use-after-free in tracing

Summary

The implementation of the [`Instrumented::into_inner`] method in affected versions of this crate contains undefined behavior due to incorrect use of [`std::mem::forget`] The function creates `*const` pointers to `self`, calls [`mem::forget(self)`][`std::mem::forget`], and then moves values out of those pointers using [`std::ptr::read`].

```rust // To manually destructure `Instrumented` without `Drop`, we // move it into a ManuallyDrop and use pointers to its fields let span: *const Span = &this.span; let inner: *const ManuallyDrop<T> = &this.inner; mem::forget(self); // SAFETY: Those pointers are valid for reads, because `Drop` didn't // run, and properly aligned, because `Instrumented` isn't // `#[repr(packed)]`. let _span = unsafe { span.read() }; let inner = unsafe { inner.read() }; ```

However, the [`mem::forget` documentation][`std::mem::forget`] states:

> Any resources the value manages, such as heap memory or a file handle, will > linger forever in an unreachable state. **However, it does not guarantee that > pointers to this memory will remain valid.**

This means that these pointers are no longer valid. This could result in a stack use-after-free if LLVM chooses to reuse `self`'s stack slot for a rebinding after the call to [`std::mem::forget`].

This undefined behavior has not been observed to cause miscompilation as of Rust 1.73.0. However, any use of this method with the affected versions of `tracing` are unsound.

The flaw was corrected in commit [20a1762] ([PR #2765]) by replacing the use of [`std::mem::forget`] with `std::mem::ManuallyDrop`, ensuring that the stack slot is not reused and the pointers remain valid when they are read. The fix is published in `tracing` [v0.1.40]. Affected versions have been yanked from crates.io.

Thanks to [Taylor Cramer] and [Manish Goregaokar] for finding and correcting this issue!

[`Instrumented::into_inner`]: https://docs.rs/tracing/latest/tracing/instrument/struct.Instrumented.html#method.into_inner [`std::mem::forget`]: https://doc.rust-lang.org/std/mem/fn.forget.html [`std::ptr::read`]: https://doc.rust-lang.org/std/primitive.pointer.html#method.read-1 [20a1762]: https://github.com/tokio-rs/tracing/commit/20a1762b3fd5f1fafead198fd18e469c68683721 [PR #2765]: https://github.com/tokio-rs/tracing/pull/2765 [v0.1.40]: https://crates.io/crates/tracing/0.1.40 [Taylor Cramer]: https://github.com/cramertj [Manish Goregaokar]: https://github.com/manishearth