[PATCH 2/5] hwrng: core - Fix current_rng init/cleanup race yet again

Herbert Xu
2014-12-23T05:41:57Z

The kref solution is still buggy because we were only focusing
on the register/unregister race. The same race affects the
setting of current_rng through sysfs.

This patch fixes it by using kref_get_unless_zero.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
---

drivers/char/hw_random/core.c | 14 +++++++++++---
1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
index 3dba2cf..42827fd 100644
--- a/drivers/char/hw_random/core.c
+++ b/drivers/char/hw_random/core.c
@@ -105,7 +105,6 @@ static inline void cleanup_rng(struct kref *kref)
static void set_current_rng(struct hwrng *rng)
{
BUG_ON(!mutex_is_locked(&rng_mutex));
- kref_get(&rng->ref);
current_rng = rng;
}

@@ -150,6 +149,9 @@ static void put_rng(struct hwrng *rng)

static inline int hwrng_init(struct hwrng *rng)
{
+ if (kref_get_unless_zero(&rng->ref))
+ goto skip_init;
+
if (rng->init) {
int ret;

@@ -157,6 +159,11 @@ static inline int hwrng_init(struct hwrng *rng)
if (ret)
return ret;
}
+
+ kref_init(&rng->ref);
+ reinit_completion(&rng->cleanup_done);
+
+skip_init:
add_early_randomness(rng);

current_quality = rng->quality ? : default_quality;
@@ -467,6 +474,9 @@ int hwrng_register(struct hwrng *rng)
goto out_unlock;
}

+ init_completion(&rng->cleanup_done);
+ complete(&rng->cleanup_done);
+
old_rng = current_rng;
err = 0;
if (!old_rng) {
@@ -494,8 +504,6 @@ int hwrng_register(struct hwrng *rng)
add_early_randomness(rng);
}

- init_completion(&rng->cleanup_done);
-
out_unlock:
mutex_unlock(&rng_mutex);