPU
writes the address of the called function into the \ac{ip}. When the function
returns, after restoring the old \ac{ip} from the stack, the execution continues
from where the function call occurred earlier. 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}
@ -177,23 +182,23 @@ This is only one of several types and exploitation techniques. Others include
\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}.
attackers code, when called~\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.
small buffer is allocated~\cite{Detection2018}. Later the buffer is indexed
with a big integer and performs a read or write 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
This paper does not 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.
The most trivial kind 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
@ -218,7 +223,7 @@ problems introduced by \acp{bof} and tries to answer the following questions:
\acp{bof}?
\item How realistic is it for developers to use the technique in real-world
code? Can it be introduced incrementally?
code? Is an incremental introduction possible?
\end{itemize}
@ -228,12 +233,12 @@ 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
base. From there on, the author performed a snowball system search with
combinations of the keywords \enquote{buffer}, \enquote{overflow},
\enquote{detection}, \enquote{prevention} and \enquote{dependent typing} using
\url{https://scholar.google.com/}.
Results are evaluated and prioritized using the following criteria:
Evaluation and prioritization of results is done using the following criteria:
\begin{itemize}
@ -264,49 +269,50 @@ 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.
runtime. Almost any language that comes with a managed runtime, uses \ac{rbc}.
For this technique to be effective effective in general, writes to raw pointers
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.
feasible. Still, if done correctly and consequently, there will be no \ac{bof}
vulnerabilities. \Ac{dos} might is still possible depending on how invalid
indexing is handled, because the program might terminate 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
Since stack based \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}
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.
stack based \acp{bof}, in the described form, it does not protect against
\ac{vmt} or \ac{plt} overwrites. An extension could be made to also protect the
\ac{plt} and \ac{vmt} but custom constructs using function pointers would remain
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}, it 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
When returning, a check is performed, to confirm, the canary is intact, if it is
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.
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 there is no need for modification of the code base.
\subsection{Type System Solutions}
@ -314,15 +320,16 @@ compiler extension so no modification of the code base is needed.
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}.
compilation of any valid C code is possible using the extension and incremental
improvement of the codebase is possible. If the type extension is advanced
enough, the additional information might form the base for a formal
verification. In some cases, inference of the type extensions is
possible~\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.
a compile-time solution, it affects the compile-time but has no negative effect
on the runtime.
\subsection{Address Space Layout Randomization}
@ -348,15 +355,15 @@ 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
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
This is achieved 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, known as \ac{rop} chains, are Turing complete~\cite{Rop2007}, so
in theory it is possible to construct any imaginable payload, as long as the
exploited program contains enough gadgets and the overflowing buffer has enough
space.
@ -367,22 +374,23 @@ space.
\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} has proven effective and sees wide use in production. Most major
operating systems implement this technique~\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.
program in memory. Since there is no additional work required 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.
The rise of code reuse exploits like \ac{rop} and ret-to-libc, shows the
ineffectiveness of w\^{}x protection. 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).
@ -398,7 +406,7 @@ might introduce other problems.
\subsection{State of the Art}
Operating systems started to compile C code to \ac{pie} by
Operating systems started to compile C code to \acp{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.
@ -406,7 +414,7 @@ makes it hard to write general exploits for modern operating systems.
To check the current state, the author investigates, which mitigations are
enabled by default in the latest release (9.2) of the \ac{gcc} and the latest
commit of the LLVM-project (\mintinline[breaklines]{shell}{181ab91efc9}) by
compiling both compilers using the default configuration. The experiments are
building both compilers 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 compile the source codes:
@ -441,12 +449,13 @@ kernel. The following commands compile the source codes:
The \mintinline{shell}{build}, \mintinline{shell}{host} and
\mintinline{shell}{target} parameters in~\cref{lst:gcc} 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. \mintinline{shell}{CMAKE_BUILD_TYPE=Release}
creates a release build of the clang compiler (see~\cref{lst:clang}).
32-bit support, which is not needed for this experiment. The
\mintinline{sh}{-j8} flag only tells make to use all 8 available cores for
compilation. \mintinline{shell}{CMAKE_BUILD_TYPE=Release} creates a release
build of the clang compiler (see~\cref{lst:clang}).
The fresh builds of \ac{gcc} and clang compile the code from~\cref{lst:vuln} to
check which mitigations are enabled by default. Using
check which mitigations are enabled by default. After 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
@ -506,40 +515,6 @@ 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}
\printbibliography{}
% \bibliographystyle{IEEEtran}
% \bibliography{bibliography}

5
work/acronyms.tex
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@ -62,3 +62,8 @@
short = DEP,
long = data execution prevention
}
\DeclareAcronym{vmt}{
short = VMT,
long = virtual method table
}