diff --git a/work/01paper.pdf b/work/01paper.pdf index 20c53fd..3b16b7e 100644 Binary files a/work/01paper.pdf and b/work/01paper.pdf differ diff --git a/work/01paper.tex b/work/01paper.tex index 8536b7b..ee8863a 100644 --- a/work/01paper.tex +++ b/work/01paper.tex @@ -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. +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 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} @@ -343,20 +350,20 @@ program. 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 +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 +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} diff --git a/work/acronyms.tex b/work/acronyms.tex index ac10b83..0ff05ec 100644 --- a/work/acronyms.tex +++ b/work/acronyms.tex @@ -62,3 +62,8 @@ short = DEP, long = data execution prevention } + +\DeclareAcronym{vmt}{ + short = VMT, + long = virtual method table +}