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

[Torsten Duwe]

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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