corcho libre: Exploiting Golang Unsafe Pointers

There are situations when c interacts with golang for example in a library, and its possible to exploit a golang function writing raw memory using an unsafe.Pointer() parameter.

When golang receive a null terminated string on a *C.Char parameter, can be converted to golang s tring with s2 := C.GoString(s1) we can do string operations with s2 safelly if the null byte is there.

When golang receives a pointer to a buffer on an unsafe.Pointer() and the length of the buffer on a C.int, if the length is not cheated can be converted to a []byte safelly with b := C.GoBytes(buf,sz)

Buuut what happens if golang receives a pointer to a buffer on an unsafe.Pointer() and is an OUT variable? the golang routine has to write on this pointer unsafelly for example we can create a golangs memcpy in the following way:

We convert to uintptr for indexing the pointer and then convert again to pointer casted to a byte pointer dereferenced and every byte is writed in this way.

If b is controlled, the memory can be written and the return pointer of main.main or whatever function can be modified.