1// Copyright 2018 Developers of the Rand project.
2// Copyright 2013 The Rust Project Developers.
3//
4// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
5// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
6// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
7// option. This file may not be copied, modified, or distributed
8// except according to those terms.
9
10//! A wrapper around another PRNG that reseeds it after it
11//! generates a certain number of random bytes.
12
13use core::mem::size_of;
14
15use rand_core::block::{BlockRng, BlockRngCore};
16use rand_core::{CryptoRng, Error, RngCore, SeedableRng};
17
18/// A wrapper around any PRNG that implements [`BlockRngCore`], that adds the
19/// ability to reseed it.
20///
21/// `ReseedingRng` reseeds the underlying PRNG in the following cases:
22///
23/// - On a manual call to [`reseed()`].
24/// - After `clone()`, the clone will be reseeded on first use.
25/// - When a process is forked on UNIX, the RNGs in both the parent and child
26/// processes will be reseeded just before the next call to
27/// [`BlockRngCore::generate`], i.e. "soon". For ChaCha and Hc128 this is a
28/// maximum of fifteen `u32` values before reseeding.
29/// - After the PRNG has generated a configurable number of random bytes.
30///
31/// # When should reseeding after a fixed number of generated bytes be used?
32///
33/// Reseeding after a fixed number of generated bytes is never strictly
34/// *necessary*. Cryptographic PRNGs don't have a limited number of bytes they
35/// can output, or at least not a limit reachable in any practical way. There is
36/// no such thing as 'running out of entropy'.
37///
38/// Occasionally reseeding can be seen as some form of 'security in depth'. Even
39/// if in the future a cryptographic weakness is found in the CSPRNG being used,
40/// or a flaw in the implementation, occasionally reseeding should make
41/// exploiting it much more difficult or even impossible.
42///
43/// Use [`ReseedingRng::new`] with a `threshold` of `0` to disable reseeding
44/// after a fixed number of generated bytes.
45///
46/// # Limitations
47///
48/// It is recommended that a `ReseedingRng` (including `ThreadRng`) not be used
49/// from a fork handler.
50/// Use `OsRng` or `getrandom`, or defer your use of the RNG until later.
51///
52/// # Error handling
53///
54/// Although unlikely, reseeding the wrapped PRNG can fail. `ReseedingRng` will
55/// never panic but try to handle the error intelligently through some
56/// combination of retrying and delaying reseeding until later.
57/// If handling the source error fails `ReseedingRng` will continue generating
58/// data from the wrapped PRNG without reseeding.
59///
60/// Manually calling [`reseed()`] will not have this retry or delay logic, but
61/// reports the error.
62///
63/// # Example
64///
65/// ```
66/// use rand::prelude::*;
67/// use rand_chacha::ChaCha20Core; // Internal part of ChaChaRng that
68/// // implements BlockRngCore
69/// use rand::rngs::OsRng;
70/// use rand::rngs::adapter::ReseedingRng;
71///
72/// let prng = ChaCha20Core::from_entropy();
73/// let mut reseeding_rng = ReseedingRng::new(prng, 0, OsRng);
74///
75/// println!("{}", reseeding_rng.gen::<u64>());
76///
77/// let mut cloned_rng = reseeding_rng.clone();
78/// assert!(reseeding_rng.gen::<u64>() != cloned_rng.gen::<u64>());
79/// ```
80///
81/// [`BlockRngCore`]: rand_core::block::BlockRngCore
82/// [`ReseedingRng::new`]: ReseedingRng::new
83/// [`reseed()`]: ReseedingRng::reseed
84#[derive(Debug)]
85pub struct ReseedingRng<R, Rsdr>(BlockRng<ReseedingCore<R, Rsdr>>)
86where
87 R: BlockRngCore + SeedableRng,
88 Rsdr: RngCore;
89
90impl<R, Rsdr> ReseedingRng<R, Rsdr>
91where
92 R: BlockRngCore + SeedableRng,
93 Rsdr: RngCore,
94{
95 /// Create a new `ReseedingRng` from an existing PRNG, combined with a RNG
96 /// to use as reseeder.
97 ///
98 /// `threshold` sets the number of generated bytes after which to reseed the
99 /// PRNG. Set it to zero to never reseed based on the number of generated
100 /// values.
101 pub fn new(rng: R, threshold: u64, reseeder: Rsdr) -> Self {
102 ReseedingRng(BlockRng::new(ReseedingCore::new(rng, threshold, reseeder)))
103 }
104
105 /// Reseed the internal PRNG.
106 pub fn reseed(&mut self) -> Result<(), Error> {
107 self.0.core.reseed()
108 }
109}
110
111// TODO: this should be implemented for any type where the inner type
112// implements RngCore, but we can't specify that because ReseedingCore is private
113impl<R, Rsdr: RngCore> RngCore for ReseedingRng<R, Rsdr>
114where
115 R: BlockRngCore<Item = u32> + SeedableRng,
116 <R as BlockRngCore>::Results: AsRef<[u32]> + AsMut<[u32]>,
117{
118 #[inline(always)]
119 fn next_u32(&mut self) -> u32 {
120 self.0.next_u32()
121 }
122
123 #[inline(always)]
124 fn next_u64(&mut self) -> u64 {
125 self.0.next_u64()
126 }
127
128 fn fill_bytes(&mut self, dest: &mut [u8]) {
129 self.0.fill_bytes(dest)
130 }
131
132 fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error> {
133 self.0.try_fill_bytes(dest)
134 }
135}
136
137impl<R, Rsdr> Clone for ReseedingRng<R, Rsdr>
138where
139 R: BlockRngCore + SeedableRng + Clone,
140 Rsdr: RngCore + Clone,
141{
142 fn clone(&self) -> ReseedingRng<R, Rsdr> {
143 // Recreating `BlockRng` seems easier than cloning it and resetting
144 // the index.
145 ReseedingRng(BlockRng::new(self.0.core.clone()))
146 }
147}
148
149impl<R, Rsdr> CryptoRng for ReseedingRng<R, Rsdr>
150where
151 R: BlockRngCore + SeedableRng + CryptoRng,
152 Rsdr: RngCore + CryptoRng,
153{
154}
155
156#[derive(Debug)]
157struct ReseedingCore<R, Rsdr> {
158 inner: R,
159 reseeder: Rsdr,
160 threshold: i64,
161 bytes_until_reseed: i64,
162 fork_counter: usize,
163}
164
165impl<R, Rsdr> BlockRngCore for ReseedingCore<R, Rsdr>
166where
167 R: BlockRngCore + SeedableRng,
168 Rsdr: RngCore,
169{
170 type Item = <R as BlockRngCore>::Item;
171 type Results = <R as BlockRngCore>::Results;
172
173 fn generate(&mut self, results: &mut Self::Results) {
174 let global_fork_counter = fork::get_fork_counter();
175 if self.bytes_until_reseed <= 0 || self.is_forked(global_fork_counter) {
176 // We get better performance by not calling only `reseed` here
177 // and continuing with the rest of the function, but by directly
178 // returning from a non-inlined function.
179 return self.reseed_and_generate(results, global_fork_counter);
180 }
181 let num_bytes = results.as_ref().len() * size_of::<Self::Item>();
182 self.bytes_until_reseed -= num_bytes as i64;
183 self.inner.generate(results);
184 }
185}
186
187impl<R, Rsdr> ReseedingCore<R, Rsdr>
188where
189 R: BlockRngCore + SeedableRng,
190 Rsdr: RngCore,
191{
192 /// Create a new `ReseedingCore`.
193 fn new(rng: R, threshold: u64, reseeder: Rsdr) -> Self {
194 use ::core::i64::MAX;
195 fork::register_fork_handler();
196
197 // Because generating more values than `i64::MAX` takes centuries on
198 // current hardware, we just clamp to that value.
199 // Also we set a threshold of 0, which indicates no limit, to that
200 // value.
201 let threshold = if threshold == 0 {
202 MAX
203 } else if threshold <= MAX as u64 {
204 threshold as i64
205 } else {
206 MAX
207 };
208
209 ReseedingCore {
210 inner: rng,
211 reseeder,
212 threshold: threshold as i64,
213 bytes_until_reseed: threshold as i64,
214 fork_counter: 0,
215 }
216 }
217
218 /// Reseed the internal PRNG.
219 fn reseed(&mut self) -> Result<(), Error> {
220 R::from_rng(&mut self.reseeder).map(|result| {
221 self.bytes_until_reseed = self.threshold;
222 self.inner = result
223 })
224 }
225
226 fn is_forked(&self, global_fork_counter: usize) -> bool {
227 // In theory, on 32-bit platforms, it is possible for
228 // `global_fork_counter` to wrap around after ~4e9 forks.
229 //
230 // This check will detect a fork in the normal case where
231 // `fork_counter < global_fork_counter`, and also when the difference
232 // between both is greater than `isize::MAX` (wrapped around).
233 //
234 // It will still fail to detect a fork if there have been more than
235 // `isize::MAX` forks, without any reseed in between. Seems unlikely
236 // enough.
237 (self.fork_counter.wrapping_sub(global_fork_counter) as isize) < 0
238 }
239
240 #[inline(never)]
241 fn reseed_and_generate(
242 &mut self, results: &mut <Self as BlockRngCore>::Results, global_fork_counter: usize,
243 ) {
244 #![allow(clippy::if_same_then_else)] // false positive
245 if self.is_forked(global_fork_counter) {
246 info!("Fork detected, reseeding RNG");
247 } else {
248 trace!("Reseeding RNG (periodic reseed)");
249 }
250
251 let num_bytes = results.as_ref().len() * size_of::<<R as BlockRngCore>::Item>();
252
253 if let Err(e) = self.reseed() {
254 warn!("Reseeding RNG failed: {}", e);
255 let _ = e;
256 }
257 self.fork_counter = global_fork_counter;
258
259 self.bytes_until_reseed = self.threshold - num_bytes as i64;
260 self.inner.generate(results);
261 }
262}
263
264impl<R, Rsdr> Clone for ReseedingCore<R, Rsdr>
265where
266 R: BlockRngCore + SeedableRng + Clone,
267 Rsdr: RngCore + Clone,
268{
269 fn clone(&self) -> ReseedingCore<R, Rsdr> {
270 ReseedingCore {
271 inner: self.inner.clone(),
272 reseeder: self.reseeder.clone(),
273 threshold: self.threshold,
274 bytes_until_reseed: 0, // reseed clone on first use
275 fork_counter: self.fork_counter,
276 }
277 }
278}
279
280impl<R, Rsdr> CryptoRng for ReseedingCore<R, Rsdr>
281where
282 R: BlockRngCore + SeedableRng + CryptoRng,
283 Rsdr: RngCore + CryptoRng,
284{
285}
286
287
288#[cfg(all(unix, not(target_os = "emscripten")))]
289mod fork {
290 use core::sync::atomic::{AtomicUsize, Ordering};
291 use std::sync::Once;
292
293 // Fork protection
294 //
295 // We implement fork protection on Unix using `pthread_atfork`.
296 // When the process is forked, we increment `RESEEDING_RNG_FORK_COUNTER`.
297 // Every `ReseedingRng` stores the last known value of the static in
298 // `fork_counter`. If the cached `fork_counter` is less than
299 // `RESEEDING_RNG_FORK_COUNTER`, it is time to reseed this RNG.
300 //
301 // If reseeding fails, we don't deal with this by setting a delay, but just
302 // don't update `fork_counter`, so a reseed is attempted as soon as
303 // possible.
304
305 static RESEEDING_RNG_FORK_COUNTER: AtomicUsize = AtomicUsize::new(0);
306
307 pub fn get_fork_counter() -> usize {
308 RESEEDING_RNG_FORK_COUNTER.load(Ordering::Relaxed)
309 }
310
311 extern "C" fn fork_handler() {
312 // Note: fetch_add is defined to wrap on overflow
313 // (which is what we want).
314 RESEEDING_RNG_FORK_COUNTER.fetch_add(1, Ordering::Relaxed);
315 }
316
317 pub fn register_fork_handler() {
318 static REGISTER: Once = Once::new();
319 REGISTER.call_once(|| {
320 // Bump the counter before and after forking (see #1169):
321 let ret = unsafe { libc::pthread_atfork(
322 Some(fork_handler),
323 Some(fork_handler),
324 Some(fork_handler),
325 ) };
326 if ret != 0 {
327 panic!("libc::pthread_atfork failed with code {}", ret);
328 }
329 });
330 }
331}
332
333#[cfg(not(all(unix, not(target_os = "emscripten"))))]
334mod fork {
335 pub fn get_fork_counter() -> usize {
336 0
337 }
338 pub fn register_fork_handler() {}
339}
340
341
342#[cfg(feature = "std_rng")]
343#[cfg(test)]
344mod test {
345 use super::ReseedingRng;
346 use crate::rngs::mock::StepRng;
347 use crate::rngs::std::Core;
348 use crate::{Rng, SeedableRng};
349
350 #[test]
351 fn test_reseeding() {
352 let mut zero = StepRng::new(0, 0);
353 let rng = Core::from_rng(&mut zero).unwrap();
354 let thresh = 1; // reseed every time the buffer is exhausted
355 let mut reseeding = ReseedingRng::new(rng, thresh, zero);
356
357 // RNG buffer size is [u32; 64]
358 // Debug is only implemented up to length 32 so use two arrays
359 let mut buf = ([0u32; 32], [0u32; 32]);
360 reseeding.fill(&mut buf.0);
361 reseeding.fill(&mut buf.1);
362 let seq = buf;
363 for _ in 0..10 {
364 reseeding.fill(&mut buf.0);
365 reseeding.fill(&mut buf.1);
366 assert_eq!(buf, seq);
367 }
368 }
369
370 #[test]
371 fn test_clone_reseeding() {
372 #![allow(clippy::redundant_clone)]
373
374 let mut zero = StepRng::new(0, 0);
375 let rng = Core::from_rng(&mut zero).unwrap();
376 let mut rng1 = ReseedingRng::new(rng, 32 * 4, zero);
377
378 let first: u32 = rng1.gen();
379 for _ in 0..10 {
380 let _ = rng1.gen::<u32>();
381 }
382
383 let mut rng2 = rng1.clone();
384 assert_eq!(first, rng2.gen::<u32>());
385 }
386}
387