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

Rusty Russell
2014-12-26T00:31:32Z

Herbert Xu <herbert@gondor.apana.org.au> writes:
> 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>

This patch scares me a little!

I'll have to pull the tree to review it properly, but the theory was
that the reference count was counting users of the rng. Now I don't
know what it's counting:

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

OK, so this skip_init branch is triggered when the rng is being
shut down as it's no longer current_rng?

> +
> + kref_init(&rng->ref);
> + reinit_completion(&rng->cleanup_done);
> +
> +skip_init:
> add_early_randomness(rng);

Then we use it to add randomness?

>
> 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);
> +

This code smells very bad.

Cheers,
Rusty.