random: fix data race on crng init time

_extract_crng() does plain loads of crng->init_time and
crng_global_init_time, which causes undefined behavior if
crng_reseed() and RNDRESEEDCRNG modify these corrently.

Use READ_ONCE() and WRITE_ONCE() to make the behavior defined.

Don't fix the race on crng->init_time by protecting it with crng->lock,
since it's not a problem for duplicate reseedings to occur.  I.e., the
lockless access with READ_ONCE() is fine.

Fixes: d848e5f8 ("random: add new ioctl RNDRESEEDCRNG")
Fixes: e192be9d ("random: replace non-blocking pool with a Chacha20-based CRNG")
Cc: stable@vger.kernel.org
Signed-off-by: NEric Biggers <ebiggers@google.com>
Acked-by: NPaul E. McKenney <paulmck@kernel.org>
Signed-off-by: NJason A. Donenfeld <Jason@zx2c4.com>

drivers/char/random.c

@@ -991,7 +991,7 @@ static void crng_reseed(struct crng_state *crng, struct entropy_store *r)
 		crng->state[i+4] ^= buf.key[i] ^ rv;
 	}
 	memzero_explicit(&buf, sizeof(buf));
-	crng->init_time = jiffies;
+	WRITE_ONCE(crng->init_time, jiffies);
 	spin_unlock_irqrestore(&crng->lock, flags);
 	if (crng == &primary_crng && crng_init < 2) {
 		invalidate_batched_entropy();
@@ -1017,12 +1017,15 @@ static void crng_reseed(struct crng_state *crng, struct entropy_store *r)
 static void _extract_crng(struct crng_state *crng,
 			  __u8 out[CHACHA_BLOCK_SIZE])
 {
-	unsigned long v, flags;
+	unsigned long v, flags, init_time;
 
-	if (crng_ready() &&
-	    (time_after(crng_global_init_time, crng->init_time) ||
-	     time_after(jiffies, crng->init_time + CRNG_RESEED_INTERVAL)))
-		crng_reseed(crng, crng == &primary_crng ? &input_pool : NULL);
+	if (crng_ready()) {
+		init_time = READ_ONCE(crng->init_time);
+		if (time_after(READ_ONCE(crng_global_init_time), init_time) ||
+		    time_after(jiffies, init_time + CRNG_RESEED_INTERVAL))
+			crng_reseed(crng, crng == &primary_crng ?
+				    &input_pool : NULL);
+	}
 	spin_lock_irqsave(&crng->lock, flags);
 	if (arch_get_random_long(&v))
 		crng->state[14] ^= v;
@@ -1962,7 +1965,7 @@ static long random_ioctl(struct file *f, unsigned int cmd, unsigned long arg)
 		if (crng_init < 2)
 			return -ENODATA;
 		crng_reseed(&primary_crng, &input_pool);
-		crng_global_init_time = jiffies - 1;
+		WRITE_ONCE(crng_global_init_time, jiffies - 1);
 		return 0;
 	default:
 		return -EINVAL;