From: delan@... Date: 2016-05-04T02:07:18+00:00 Subject: [ruby-core:75343] [Ruby trunk Bug#9569] SecureRandom should try /dev/urandom first Issue #9569 has been updated by Delan Azabani. For anyone reading this thread after me: * `SecureRandom.gen_random` calls `OpenSSL::Random.random_bytes` before falling back on `Random.raw_seed` [1] * `Random.raw_seed` calls `fill_random_bytes` [2] * `fill_random_bytes` calls `fill_random_bytes_syscall` before falling back on `fill_random_bytes_urandom` [3] * `fill_random_bytes_syscall` is compiled to wrap `CryptGenRandom` if available, falling back on `getrandom(2)` [4] * `fill_random_bytes_urandom` reads from `/dev/urandom` if it considers the device reasonable [5] It���s good to see that `fill_random_bytes` already prefers `getrandom(2)` over `/dev/urandom`, as `getrandom(2)` blocks only if it has been called so early in the boot process that there isn���t enough *initial* entropy [6], and it isn���t susceptible to file descriptor exhaustion attacks [7]. Changing the order of `SecureRandom.gen_random` should be enough to fix this bug, but I would also suggest adding `arc4random(3)` or `getentropy(2)` to the collection of system calls tried by `fill_random_bytes_syscall`, which will mitigate file descriptor exhaustion attacks for OpenBSD users. 1. https://github.com/ruby/ruby/blob/8ef6dacb248876b444595a26ea78c35eb07a188b/lib/securerandom.rb#L50-78 2. https://github.com/ruby/ruby/blob/8ef6dacb248876b444595a26ea78c35eb07a188b/random.c#L620-635 3. https://github.com/ruby/ruby/blob/8ef6dacb248876b444595a26ea78c35eb07a188b/random.c#L549-555 4. https://github.com/ruby/ruby/blob/8ef6dacb248876b444595a26ea78c35eb07a188b/random.c#L493-547 5. https://github.com/ruby/ruby/blob/8ef6dacb248876b444595a26ea78c35eb07a188b/random.c#L444-481 6. http://www.2uo.de/myths-about-urandom/ 7. https://lwn.net/Articles/606141/ 8. http://man.openbsd.org/OpenBSD-current/man3/arc4random.3 9. http://man.openbsd.org/OpenBSD-current/man2/getentropy.2 ---------------------------------------- Bug #9569: SecureRandom should try /dev/urandom first https://bugs.ruby-lang.org/issues/9569#change-58472 * Author: Corey Csuhta * Status: Rejected * Priority: Normal * Assignee: ruby-core * ruby -v: * Backport: ---------------------------------------- Right now, `SecureRandom.random_bytes` tries to detect an OpenSSL to use before it tries to detect `/dev/urandom`. I think it should be the other way around. In both cases, you just need random bytes to unpack, so SecureRandom could skip the middleman (and [second point of failure](http://sockpuppet.org/blog/2014/02/25/safely-generate-random-numbers/)) and just talk to `/dev/urandom` directly if it's available. Is this a case of just re-ordering the two code chunks so that `/dev/urandom` is tried first? Relevant lines: https://github.com/ruby/ruby/blob/trunk/lib/securerandom.rb#L59-L90 -- https://bugs.ruby-lang.org/ Unsubscribe: