hwrng: core - Always drop the RNG in hwrng_unregister()

enable_best_rng() is used in hwrng_unregister() to switch away from the
currently active RNG, if that is the one currently being removed.
However enable_best_rng() might fail, if the next RNG's init routine
fails. In that case enable_best_rng() will return an error code and
the currently active RNG will remain active.
After unregistering this might lead to crashes due to use-after-free.

Fix this by dropping the currently active RNG, if enable_best_rng()
failed. This will result in no RNG to be active, if the next-best
one failed to initialize.

This problem was introduced by 142a27f

Fixes: 142a27f ("hwrng: core - Reset user selected rng by...")
Reported-by: Wirz <spam@lukas-wirz.de>
Tested-by: Wirz <spam@lukas-wirz.de>
Signed-off-by: Michael Büsch <m@bues.ch>
Cc: stable@vger.kernel.org
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

drivers/char/hw_random/core.c

@@ -516,11 +516,18 @@ EXPORT_SYMBOL_GPL(hwrng_register);
 
 void hwrng_unregister(struct hwrng *rng)
 {
+	int err;
+
 	mutex_lock(&rng_mutex);
 
 	list_del(&rng->list);
-	if (current_rng == rng)
-		enable_best_rng();
+	if (current_rng == rng) {
+		err = enable_best_rng();
+		if (err) {
+			drop_current_rng();
+			cur_rng_set_by_user = 0;
+		}
+	}
 
 	if (list_empty(&rng_list)) {
 		mutex_unlock(&rng_mutex);