Some library variants may not supply these symbols for your target architecture. If you are linking with the compiler’s standard libraries, you will have these symbols. On OS X, they are implemented in libSystem. The _stack_chk_guard and _stack_chk_fail symbols are normally supplied by a GCC library called libssp. Some compilers are built with -fstack-protector enabled by default, but the standard setting tends to be -fno-stack-protector by default. -fno-stack-protector: disable stack protection.-fstack-protector-all: add stack protection to all function calls.Uses register local variables (although the register keyword is outdated).Has a struct or union containing an array, regardless of array type or length.Local variable is an array, regardless of array type or length.A local variable’s address is used as part of the right hand side of an assignment or function argument.-fstack-protector-strong: increases coverage beyond -fstack-protector by inserting stack protection under the following conditions:.This can be configured by specifying a -param=ssp-buffer-size=X, where X=8 by default.-fstack-protector: add stack protection to functions with local char buffers larger than 8 bytes, or calls to alloca.GCC and Clang provide a set of related flags that control how stack protection is used in a program: If a stack buffer overflow is detected, the canary value will no longer match the value of _stack_chk_guard, and _stack_chk_fail() will be called, aborting the program. This function shall abort the function that called it with a message that a stack buffer overflow has been detected, and then halt the program via exit, abort, or a custom panic handler._stack_chk_fail, a callback function that is invoked when a stack buffer overflow is detected._stack_chk_guard, which contains the value of the stack protection canary word.The two symbols inserted by the compiler are: If ((canary = canary ^ _stack_chk_guard) != 0 ) If SSP is enabled for your program, GCC and Clang will automatically transform the function into something resembling: extern uintptr_t _stack_chk_guard The Clang and GCC ApproachĬonsider a trivial function written just for demonstration purposes: void a_function(const char* input) Note: Embedded Artistry members can find more detailed information about stack buffer overflows and stack smashing protection in the Field Atlas. Compiler flags are used to control the degree that your program is checked. These additions are made by the compiler without your involvement. A failure callback is invoked, which prints an error message and terminates the program. If it has been modified, a stack buffer overflow has occurred. Before returning from the function, the stored canary value in the stack is checked. In order to detect this case, a “canary value” is added to the stack before other values are declared. The fundamental assumption behind the approach is that most stack buffer overflows occur by writing past the end of a function’s stack frame. In this article, we’ll discuss stack smashing protection (SSP), related compiler flags, and an implementation that is suitable for use on microcontrollers.Ĭlang, GCC, and related compilers implement stack smashing protection (SSP) using StackGuard. This support can be enabled in our programs by setting a few compiler flags. When conditions are right, an attacker can insert executable instructions into the program and gain unintended access to a system.Ĭlang, GCC, and other related compilers provide built-in support for detecting stack buffer overflows and aborting a program if one is detected. An attacker can deliberately trigger a stack buffer overflow as part of a “stack smashing” attack. These errors also create potential security vulnerabilities in our programs. A stack buffer overflow occurs when a program writes to a memory address on the stack which is outside of its current stack frame, often triggered by a buffer overflow on a local stack variable. Stack buffer overflows are a category of error that can wreak havoc on our programs, resulting in sporadic crashes or strange and unexpected program behaviors.
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