use-after-free in tracing
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This means that these pointers are no longer valid.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
