# computer misuse: rust vtable patching

this is the first in a loosely-affiliated series of posts about wildly misusing what is available for us on the computer. setting the tone some, this first post is about an old bit of rust vtable patching i hacked together years ago.

the claim at the time was that rust has no affordances for monkey-patching, and i decided to prove otherwise\* as much as possible\*\*. in case you're fluent in rust and x86, just posting the code might be fine. afterward, i'll talk about it a bit.

```
trait Person {
    fn greeting(&self) -> StateOfMind;
    fn name(&self) -> &'static str;
}

struct Ixi { }
struct Katie { }

#[derive(Debug)]
enum StateOfMind {
    GoodMorning,
    HappyBirthday,
}

impl Person for Ixi {
    fn greeting(&self) -> StateOfMind {
        StateOfMind::GoodMorning
    }
    fn name(&self) -> &'static str { "ixi" }
}

impl Person for Katie {
    fn greeting(&self) -> StateOfMind {
        StateOfMind::HappyBirthday
    }
    fn name(&self) -> &'static str { "katie" }
}

#[inline(never)]
fn greet(you: &dyn Person) {
    println!("{} says {:?} to you!", you.name(), you.greeting());
}

#[inline(never)]
fn correct_ixi() {
    unsafe {
        // gotta reprogram ixi real quick
        let ixi_in_a_box: Box<dyn Person> = Box::new(Ixi {});
        #[inline(never)]
        unsafe fn inner_takes_dyn(v: Box<dyn Person>) {
            if let Some(vtable_offset) = get_greeting_offset() {
                // ok we know where in the vtable to patch, just not where the vtable itself is yet.

                // v is a pointer to (data, vtable)
                let dyn_thingie: [*const usize; 2] = std::mem::transmute(v);
                let vtable = dyn_thingie[1];
                let table_entry = vtable_offset as usize / std::mem::size_of::<usize>();
                // get a pointer to the entry in the vtable we want to fix
                let vtable_ptr = (vtable as *mut usize).offset(table_entry as isize);
                let katie_greet = get_katie_greet_address()
                    .expect("if we got here we can read ixi's vtable, and the same should work for katie");
                    
                // memory mapping will have this as read-only. gotta fix that...
                libc::mprotect(
                    (vtable_ptr as usize & !(4095)) as *mut _,
                    4096 * 2,
                    7, // READ | WRITE
                );

                // and commit a crime
                std::ptr::write_volatile(vtable_ptr, katie_greet);

                // clean up after though
                libc::mprotect(
                    (vtable_ptr as usize & !(4095)) as *mut _,
                    4096 * 2,
                    1, // READ | WRITE
                );
            } else {
                panic!("well we need a vtable offset");
            }
        }

        inner_takes_dyn(ixi_in_a_box);
    }
}

// just get the offset of `greeting` in the `Person` vtable
#[inline(never)]
unsafe fn get_greeting_offset() -> Option<u32> {
    // the function is read, so this will be compiled.
    // because `greeting` does not return a type with a destructor, it
    // will end with a tail call to the function we want. the offset
    // of that tail call is the magic number for the offset of the
    // function to replace in the vtable.
    //
    // in debug builds, this ends with `call; pop; ret`. look for both.
    // in both cases these are preceeded by `mov rdi, rax`, spelled
    // `4889c7`.
    fn do_greeting(b: &Box<dyn Person>) {
        b.greeting();
    }
    let f = do_greeting as *const u8;
    std::ptr::read_volatile(f);
    
    // find the magic sigil. 0 indicates we couldn't find it!
    let mut i = 1;
    let mut vtable_offset = None;
    while i < 100 {
        let curr = f.offset(i);
        if *curr == 0x48 && *(curr.offset(1)) == 0x89 && *(curr.offset(2)) == 0xc7 {
            // now the offset is encoded in one of two ways: either an 8-bit
            // offset if the vtable is tiny, or a 32-bit offset if it's larger.
            //
            // 8-bit offst has modrm bits of     01_rrr_mmm
            // 32-bit offset has modrm bits of   10_rrr_mmm
            // for example, "call [rcx + 0x18]" is spelled
            // ff 61 18
            //  ^  ^  ^-- offset we want
            //  |  ------ modrm 01_000_001, 8-bit displacement off reg 001 (rcx), r=0 (call)
            //  --------- opcode for call/jmp [mem]
            let modrm: u8 = *curr.offset(4);
            let offset = curr.offset(5); // we might not actually read this, depending on mod bits
            match modrm >> 6 {
                0b00 => {
                    // no offset - this is just `call [reg]`
                    vtable_offset = Some(0);
                }
                0b01 => {
                    // 8-bit offset
                    vtable_offset = Some(std::ptr::read_unaligned(offset) as u32);
                }
                0b10 => {
                    // 32-bit offset
                    vtable_offset = Some(std::ptr::read_unaligned(offset as *const u32));
                }
                0b11 => {
                    // this is actually `{call,jmp} reg`. we'd be in a bad spot here.
                }
                _ => {
                    // these are just unreachable. if everything is well-formed, anyway.
                    unreachable!();
                }
            }
            // anyway we found the instruction so lets break and move on
            break;
        }
        i += 1;
    }
    
    vtable_offset
}

fn get_katie_greet_address() -> Option<usize> {
    // this is for the most part the same logic as correcting ixi, but instead of
    // writing to ixi's vtable, we read katie's vtable
    unsafe {
        let katie_in_a_box: Box<dyn Person> = Box::new(Katie {});
        #[inline(never)]
        unsafe fn inner_takes_dyn(v: Box<dyn Person>) -> Option<usize>{
            if let Some(vtable_offset) = get_greeting_offset() {
                // ok we know where in the vtable to read, just not where the vtable itself is yet.

                // v is a pointer to (data, vtable)
                let dyn_thingie: [*const usize; 2] = std::mem::transmute(v);
                let vtable = dyn_thingie[1];
                let table_entry = vtable_offset as usize / std::mem::size_of::<usize>();
                // get a pointer to the entry in the vtable we want to read
                let vtable_ptr = (vtable as *mut usize).offset(table_entry as isize);
                // and commit a crime
                Some(*vtable_ptr)
            } else {
                None
            }
        }

        inner_takes_dyn(katie_in_a_box)
    }
}

fn main() {
    greet(&Ixi {});
    greet(&Katie {});
    println!("wait that's not right, hold on a sec...");
    correct_ixi();
    greet(&Ixi {});
}
```

... and when you run it, you'll get ... the wrong thing. gotta figure it out and fix.

<h1 id="tldr">tl;dr? is rust bad?</h1>

<a href="/index.html">index</a>

<h3 id="ntdll_RtlQueryPerformanceCounter">ps: some windows stuff</h3>

<div class="codebox">
#eval radare2 -q -c 'pd 43 @ 0x180040150' ./now_what/ntdll.dll | aha --no-header --stylesheet
</div>