random: throttle hwrng writes if no entropy is credited

If a hwrng source does not provide an entropy estimate, it currently
does not contribute at all to the CRNG. In order to help fix this, in
case add_hwgenerator_randomness() is called with the entropy parameter
set to zero, go to sleep until one reseed interval has passed.

While the hwrng thread currently only runs under conditions where this
is non-zero, this change is not harmful and prepares for future updates
to the hwrng core.

Cc: Herbert Xu <herbert@gondor.apana.org.au>
Reviewed-by: Dominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 16e0c5f6cf2fa4fa2c798fc7268b43ce2d268cb6..520a385c7dab8df9c6a04dc693d995b26c0bad42 100644 (file)
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -865,9 +865,9 @@ void add_hwgenerator_randomness(const void *buf, size_t len, size_t entropy)
 
        /*
         * Throttle writing to once every reseed interval, unless we're not yet
-        * initialized.
+        * initialized or no entropy is credited.
         */
-       if (!kthread_should_stop() && crng_ready())
+       if (!kthread_should_stop() && (crng_ready() || !entropy))
                schedule_timeout_interruptible(crng_reseed_interval());
 }
 EXPORT_SYMBOL_GPL(add_hwgenerator_randomness);