Re: [PATCH 3/5] hw_random: fix unregister race.

Amos Kong
2014-11-03T15:25:30Z

On Tue, Oct 21, 2014 at 10:15:23PM +0800, Herbert Xu wrote:
> On Thu, Sep 18, 2014 at 12:18:24PM +0930, Rusty Russell wrote:
> > The previous patch added one potential problem: we can still be
> > reading from a hwrng when it's unregistered. Add a wait for zero
> > in the hwrng_unregister path.
> >
> > Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
> > ---
> > drivers/char/hw_random/core.c | 5 +++++
> > 1 file changed, 5 insertions(+)
> >
> > diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
> > index dc9092a1075d..b4a21e9521cf 100644
> > --- a/drivers/char/hw_random/core.c
> > +++ b/drivers/char/hw_random/core.c
> > @@ -60,6 +60,7 @@ static DEFINE_MUTEX(rng_mutex);
> > static DEFINE_MUTEX(reading_mutex);
> > static int data_avail;
> > static u8 *rng_buffer, *rng_fillbuf;
> > +static DECLARE_WAIT_QUEUE_HEAD(rng_done);
> > static unsigned short current_quality;
> > static unsigned short default_quality; /* = 0; default to "off" */
> >
> > @@ -98,6 +99,7 @@ static inline void cleanup_rng(struct kref *kref)
> >
> > if (rng->cleanup)
> > rng->cleanup(rng);

rng->cleanup_done = true;

> > + wake_up_all(&rng_done);
> > }
> >
> > static void set_current_rng(struct hwrng *rng)
> > @@ -529,6 +531,9 @@ void hwrng_unregister(struct hwrng *rng)
> > }
> >
> > mutex_unlock(&rng_mutex);
> > +
> > + /* Just in case rng is reading right now, wait. */
> > + wait_event(rng_done, atomic_read(&rng->ref.refcount) == 0);

Hi Rusty,

After initializing (kref_init()), the refcount is 1, so we need one
more kref_put() after each drop_current_rng() to release last
reference count, then cleanup function will be called.

> While it's obviously better than what we have now, I don't believe
> this is 100% safe as the cleanup function might still be running
> even after the ref count hits zero. Once we return from this function
> the module may be unloaded so we need to ensure that nothing is
> running at this point.

I found wait_event() can still pass and finish unregister even cleanup
function isn't called (wake_up_all() isn't called). So I added a flag
cleanup_done to indicate that the rng device is cleaned up.

+ /* Just in case rng is reading right now, wait. */
+ wait_event(rng_done, rng->cleanup_done &&
+ atomic_read(&rng->ref.refcount) == 0);

I will post the new v4 later.

> Cheers,
> --
> Email: Herbert Xu <herbert@gondor.apana.org.au>
> Home Page: http://gondor.apana.org.au/~herbert/
> PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

--
Amos.