tokio::io::unix::AsyncFd provides safe constructors (new, with_interest, try_new, and try_with_interest) that accept any T: AsRawFd rather than requiring proof of ownership or validity of the file descriptor. Because RawFd implements AsRawFd, safe code can construct an AsyncFd from an arbitrary or closed file descriptor, or close/replace the underlying file descriptor while the AsyncFd is still alive.
This violates I/O safety in two ways:
AsyncFd<T> implements AsFd for all T: AsRawFd by calling BorrowedFd::borrow_raw(self.as_raw_fd()), allowing safe code to obtain an arbitrary BorrowedFd<'_> without owning the file descriptor.
Tokio's I/O driver stores a pointer to an internal allocation in the epoll/kqueue user data for registered file descriptors. If the underlying file descriptor is duplicated and then closed while an AsyncFd is alive, and the OS reuses that file descriptor number for a another IO resource on the same runtime, dropping the first AsyncFd deregisters the reused file descriptor number and frees the first while the original file description remains registered in epoll, leading to a use-after-free when a readiness event fires.
Starting in Tokio 1.51.6, 1.53.3, and 1.54.0, AsyncFd::new, AsyncFd::with_interest, AsyncFd::try_new, and AsyncFd::try_with_interest are deprecated in favor of the unsafe constructors AsyncFd::register and AsyncFd::register_with_interest. Callers must ensure that the file descriptor remains valid, open, and unchanged for the lifetime of the AsyncFd.
> `AsyncFd::new` and related constructors violate I/O safety
| Details | |
| ------------------- | ---------------------------------------------- |
| Status | unsound |
| Package | `tokio` |
| Version | `1.53.1` |
| URL | [https://github.com/tokio-rs/tokio/pull/8587](https://github.com/tokio-rs/tokio/pull/8587) |
| Date | 2026-10-13 |
`tokio::io::unix::AsyncFd` provides safe constructors (`new`, `with_interest`, `try_new`, and `try_with_interest`) that accept any `T: AsRawFd` rather than requiring proof of ownership or validity of the file descriptor. Because `RawFd` implements `AsRawFd`, safe code can construct an `AsyncFd` from an arbitrary or closed file descriptor, or close/replace the underlying file descriptor while the `AsyncFd` is still alive.
This violates I/O safety in two ways:
1. `AsyncFd<T>` implements `AsFd` for all `T: AsRawFd` by calling `BorrowedFd::borrow_raw(self.as_raw_fd())`, allowing safe code to obtain an arbitrary `BorrowedFd<'_>` without owning the file descriptor.
2. Tokio's I/O driver stores a pointer to an internal allocation in the `epoll`/`kqueue` user data for registered file descriptors. If the underlying file descriptor is duplicated and then closed while an `AsyncFd` is alive, and the OS reuses that file descriptor number for a another IO resource on the same runtime, dropping the first `AsyncFd` deregisters the reused file descriptor number and frees the first while the original file description remains registered in `epoll`, leading to a use-after-free when a readiness event fires.
Starting in Tokio 1.51.6, 1.53.3, and 1.54.0, `AsyncFd::new`, `AsyncFd::with_interest`, `AsyncFd::try_new`, and `AsyncFd::try_with_interest` are deprecated in favor of the `unsafe` constructors `AsyncFd::register` and `AsyncFd::register_with_interest`. Callers must ensure that the file descriptor remains valid, open, and unchanged for the lifetime of the `AsyncFd`.
See [advisory page](https://rustsec.org/advisories/RUSTSEC-2026-0386.html) for additional details.
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tokio1.53.1tokio::io::unix::AsyncFdprovides safe constructors (new,with_interest,try_new, andtry_with_interest) that accept anyT: AsRawFdrather than requiring proof of ownership or validity of the file descriptor. BecauseRawFdimplementsAsRawFd, safe code can construct anAsyncFdfrom an arbitrary or closed file descriptor, or close/replace the underlying file descriptor while theAsyncFdis still alive.This violates I/O safety in two ways:
AsyncFd<T>implementsAsFdfor allT: AsRawFdby callingBorrowedFd::borrow_raw(self.as_raw_fd()), allowing safe code to obtain an arbitraryBorrowedFd<'_>without owning the file descriptor.epoll/kqueueuser data for registered file descriptors. If the underlying file descriptor is duplicated and then closed while anAsyncFdis alive, and the OS reuses that file descriptor number for a another IO resource on the same runtime, dropping the firstAsyncFdderegisters the reused file descriptor number and frees the first while the original file description remains registered inepoll, leading to a use-after-free when a readiness event fires.Starting in Tokio 1.51.6, 1.53.3, and 1.54.0,
AsyncFd::new,AsyncFd::with_interest,AsyncFd::try_new, andAsyncFd::try_with_interestare deprecated in favor of theunsafeconstructorsAsyncFd::registerandAsyncFd::register_with_interest. Callers must ensure that the file descriptor remains valid, open, and unchanged for the lifetime of theAsyncFd.See advisory page for additional details.