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.
The return addres can be pinpointed, for example 0x41 buffer 0x42 address:
We can reproduce it simulating the buffer from golang in this way:
we can dump the address of a function and redirect the execution to it:
In this way it’s possible to build a rop chain using golang runtime to unprotect a shellcode.