[PATCH v2 0/2] crypto hwrng consider quality value, remember user choice

[PATCH v2 0/2] crypto hwrng consider quality value, remember user choice

[Harald Freudenberger]

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/rng_current) the decision for the
best quality should be suppressed.

This two patches make 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.

Patch 1 introduces the sorted list of registered rngs and the
always use the best quality behavior.

Patch 2 makes hwrng remember that the user has selected an
rng via echo to /sys/class/misc/hw_random/rng_current and
adds a new sysfs attribute file 'rng_selected' to the rng core.

Harald Freudenberger (2):
  crypto: hwrng use rng source with best quality
  crypto: hwrng remember rng chosen by user

 drivers/char/hw_random/core.c | 42 ++++++++++++++++++++++++++++++++++++------
 1 file changed, 36 insertions(+), 6 deletions(-)

-- 
2.7.4

[PATCH 1/2] crypto: hwrng use rng source with best quality

[Harald Freudenberger]

This patch rewoks the hwrng to always use the
rng source with best entropy quality.

On registation and unregistration the hwrng now
tries to choose the best (= highest quality value)
rng source. The handling of the internal list
of registered rng sources is now always sorted
by quality and the top most rng chosen.

Signed-off-by: Harald Freudenberger <freude@linux.vnet.ibm.com>
Reviewed-by: PrasannaKumar Muralidharan <prasannatsmkumar@gmail.com>
---
 drivers/char/hw_random/core.c | 25 +++++++++++++++++++------
 1 file changed, 19 insertions(+), 6 deletions(-)

diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
index 503a41d..e9dda16 100644
--- a/drivers/char/hw_random/core.c
+++ b/drivers/char/hw_random/core.c
@@ -29,6 +29,7 @@
 
 static struct hwrng *current_rng;
 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);
@@ -417,6 +418,7 @@ int hwrng_register(struct hwrng *rng)
 {
 	int err = -EINVAL;
 	struct hwrng *old_rng, *tmp;
+	struct list_head *rng_list_ptr;
 
 	if (!rng->name || (!rng->data_read && !rng->read))
 		goto out;
@@ -432,14 +434,25 @@ 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(rng_list_ptr, &rng_list) {
+		tmp = list_entry(rng_list_ptr, struct hwrng, list);
+		if (tmp->quality < rng->quality)
+			break;
+	}
+	list_add_tail(&rng->list, rng_list_ptr);
+
 	old_rng = current_rng;
 	err = 0;
-	if (!old_rng) {
+	if (!old_rng || (rng->quality > old_rng->quality)) {
+		/*
+		 * Set new rng as current as the new rng source
+		 * provides better entropy 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 +479,12 @@ void hwrng_unregister(struct hwrng *rng)
 	list_del(&rng->list);
 	if (current_rng == rng) {
 		drop_current_rng();
+		/* 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

[PATCH 2/2] crypto: hwrng remember rng chosen by user

[Harald Freudenberger]

When a user chooses a rng source via sysfs attribute
this rng should be sticky, even when other sources
with better quality to register. This patch introduces
a simple way to remember the user's choice. This is
reflected by a new sysfs attribute file 'rng_selected'
which shows if the current rng has been chosen by
userspace. The new attribute file shows '1' for user
selected rng and '0' otherwise.

Signed-off-by: Harald Freudenberger <freude@linux.vnet.ibm.com>
Reviewed-by: PrasannaKumar Muralidharan <prasannatsmkumar@gmail.com>
---
 drivers/char/hw_random/core.c | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
index e9dda16..9701ac7 100644
--- a/drivers/char/hw_random/core.c
+++ b/drivers/char/hw_random/core.c
@@ -28,6 +28,8 @@
 #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);
@@ -304,6 +306,7 @@ static ssize_t hwrng_attr_current_store(struct device *dev,
 	list_for_each_entry(rng, &rng_list, list) {
 		if (sysfs_streq(rng->name, buf)) {
 			err = 0;
+			cur_rng_set_by_user = 1;
 			if (rng != current_rng)
 				err = set_current_rng(rng);
 			break;
@@ -352,16 +355,27 @@ static ssize_t hwrng_attr_available_show(struct device *dev,
 	return strlen(buf);
 }
 
+static ssize_t hwrng_attr_selected_show(struct device *dev,
+					struct device_attribute *attr,
+					char *buf)
+{
+	return snprintf(buf, PAGE_SIZE, "%d\n", cur_rng_set_by_user);
+}
+
 static DEVICE_ATTR(rng_current, S_IRUGO | S_IWUSR,
 		   hwrng_attr_current_show,
 		   hwrng_attr_current_store);
 static DEVICE_ATTR(rng_available, S_IRUGO,
 		   hwrng_attr_available_show,
 		   NULL);
+static DEVICE_ATTR(rng_selected, S_IRUGO,
+		   hwrng_attr_selected_show,
+		   NULL);
 
 static struct attribute *rng_dev_attrs[] = {
 	&dev_attr_rng_current.attr,
 	&dev_attr_rng_available.attr,
+	&dev_attr_rng_selected.attr,
 	NULL
 };
 
@@ -444,10 +458,12 @@ int hwrng_register(struct hwrng *rng)
 
 	old_rng = current_rng;
 	err = 0;
-	if (!old_rng || (rng->quality > old_rng->quality)) {
+	if (!old_rng ||
+	    (!cur_rng_set_by_user && rng->quality > old_rng->quality)) {
 		/*
 		 * Set new rng as current as the new rng source
-		 * provides better entropy quality.
+		 * provides better entropy quality and was not
+		 * chosen by userspace.
 		 */
 		err = set_current_rng(rng);
 		if (err)
@@ -479,6 +495,7 @@ 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 *new_rng;
-- 
2.7.4

Re: [PATCH v2 0/2] crypto hwrng consider quality value, remember user choice

[Herbert Xu]

On Tue, Jul 11, 2017 at 09:36:21AM +0200, Harald Freudenberger wrote:
> 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/rng_current) the decision for the
> best quality should be suppressed.
> 
> This two patches make 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.
> 
> Patch 1 introduces the sorted list of registered rngs and the
> always use the best quality behavior.
> 
> Patch 2 makes hwrng remember that the user has selected an
> rng via echo to /sys/class/misc/hw_random/rng_current and
> adds a new sysfs attribute file 'rng_selected' to the rng core.
> 
> Harald Freudenberger (2):
>   crypto: hwrng use rng source with best quality
>   crypto: hwrng remember rng chosen by user

All patches applied.  Thanks.
-- 
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