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

commit 837bf7cc3b7504385ae0e829c72e470dfc27cf6c upstream.

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

Fixes: 142a27f0a731 ("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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
index 91bb98c42a1ca76376ae0db4b43fd7a89fca27b0..aaf9e5afaad435e2342a15fc963aa91367079957 100644 (file)
--- 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);