This makes it vulnerable against an overflow that writes past the end of any of the local variables of the function, like buffer in the example above. When entering a function, the return address will be stored before any local variables on the stack, and therefore has the highest memory address. On most platforms the stack grows downwards, towards zero 1. One typical example for the latter is the handling of user data without proper checks of the limits of data structures: The write can happen on purpose by using pointer arithmetic, or by coding errors. But by writing to memory beyond those variables it is possible to change the return address, and thus alter the control flow of the program. This is very much transparent to the programmer. The stack also stores all variables local to that function. When calling a function, the stack stores the address where execution must resume once the functions returns. Let’s recap: The stack is a memory region that stores data. Perhaps the most dangerous class of memory access errors are stack smashing bugs. Please subscribe via Email or RSS on the right to get notified of the upcoming posts! Stack protector Stack protector will be discussed in this post, other techniques will follow: Executable-space protection, ASLR, RELRO/BIND_NOW, Fortify, RPATH/RUNPATH. We will discuss Linux ELF only (including x86-64 and PowerPC). These techniques are not only meant as a protection against the exploitation of memory errors, but they are most useful for that class of errors. But in today’s world, with many programs exposed to the internet in some form, and exploits doing more and more damage due to more data being handled, and more devices running your software, it is essential to harden your binaries! This may be partly due to the C/C++ mantra of “You pay only for what you need”, partly due to compatibility fears and inertia. You may be surprised to learn that in some toolchains these are not enabled by default. In all these cases hardening provides a safety net to avoid the worst, even when a bug slips into the code.īelow we will discuss several useful hardening techniques.
#STACK GUARD STACK SMASHING DETECTED CODE#
Furthermore there is a large body of legacy code, C code, system code and external libraries that sometimes cannot be audited for correctness. But most of the time you will not be aware of the bug: It might just slip through code review, or might only be triggered under very complex circumstances which no reviewer considered. Now, you might argue that the best way to prevent attacks is to fix the buggy code to make the exploit impossible. This is not strictly necessary since program execution can just be redirected to some code already loaded into memory (“ return-oriented programming“). Possibly inject some code as part of the input (in assembly form) to which execution can be redirected.In most cases the return address on the stack is overwritten to point to code the attacker wants to run. Provide input to the program which makes the buggy code overwrite critical parts of memory so that program execution is redirected.Find a piece of code that can write beyond the bounds of a data structure, preferably on the stack.The basics of an attackĮxploiting a memory access error works as follows: Hardening improves the resiliency of a program against attacks trying to exploit such memory access errors, and other kinds of errors. A large part of the vulnerabilities stem from memory access errors, which happen easily in C/C++ due to the direct, unchecked access that is possible. Nowadays these designs haunt us: The past two decades have uncovered a lot of security vulnerabilities in C/C++ programs, and with many of these programs processing user input from the internet, they can often be exploited easily. Attacks on computer systems were rare, since there was not much incentive to it, so there did not have to be a focus on security and robustness. When C, C++ and the Internet were conceived, they were mostly used by academics.
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