Coroutines can be assumed to be 'static despite the resume arg not being such.

[danielhenrymantilla]

I tried this code:

#![forbid(unsafe_code)]
#![feature(coroutine_trait, coroutines)]

use ::core::ops::Coroutine;
use ::std::{thread, time};

fn demo<'not_static>(s: &'not_static str) -> thread::JoinHandle<()> {
    let mut generator = Box::pin({
        #[coroutine]
        move |_ctx| {
            let ctx: &'not_static str = yield;
            yield;
            dbg!(ctx);
        }
    });
    // PROBLEM: `typeof(generator) : 'static`!
    let _assert_usable_for_static: &dyn ::core::any::Any = &generator;

    // exploit:
    generator.as_mut().resume("");
    generator.as_mut().resume(s); // <- generator hoards it as `let ctx`.
    thread::spawn(move || {
        thread::sleep(time::Duration::from_millis(200));
        generator.as_mut().resume(""); // <- resumes from the last `yield`, running `dbg!(ctx)`.
    })
}

fn main() {
    let local = String::from("...");
    let thread = demo(&local);
    drop(local);
    let _unrelated = String::from("UAF");
    thread.join().unwrap();
}

• Demo: godbolt permalink
• Demo: on current playground

I expected to see this happen:

Compilation error because 'not_static : 'static would be needed for that spawn() (or Any) to pass.

Instead, this happened:

No compilation error, thus unsoundness ensues (coroutine prints UAF in the current playground, thereby proving it is reading unrelated/UAF memory. Miri reports UB).

Meta

• rustc --version --verbose:

  2024-10-23 4f2f477fded0a47b21ed
  

@rustbot label +I-unsound +A-coroutines +F-coroutines +F-coroutine_trait +requires-nightly

[rustbot]

Error: Parsing relabel command in comment failed: ...' label add' | error: a label delta at >| ': +I-unsou'...

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[compiler-errors]

Reduced a bit:

#![forbid(unsafe_code)]
#![feature(coroutine_trait, coroutines)]

use std::ops::Coroutine;
use std::pin::Pin;

fn demo<'not_static>(s: &'not_static str) -> Pin<Box<impl Coroutine<&'not_static str> + 'static>> {
    let mut generator = Box::pin({
        #[coroutine]
        move |ctx: &'not_static str| {
            yield;
            dbg!(ctx);
        }
    });
    generator.as_mut().resume(s);
    generator
}

fn main() {
    let local = String::from("...");
    let mut coro = demo(&local);
    drop(local);
    let _unrelated = String::from("UAF");
    coro.as_mut().resume("");
}

[compiler-errors]

Given that I fixed #119563, I can also put up a fix for this too. I see where to fix, just need to write up justification for why it's sufficient.

@rustbot claim