[PATCH 6/6] hw_random: credit entropy for low quality sources of randomness

[Dominik Brodowski <linux@dominikbrodowski.net>]

In case the entropy quality is low, there may be less than one bit to
credit in the call to add_hwgenerator_randomness(): The number of bytes
returned by rng_get_data() multiplied by the current quality (in entropy
bits per 1024 bits of input) must be larger than 128 to credit at least
one bit. However, imx-rngc.c sets the quality to 19, but may return less
than 32 bytes; hid_u2fzero.c sets the quality to 1; and users may override
the quality setting manually.

In case there is less than one bit to credit, keep track of it and add
that credit to the next iteration.

Cc: Matt Mackall <mpm@selenic.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>
---
 drivers/char/hw_random/core.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
index bc9f95cbac92..6d7f05641c7c 100644
--- a/drivers/char/hw_random/core.c
+++ b/drivers/char/hw_random/core.c
@@ -427,6 +427,7 @@ static int hwrng_fillfn(void *unused)
 	long rc;
 
 	while (!kthread_should_stop()) {
+		size_t entropy, entropy_credit = 0; /* in 1/1024 of a bit */
 		struct hwrng *rng;
 
 		if (!current_quality)
@@ -445,9 +446,17 @@ static int hwrng_fillfn(void *unused)
 			msleep_interruptible(10000);
 			continue;
 		}
+
+		/* If we cannot credit at least one bit of entropy,
+		 * keep track of the remainder for the next iteration
+		 */
+		entropy = rc * current_quality * 8 + entropy_credit;
+		if ((entropy >> 10) == 0)
+			entropy_credit = entropy;
+
 		/* Outside lock, sure, but y'know: randomness. */
 		add_hwgenerator_randomness((void *)rng_fillbuf, rc,
-					   rc * current_quality * 8 >> 10);
+					   entropy >> 10);
 	}
 	hwrng_fill = NULL;
 	return 0;
-- 
2.34.1