s2n-tls has undefined behavior at process exit

Summary

### Impact s2n-tls uses the Linux atexit function to register functions that clean up the global state when the process exits. In multi-threaded environments, the atexit handler may clean up state which is still in use by other threads. When this occurs, the exiting process may experience a segmentation fault or other undefined behavior.

Customers of AWS services do not need to take action. Applications using s2n-tls should upgrade to the most recent release of s2n-tls.

**Impacted versions**: < v1.5.9.

### Patches The patch commit [493b771](https://github.com/aws/s2n-tls/commit/493b77167dc367c394de23cfe78a029298e2a254) is included in s2n-tls v1.5.9 [1]

### Workarounds The atexit handler may be disabled by calling `s2n_disable_atexit()` prior to initializing s2n-tls. The atexit handler is off by default in the patched versions. For further details, refer to [s2n-tls Usage Guide: Initialization and Teardown](https://github.com/aws/s2n-tls/blob/main/docs/usage-guide/topics/ch02-initialization.md).

If you have any questions or comments about this advisory, we ask that you contact AWS/Amazon Security via our vulnerability reporting page [2] or directly via email to [aws-security@amazon.com](mailto:aws-security@amazon.com). Please do not create a public GitHub issue.

[1] https://github.com/aws/s2n-tls/releases/tag/v1.5.9 [2] Vulnerability reporting page: https://aws.amazon.com/security/vulnerability-reporting