20.09.2019 22:52, Andy Lutomirski пишет:
I think that, given existing software, we should make two or three changes to fix the basic problems here: 1. Add GRND_INSECURE: at least let new applications do the right thing going forward. 2. Fix what is arguably a straight up kernel bug, not even an ABI issue: when a user program is blocking in getrandom(..., 0), the kernel happily sits there doing absolutely nothing and deadlocks the system as a result. This IMO isn't an ABI issue -- it's an implementation problem. How about we make getrandom() (probably actually wait_for_random_bytes()) do something useful to try to seed the RNG if the system is otherwise not doing IO. 3. Optionally, entirely in user code: Get glibc to add new *library* functions: getentropy_secure_blocking() and getentropy_insecure() or whatever they want to call them. Deprecate getentropy(). I think #2 is critical. Right now, suppose someone has a system that neets to do a secure network request (a la Red Hat's Clevis). I have no idea what Clevis actually does, but it wouldn't be particularly crazy to do a DH exchange or sign with an EC key to ask some network server to help unlock a dm-crypt volume. If the system does this at boot, it needs to use getrandom(..., 0), GRND_EXPLICIT, or whatever, because it NEEDS a secure random number. No about of ABI fiddling will change this. The kernel should *work* in this case rather than deadlocking.
Let me express a little bit of disagreement with the logic here.I do agree that #2 is critical, and the Clevis use case is a perfect example why it is important. I doubt that it is solvable without trusting jitter entropy, or without provoking a dummy read on a random block device, just for timings, or maybe some other interaction with the external world - but Willy already said "it seems fishy". However, _if_ it is solved, then we don't need GRND_INSECURE, because solving #2 is equivalent to magically making secure random numbers always available.
-- Alexander E. Patrakov
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