CVE-2020-15254: 
Rust vulnerability analysis and mitigation

CVE-2020-15254 affects crossbeam-channel version 0.4.3, a Rust crate for concurrent programming. The vulnerability was discovered in June 2020 and disclosed in October 2020. The issue affects the bounded channel implementation in the crossbeam-channel crate (Mozilla Security, Github Advisory).

The vulnerability stems from incorrect assumptions in the bounded channel implementation. The code assumed that Vec::from_iter allocates capacity exactly equal to the number of iterator elements. However, Vec::from_iter does not guarantee this and may allocate extra memory. When the bounded channel's destructor reconstructs Vec from the raw pointer based on these incorrect assumptions, it leads to undefined behavior by deallocating memory with incorrect capacity when Vec::from_iter has allocated different sizes than the number of iterator elements (Github Issue).

The undefined behavior caused by this vulnerability led to corruption of jemalloc memory structures and ultimately resulted in deadlocks in some cases. This was observed in practice and affected real-world applications (Github PR, Github Issue).

The vulnerability has been confirmed to be exploitable in practice, with documented cases of memory corruption and system deadlocks. The issue was discovered through automated testing using Miri, which reported incorrect memory deallocation patterns (Github Issue).

The vulnerability was fixed in crossbeam-channel version 0.4.4. The fix involved using Box<[T]> instead of Vec<T> to initialize and drop ArrayQueue, which guarantees exact capacity and resolves the undefined behavior. Users are strongly recommended to upgrade to version 0.4.4 or later (Github Advisory).

The vulnerability received attention from the Rust community and affected several major projects. Mozilla Firefox included fixes for this vulnerability in their Firefox 82 release, highlighting its significance in production environments (Mozilla Security).