From: Harald Freudenberger <freude@linux.vnet.ibm.com>
Subject: [PATCH] hwrng: Make hwrng choose rng source by quality.
Date: Tue, 27 Jun 2017 08:01:30 +0200
Message-ID: <1498543290-17824-1-git-send-email-freude@linux.vnet.ibm.com>
Cc: schwidefsky@de.ibm.com,
        Harald Freudenberger <freude@linux.vnet.ibm.com>
To: linux-crypto@vger.kernel.org
Sender: linux-crypto-owner@vger.kernel.org

The hwrng core implementation currently doesn't consider the
quality field of the struct hwrng. So the first registered rng
is the winner and further rng sources even with much better
quality are ignored.

The behavior should be that always the best rng with the highest
quality rate should be used as current rng source. Only if the
user explicitly chooses a rng source (via writing a rng name
to /sys/class/misc/hw_random) the decision for the best quality
should be suppressed.

This patch makes hwrng always hold a list of registered rng
sources sorted decreasing by quality. On registration of a new
hwrng source the list is updated and if the current rng source
was not chosen by user and the new rng provides better quality
set as new current rng source. Similar on unregistration of an
rng, if it was the current used rng source the one with the
next highest quality is used. If a rng source has been set via
sysfs from userland as long as this one doesn't unregister
it is kept as current rng regardless of registration of 'better'
rng sources.

Signed-off-by: Harald Freudenberger <freude@linux.vnet.ibm.com>
---
 drivers/char/hw_random/core.c | 31 +++++++++++++++++++++++++------
 1 file changed, 25 insertions(+), 6 deletions(-)

diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
index 503a41d..7fe47f8 100644
--- a/drivers/char/hw_random/core.c
+++ b/drivers/char/hw_random/core.c
@@ -28,7 +28,10 @@
 #define RNG_MODULE_NAME		"hw_random"
 
 static struct hwrng *current_rng;
+/* the current rng has been explicitly chosen by user via sysfs */
+static int cur_rng_set_by_user;
 static struct task_struct *hwrng_fill;
+/* list of registered rngs, sorted decending by quality */
 static LIST_HEAD(rng_list);
 /* Protects rng_list and current_rng */
 static DEFINE_MUTEX(rng_mutex);
@@ -308,6 +311,8 @@ static ssize_t hwrng_attr_current_store(struct device *dev,
 			break;
 		}
 	}
+	if (!err)
+		cur_rng_set_by_user = 1;
 	mutex_unlock(&rng_mutex);
 
 	return err ? : len;
@@ -417,6 +422,7 @@ int hwrng_register(struct hwrng *rng)
 {
 	int err = -EINVAL;
 	struct hwrng *old_rng, *tmp;
+	struct list_head *ptr;
 
 	if (!rng->name || (!rng->data_read && !rng->read))
 		goto out;
@@ -432,14 +438,26 @@ int hwrng_register(struct hwrng *rng)
 	init_completion(&rng->cleanup_done);
 	complete(&rng->cleanup_done);
 
+	/* rng_list is sorted by decreasing quality */
+	list_for_each(ptr, &rng_list) {
+		tmp = list_entry(ptr, struct hwrng, list);
+		if (tmp->quality < rng->quality)
+			break;
+	}
+	list_add_tail(&rng->list, ptr);
+
 	old_rng = current_rng;
 	err = 0;
-	if (!old_rng) {
+	if (!old_rng ||
+	    (!cur_rng_set_by_user && rng->quality > old_rng->quality)) {
+		/*
+		 * Set new rng as current if no current rng or rng was
+		 * not chosen by user and the new one has better quality.
+		 */
 		err = set_current_rng(rng);
 		if (err)
 			goto out_unlock;
 	}
-	list_add_tail(&rng->list, &rng_list);
 
 	if (old_rng && !rng->init) {
 		/*
@@ -466,12 +484,13 @@ void hwrng_unregister(struct hwrng *rng)
 	list_del(&rng->list);
 	if (current_rng == rng) {
 		drop_current_rng();
+		cur_rng_set_by_user = 0;
+		/* rng_list is sorted by quality, use the best (=first) one */
 		if (!list_empty(&rng_list)) {
-			struct hwrng *tail;
-
-			tail = list_entry(rng_list.prev, struct hwrng, list);
+			struct hwrng *new_rng;
 
-			set_current_rng(tail);
+			new_rng = list_entry(rng_list.next, struct hwrng, list);
+			set_current_rng(new_rng);
 		}
 	}
 
-- 
2.7.4