10b1a17594
There is no functional change for the existing Fortuna random(4) implementation, which remains the default in GENERIC. In the FenestrasX model, when the root CSPRNG is reseeded from pools due to an (infrequent) timer, child CSPRNGs can cheaply detect this condition and reseed. To do so, they just need to track an additional 64-bit value in the associated state, and compare it against the root seed version (generation) on random reads. This revision integrates arc4random(9) into that model without substantially changing the design or implementation of arc4random(9). The motivation is that arc4random(9) is immediately reseeded when the backing random(4) implementation has additional entropy. This is arguably most important during boot, when fenestrasX is reseeding at 1, 3, 9, 27, etc., second intervals. Today, arc4random(9) has a hardcoded 300 second reseed window. Without this mechanism, if arc4random(9) gets weak entropy during initial seed (and arc4random(9) is used early in boot, so this is quite possible), it may continue to emit poorly seeded output for 5 minutes. The FenestrasX push-reseed scheme corrects consumers, like arc4random(9), as soon as possible. Reviewed by: markm Approved by: csprng (markm) Differential Revision: https://reviews.freebsd.org/D22838
54 lines
2.3 KiB
C
54 lines
2.3 KiB
C
/*-
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* SPDX-License-Identifier: BSD-2-Clause-FreeBSD
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*
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* Copyright (c) 2019 Conrad Meyer <cem@FreeBSD.org>
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*
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* Redistribution and use in source and binary forms, with or without
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* modification, are permitted provided that the following conditions
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* are met:
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* 1. Redistributions of source code must retain the above copyright
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* notice, this list of conditions and the following disclaimer.
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* 2. Redistributions in binary form must reproduce the above copyright
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* notice, this list of conditions and the following disclaimer in the
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* documentation and/or other materials provided with the distribution.
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*
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* THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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* ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
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* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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* SUCH DAMAGE.
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*
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* $FreeBSD$
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*/
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#pragma once
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#include <sys/systm.h>
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/*
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* The root BRNG seed version, or generation.
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*
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* FenestrasX-aware downstream CSPRNGs (i.e., arc4random(9)) should track the
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* generation number they seeded from, using the read_random_key(9) API below.
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* If their current seed version is older than the root generation, they should
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* reseed before producing output.
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*
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* The variable is read-only outside of the fenestrasX implementation and
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* should be accessed using 'atomic_load_acq_64(&fxrng_root_generation)'.
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* Reseeds are extremely infrequent, so callers may wish to hint to the
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* compiler that a matching generation is the expected case, with
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* __predict_true() or __predict_false().
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*/
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extern uint64_t __read_mostly fxrng_root_generation;
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/*
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* A routine for generating seed/key material
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* Bypasses random(4) for now, but conceivably could be incorporated into that.
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*/
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void read_random_key(void *buf, size_t nbytes, uint64_t *seed_version_out);
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