Integer overflow caused by multiply in munmap()

[Marsman1996]

There is a multiply with overflow problem in kernel/src/syscall/munmap.rs, munmap(), Maestro.
The div_ceil() rounds the result towards positive infinity.
As a result, when user calls munmap with large length (i.e., 0xfffffff0), the following multiplication operation will cause an integer overflow problem.

maestro/kernel/src/syscall/munmap.rs

Lines 45 to 46 in e7ebdfa

	let pages = length.div_ceil(PAGE_SIZE);
	let length = pages * PAGE_SIZE;

For example:

use std::usize;

fn main() {
    let length: usize = usize::MAX;
    let page_size = 0x1000;
    let pages = length.div_ceil(page_size);
    let length = pages * page_size;
    println!("length = 0x{length:x}");
}

thread 'main' panicked at src/main.rs:7:18:
attempt to multiply with overflow

[llenotre]

Thank you for the report! I am putting this on my todolist