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clarifications to randomization documentation

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Daniel Micay 2019-06-23 19:20:16 -04:00
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README.md
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@ -313,17 +313,18 @@ was a bit less important and if a core goal was finding latent bugs.
The current implementation of random number generation for randomization-based
mitigations is based on generating a keystream from a stream cipher (ChaCha8)
in small chunks. A separate CSPRNG is used for each small size class, large
allocations, etc. in order to fit into the existing fine-grained locking model
without needing to waste memory per thread by having the CSPRNG state in Thread
Local Storage. Similarly, it's protected via the same approach taken for the
rest of the metadata. The stream cipher is regularly reseeded from the OS to
provide backtracking and prediction resistance with a negligible cost. The
reseed interval simply needs to be adjusted to the point that it stops
registering as having any significant performance impact. The performance
impact on recent Linux kernels is primarily from the high cost of system calls
and locking since the implementation is quite efficient (ChaCha20), especially
for just generating the key and nonce for another stream cipher (ChaCha8).
in small chunks. Separate CSPRNGs are used for each small size class in each
arena, large allocations and initialization in order to fit into the
fine-grained locking model without needing to waste memory per thread by
having the CSPRNG state in Thread Local Storage. Similarly, it's protected via
the same approach taken for the rest of the metadata. The stream cipher is
regularly reseeded from the OS to provide backtracking and prediction
resistance with a negligible cost. The reseed interval simply needs to be
adjusted to the point that it stops registering as having any significant
performance impact. The performance impact on recent Linux kernels is
primarily from the high cost of system calls and locking since the
implementation is quite efficient (ChaCha20), especially for just generating
the key and nonce for another stream cipher (ChaCha8).
ChaCha8 is a great fit because it's extremely fast across platforms without
relying on hardware support or complex platform-specific code. The security