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Author dstufft
Recipients Colm Buckley, Lukasa, Theodore Tso, alex, christian.heimes, doko, dstufft, larry, lemburg, martin.panter, matejcik, ned.deily, python-dev, rhettinger, skrah, thomas-petazzoni, vstinner, ztane
Date 2016-06-08.10:23:58
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I would greatly prefer it if we would allow os.urandom to block on Linux, at least by default. This will make os.urandom behavior similarly on most modern platforms. The cases where this is going to matter are extreme edge cases, for most users they'll just silently be a bit more secure-- important for a number of use cases of Python (think for instance, if someone has a SSH server written in Twisted that generates it's own host keys, a perfectly reasonable use of os.urandom). We've been telling people that os.urandom is the right source for generating randomness for cryptographic use for ages, and I think it is important to use the tools provided to us by the platform to best satisfy that use case by default-- in this case, getrandom() in blocking mode is the best tool provided by the Linux platform.

People writing Python code cannot expect that os.urandom will not block, because on most platforms it *will* block on intialization. However, the cases where it will block are a fairly small window, so by allowing it to block we're giving a better guarantee for very little downside-- essentially that something early on in the boot process shouldn't call os.urandom(), which is the right behavior on Linux (and any other OS) anyways.

The problem is that the Python interpreter itself (essentially) calls os.urandom() as part of it's start up sequence which makes it unsuitable for use in very early stage boot programs. In the abstract, it's not possible to fix this on every single platform without removing all use of os.urandom from Python start up (which I think would be a bad idea). I think Colm's nonblocking_urandom_noraise.patch is a reasonable trade off (perhaps not the one I would personally make, but I think it's reasonable). If we wish to ensure that Python interpreter start up never blocks on Linux without needing to supply any command line flags or environment variables, then I would strongly urge us to adopt his patch, but allow os.urandom to still block.

In other words, please let's not let systemd's design weaken the security guarantees of os.urandom (generate cryptographically secure random bytes using the best tools provided by the platform). Let's make a targeted fix.
Date User Action Args
2016-06-08 10:23:58dstufftsetrecipients: + dstufft, lemburg, rhettinger, doko, vstinner, larry, christian.heimes, matejcik, ned.deily, alex, skrah, python-dev, martin.panter, ztane, Lukasa, thomas-petazzoni, Colm Buckley, Theodore Tso
2016-06-08 10:23:58dstufftsetmessageid: <>
2016-06-08 10:23:58dstufftlinkissue26839 messages
2016-06-08 10:23:58dstufftcreate