[PATCH] hw_random: Always drop the RNG in hwrng_unregister()

["Michael Büsch" <m@bues.ch>]

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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 142a27f0a731ddcf467546960a5585970ca98e21


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

---

See this discussion for a crash in b43's hwrng caused by this problem:
https://www.spinics.net/lists/linux-wireless/msg173089.html



diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
index 91bb98c42a1c..aaf9e5afaad4 100644
--- a/drivers/char/hw_random/core.c
+++ b/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);



-- 
Michael

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