feat: continuously reseed entropy pool

Author: joboet

src/entropy.rs

@@ -7,33 +7,46 @@ use hermit_sync::InterruptTicketMutex;
 use rand_chacha::rand_core::{RngCore, SeedableRng};
 use rand_chacha::ChaCha20Rng;
 
-use crate::arch::kernel::processor::seed_entropy;
+use crate::arch::kernel::processor::{get_timer_ticks, seed_entropy};
 use crate::errno::ENOSYS;
 
+// Reseed every second for increased security while maintaining the performance of
+// the PRNG.
+const RESEED_INTERVAL: u64 = 1000000;
+
 bitflags! {
 	pub struct Flags: u32 {}
 }
 
-static POOL: InterruptTicketMutex<Option<ChaCha20Rng>> = InterruptTicketMutex::new(None);
+struct Pool {
+	rng: ChaCha20Rng,
+	last_reseed: u64,
+}
+
+static POOL: InterruptTicketMutex<Option<Pool>> = InterruptTicketMutex::new(None);
 
 /// Fills `buf` with random data, respecting the options in `flags`.
 ///
 /// Returns the number of bytes written or `-ENOSYS` if the system does not support
 /// random data generation.
 pub fn read(buf: &mut [u8], _flags: Flags) -> isize {
 	let pool = &mut *POOL.lock();
+	let now = get_timer_ticks();
 	let pool = match pool {
-		Some(pool) => pool,
-		pool @ None => {
+		Some(pool) if now.saturating_sub(pool.last_reseed) <= RESEED_INTERVAL => pool,
+		pool => {
 			if let Some(seed) = seed_entropy() {
-				pool.insert(ChaCha20Rng::from_seed(seed))
+				pool.insert(Pool {
+					rng: ChaCha20Rng::from_seed(seed),
+					last_reseed: now,
+				})
 			} else {
 				return -ENOSYS as isize;
 			}
 		}
 	};
 
-	pool.fill_bytes(buf);
+	pool.rng.fill_bytes(buf);
 	// Slice lengths are always <= isize::MAX so this return value cannot conflict
 	// with error numbers.
 	buf.len() as isize