[PATCH 4.9 151/264] hwrng: remember rng chosen by user

Greg Kroah-Hartman
2022-06-23T16:59:39Z

From: Harald Freudenberger <freude@linux.vnet.ibm.com>

commit 10a515ddb5f19a1ff0b9882c430b4427843169f3 upstream.

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>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/char/hw_random/core.c | 21 +++++++++++++++++++--
1 file changed, 19 insertions(+), 2 deletions(-)

--- a/drivers/char/hw_random/core.c
+++ b/drivers/char/hw_random/core.c
@@ -27,6 +27,8 @@
#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);
@@ -303,6 +305,7 @@ static ssize_t hwrng_attr_current_store(
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;
@@ -351,16 +354,27 @@ static ssize_t hwrng_attr_available_show
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
};

@@ -443,10 +457,12 @@ 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)
@@ -478,6 +494,7 @@ 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;