[PATCH v3 02/03]: hwrng: create filler thread

From: Torsten Duwe
Date: Mon Apr 14 2014 - 12:06:07 EST



This can be viewed as the in-kernel equivalent of hwrngd;
like FUSE it is a good thing to have a mechanism in user land,
but for some reasons (simplicity, secrecy, integrity, speed)
it may be better to have it in kernel space.

This patch creates a thread once a hwrng registers, and uses
the previously established add_hwgenerator_randomness() to feed
its data to the input pool as long as needed. A derating factor
is used to bias the entropy estimation and to disable this
mechanism entirely when set to zero.

Signed-off-by: Torsten Duwe <duwe@xxxxxxx>

---
drivers/char/hw_random/core.c | 65 ++++++++++++++++++++++++++++++++++++++++--
1 file changed, 62 insertions(+), 3 deletions(-)

--- linux/drivers/char/hw_random/core.c.orig
+++ linux/drivers/char/hw_random/core.c
@@ -39,6 +39,7 @@
#include <linux/sched.h>
#include <linux/init.h>
#include <linux/miscdevice.h>
+#include <linux/kthread.h>
#include <linux/delay.h>
#include <linux/slab.h>
#include <asm/uaccess.h>
@@ -50,10 +51,18 @@


static struct hwrng *current_rng;
+static struct task_struct *hwrng_fill;
static LIST_HEAD(rng_list);
static DEFINE_MUTEX(rng_mutex);
static int data_avail;
-static u8 *rng_buffer;
+static u8 *rng_buffer, *rng_fillbuf;
+static unsigned short derating_current = 700; /* an arbitrary 70% */
+
+module_param(derating_current, ushort, 0644);
+MODULE_PARM_DESC(derating_current,
+ "current hwrng entropy estimation per mill");
+
+static void start_khwrngd(void);

static size_t rng_buffer_size(void)
{
@@ -62,9 +71,18 @@ static size_t rng_buffer_size(void)

static inline int hwrng_init(struct hwrng *rng)
{
+ int err;
+
if (!rng->init)
return 0;
- return rng->init(rng);
+ err = rng->init(rng);
+ if (err)
+ return err;
+
+ if (derating_current > 0 && !hwrng_fill)
+ start_khwrngd();
+
+ return 0;
}

static inline void hwrng_cleanup(struct hwrng *rng)
@@ -300,6 +318,36 @@ err_misc_dereg:
goto out;
}

+static int hwrng_fillfn(void *unused)
+{
+ long rc;
+
+ while (!kthread_should_stop()) {
+ if (!current_rng)
+ break;
+ rc = rng_get_data(current_rng, rng_fillbuf,
+ rng_buffer_size(), 1);
+ if (rc <= 0) {
+ pr_warn("hwrng: no data available\n");
+ msleep_interruptible(10000);
+ continue;
+ }
+ add_hwgenerator_randomness((void *)rng_fillbuf, rc,
+ (rc*derating_current)>>10);
+ }
+ hwrng_fill = 0;
+ return 0;
+}
+
+static void start_khwrngd(void)
+{
+ hwrng_fill = kthread_run(hwrng_fillfn, NULL, "hwrng");
+ if (hwrng_fill == ERR_PTR(-ENOMEM)) {
+ pr_err("hwrng_fill thread creation failed");
+ hwrng_fill = NULL;
+ }
+}
+
int hwrng_register(struct hwrng *rng)
{
int must_register_misc;
@@ -319,6 +367,13 @@ int hwrng_register(struct hwrng *rng)
if (!rng_buffer)
goto out_unlock;
}
+ if (!rng_fillbuf) {
+ rng_fillbuf = kmalloc(rng_buffer_size(), GFP_KERNEL);
+ if (!rng_fillbuf) {
+ kfree(rng_buffer);
+ goto out_unlock;
+ }
+ }

/* Must not register two RNGs with the same name. */
err = -EEXIST;
@@ -373,8 +428,11 @@ void hwrng_unregister(struct hwrng *rng)
current_rng = NULL;
}
}
- if (list_empty(&rng_list))
+ if (list_empty(&rng_list)) {
unregister_miscdev();
+ if (hwrng_fill)
+ kthread_stop(hwrng_fill);
+ }

mutex_unlock(&rng_mutex);
}
@@ -385,6 +443,7 @@ static void __exit hwrng_exit(void)
mutex_lock(&rng_mutex);
BUG_ON(current_rng);
kfree(rng_buffer);
+ kfree(rng_fillbuf);
mutex_unlock(&rng_mutex);
}

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