Unsound Sync implementation for ImmediateIO and TransactionalIO

[Qwaz]

Hello fellow Rustacean,
we (Rust group @sslab-gatech) found a memory-safety/soundness issue in this crate while scanning Rust code on crates.io for potential vulnerabilities.

Issue Description

max7301/src/expander/immediate.rs

Lines 23 to 28 in a2084de

	unsafe impl<M, EI> Sync for ImmediateIO<M, EI>
	where
	M: IOMutex<Expander<EI>>,
	EI: ExpanderInterface,
	{
	}

max7301/src/expander/transactional.rs

Lines 65 to 70 in a2084de

	unsafe impl<M, EI> Sync for TransactionalIO<M, EI>
	where
	M: IOMutex<Expander<EI>>,
	EI: ExpanderInterface,
	{
	}

ImmediateIO and TransactionIO implement Sync for M: IOMutex<Expander<EI>>, EI: ExpanderInterface. This bound is seemingly unsound; it allows to send a reference of ImmediateIO or TransactionIO to another thread, and from there a mutable reference of Expander<EI> can be accessed through IOMutex APIs. This could result in a thread-safety violation when EI is a non-Send type.

We suggest to add an Expander<EI>: Send bound (or an equivalent EI: Send bound) to those Sync implementations, following the stdlib Mutex's Sync implementation.

[Shnatsel]

Heads up: this issue has been included in the RustSec advisory database. It will be surfaced by tools such as cargo-audit or cargo-deny from now on.

Once a fix is released to crates.io, please open a pull request to update the advisory with the patched version, or file an issue on the advisory database repository.

[edarc]

First of all thank you @Qwaz for the report, and apologies for somehow missing the original notification.

I'll test the suggested fix as soon as I am able and publish an update.

[edarc]

This is fixed in 0.2.0. Additionally, the crate now no longer has any unsafe in it. I will send a PR on the advisory shortly.