[PATCH] hwrng: core - Fix wrong quality calculation at hw rng registration

[Harald Freudenberger <freude@linux.ibm.com>]

When there are rng sources registering at the hwrng core via
hwrng_register() a struct hwrng is delivered. There is a quality
field in there which is used to decide which of the registered
hw rng sources will be used by the hwrng core.

With commit 16bdbae39428 ("hwrng: core - treat default_quality as
a maximum and default to 1024") there came in a new default of
1024 in case this field is empty and all the known hw rng sources
at that time had been reworked to not fill this field and thus
use the default of 1024.

The code choosing the 'better' hw rng source during registration
of a new hw rng source has never been adapted to this and thus
used 0 if the hw rng implementation does not fill the quality field.
So when two rng sources register, one with 0 (meaning 1024) and
the other one with 999, the 999 hw rng will be chosen.

This patch simple takes into account that a quality field value
of 0 is to be treated as 1024 and then the decision about which
hw rng to use works as expected.

Tested on s390 with two hardware rng sources: crypto cards and
trng true random generator device driver.

Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>

Reported-by: Christian Rund <Christian.Rund@de.ibm.com>
Fixes: 16bdbae39428 ("hwrng: core - treat default_quality as a maximum and default to 1024")
---
 drivers/char/hw_random/core.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
index 4084df65c9fa..993b8a1f1d19 100644
--- a/drivers/char/hw_random/core.c
+++ b/drivers/char/hw_random/core.c
@@ -525,6 +525,7 @@ static int hwrng_fillfn(void *unused)
 
 int hwrng_register(struct hwrng *rng)
 {
+	unsigned short rng_quality, cur_quality;
 	int err = -EINVAL;
 	struct hwrng *tmp;
 
@@ -545,8 +546,14 @@ int hwrng_register(struct hwrng *rng)
 	complete(&rng->cleanup_done);
 	init_completion(&rng->dying);
 
+	/* Quality field not set in struct hwrng means 1024 */
+	rng_quality = rng->quality ? rng->quality : 1024;
+	cur_quality = current_rng ?
+		(current_rng->quality ? current_rng->quality : 1024) :
+		0;
+
 	if (!current_rng ||
-	    (!cur_rng_set_by_user && rng->quality > current_rng->quality)) {
+	    (!cur_rng_set_by_user && rng_quality > cur_quality)) {
 		/*
 		 * Set new rng as current as the new rng source
 		 * provides better entropy quality and was not
-- 
2.34.1