[Security-sig] PEP 522: Allow BlockingIOError in security sensitive APIs on Linux

[Cory Benfield]

> On 22 Jun 2016, at 02:28, Nick Coghlan <ncoghlan at gmail.com> wrote:
> 
> Hi folks,
> 
> Over the weekend, Nathaniel Smith and I put together a proposal to
> allow security sensitive APIs (os.urandom, random.SystemRandom and the
> new secrets module) to throw BlockingIOError if the operating system's
> random number generator isn't ready.

In general I like this approach. One note inline below.

> Limitations on scope
> --------------------
> 
> No changes are proposed for Windows or Mac OS X systems, as neither of those
> platforms provides any mechanism to run Python code before the operating
> system random number generator has been initialized. Mac OS X goes so far as
> to kernel panic and abort the boot process if it can't properly initialize the
> random number generator (although Apple's restrictions on the supported
> hardware platforms make that exceedingly unlikely in practice).
> 
> Similarly, no changes are proposed for other \*nix systems where
> ``os.urandom()`` will currently block waiting for the system random number
> generator to be initialized, rather than returning values that are potentially
> unsuitable for use in security sensitive applications.

You may want to be careful around this point. Solaris provides a getrandom() syscall as well, that Python *does* use. Furthermore, if other *nix OSes provide a getrandom() syscall then the current Python code will favour it over the urandom fallback: care should be taken to clarify what the expected plan is in these cases.

Cory
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