Function returns address of local variable
The use of previously-freed memory can have any number of adverse consequences, ranging from the corruption of valid data, to the execution of arbitrary code, depending on the instantiation and timing of the flaw. The simplest way data corruption may occur involves the system's reuse of the freed memory. Use-after-free is a rare situation in C# since it has an internal garbage collector that will not free memory if there are still references to it, but they can still occur in unsafe blocks. The CS.LOCRET checker family finds instances in which an unsafe function provides access to the address of a local variable, outside the scope of the block, leading to issues such as dangling pointer or use-after-free.
The CS.LOCRET.GLOB checker finds instances in which a function returns the address of a local variable by writing it into a global variable.
Vulnerability and risk
Local variables are allocated on the stack, so when a function returns a pointer to the variable, it's returning a stack address. The address will be invalidated after returning from the function, so access will probably cause unexpected application behavior, typically a program crash.
Use-after-free errors have two common and sometimes overlapping causes:
- Error conditions and other exceptional circumstances.
- Confusion over which part of the program is responsible for freeing the memory.
Vulnerable code example
public static int *glob;
unsafe static int* aaa()
int *aux = stackalloc int;
glob = aux; //@ CS.LOCRET.GLOB