[PATCH 2/5] hw_random: use reference counts on each struct hwrng.

Rusty Russell
2014-09-18T02:52:07Z

current_rng holds one reference, and we bump it every time we want
to do a read from it.

This means we only hold the rng_mutex to grab or drop a reference,
so accessing /sys/devices/virtual/misc/hw_random/rng_current doesn't
block on read of /dev/hwrng.

Using a kref is overkill (we're always under the rng_mutex), but
a standard pattern.

This also solves the problem that the hwrng_fillfn thread was
accessing current_rng without a lock, which could change (eg. to NULL)
underneath it.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
---
drivers/char/hw_random/core.c | 135 ++++++++++++++++++++++++++++--------------
include/linux/hw_random.h | 2 +
2 files changed, 94 insertions(+), 43 deletions(-)

diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
index b1b6042ad85c..dc9092a1075d 100644
--- a/drivers/char/hw_random/core.c
+++ b/drivers/char/hw_random/core.c
@@ -42,6 +42,7 @@
#include <linux/delay.h>
#include <linux/slab.h>
#include <linux/random.h>
+#include <linux/err.h>
#include <asm/uaccess.h>

@@ -91,6 +92,59 @@ static void add_early_randomness(struct hwrng *rng)
add_device_randomness(bytes, bytes_read);
}

+static inline void cleanup_rng(struct kref *kref)
+{
+ struct hwrng *rng = container_of(kref, struct hwrng, ref);
+
+ if (rng->cleanup)
+ rng->cleanup(rng);
+}
+
+static void set_current_rng(struct hwrng *rng)
+{
+ BUG_ON(!mutex_is_locked(&rng_mutex));
+ kref_get(&rng->ref);
+ current_rng = rng;
+}
+
+static void drop_current_rng(void)
+{
+ BUG_ON(!mutex_is_locked(&rng_mutex));
+ if (!current_rng)
+ return;
+
+ kref_put(&current_rng->ref, cleanup_rng);
+ current_rng = NULL;
+}
+
+/* Returns ERR_PTR(), NULL or refcounted hwrng */
+static struct hwrng *get_current_rng(void)
+{
+ struct hwrng *rng;
+
+ if (mutex_lock_interruptible(&rng_mutex))
+ return ERR_PTR(-ERESTARTSYS);
+
+ rng = current_rng;
+ if (rng)
+ kref_get(&rng->ref);
+
+ mutex_unlock(&rng_mutex);
+ return rng;
+}
+
+static void put_rng(struct hwrng *rng)
+{
+ /*
+ * Hold rng_mutex here so we serialize in case they set_current_rng
+ * on rng again immediately.
+ */
+ mutex_lock(&rng_mutex);
+ if (rng)
+ kref_put(&rng->ref, cleanup_rng);
+ mutex_unlock(&rng_mutex);
+}
+
static inline int hwrng_init(struct hwrng *rng)
{
if (rng->init) {
@@ -113,12 +167,6 @@ static inline int hwrng_init(struct hwrng *rng)
return 0;
}

-static inline void hwrng_cleanup(struct hwrng *rng)
-{
- if (rng && rng->cleanup)
- rng->cleanup(rng);
-}
-
static int rng_dev_open(struct inode *inode, struct file *filp)
{
/* enforce read-only access to this chrdev */
@@ -154,21 +202,22 @@ static ssize_t rng_dev_read(struct file *filp, char __user *buf,
ssize_t ret = 0;
int err = 0;
int bytes_read, len;
+ struct hwrng *rng;

while (size) {
- if (mutex_lock_interruptible(&rng_mutex)) {
- err = -ERESTARTSYS;
+ rng = get_current_rng();
+ if (IS_ERR(rng)) {
+ err = PTR_ERR(rng);
goto out;
}
-
- if (!current_rng) {
+ if (!rng) {
err = -ENODEV;
- goto out_unlock;
+ goto out;
}

mutex_lock(&reading_mutex);
if (!data_avail) {
- bytes_read = rng_get_data(current_rng, rng_buffer,
+ bytes_read = rng_get_data(rng, rng_buffer,
rng_buffer_size(),
!(filp->f_flags & O_NONBLOCK));
if (bytes_read < 0) {
@@ -200,7 +249,6 @@ static ssize_t rng_dev_read(struct file *filp, char __user *buf,
ret += len;
}

- mutex_unlock(&rng_mutex);
mutex_unlock(&reading_mutex);

if (need_resched())
@@ -210,15 +258,16 @@ static ssize_t rng_dev_read(struct file *filp, char __user *buf,
err = -ERESTARTSYS;
goto out;
}
+
+ put_rng(rng);
}
out:
return ret ? : err;
-out_unlock:
- mutex_unlock(&rng_mutex);
- goto out;
+
out_unlock_reading:
mutex_unlock(&reading_mutex);
- goto out_unlock;
+ put_rng(rng);
+ goto out;
}

@@ -257,8 +306,8 @@ static ssize_t hwrng_attr_current_store(struct device *dev,
err = hwrng_init(rng);
if (err)
break;
- hwrng_cleanup(current_rng);
- current_rng = rng;
+ drop_current_rng();
+ set_current_rng(rng);
err = 0;
break;
}
@@ -272,17 +321,15 @@ static ssize_t hwrng_attr_current_show(struct device *dev,
struct device_attribute *attr,
char *buf)
{
- int err;
ssize_t ret;
- const char *name = "none";
+ struct hwrng *rng;

- err = mutex_lock_interruptible(&rng_mutex);
- if (err)
- return -ERESTARTSYS;
- if (current_rng)
- name = current_rng->name;
- ret = snprintf(buf, PAGE_SIZE, "%s\n", name);
- mutex_unlock(&rng_mutex);
+ rng = get_current_rng();
+ if (IS_ERR(rng))
+ return PTR_ERR(rng);
+
+ ret = snprintf(buf, PAGE_SIZE, "%s\n", rng ? rng->name : "none");
+ put_rng(rng);

return ret;
}
@@ -357,12 +404,16 @@ static int hwrng_fillfn(void *unused)
long rc;

while (!kthread_should_stop()) {
- if (!current_rng)
+ struct hwrng *rng;
+
+ rng = get_current_rng();
+ if (IS_ERR(rng) || !rng)
break;
mutex_lock(&reading_mutex);
- rc = rng_get_data(current_rng, rng_fillbuf,
+ rc = rng_get_data(rng, rng_fillbuf,
rng_buffer_size(), 1);
mutex_unlock(&reading_mutex);
+ put_rng(rng);
if (rc <= 0) {
pr_warn("hwrng: no data available\n");
msleep_interruptible(10000);
@@ -423,14 +474,13 @@ int hwrng_register(struct hwrng *rng)
err = hwrng_init(rng);
if (err)
goto out_unlock;
- current_rng = rng;
+ set_current_rng(rng);
}
err = 0;
if (!old_rng) {
err = register_miscdev();
if (err) {
- hwrng_cleanup(rng);
- current_rng = NULL;
+ drop_current_rng();
goto out_unlock;
}
}
@@ -457,22 +507,21 @@ EXPORT_SYMBOL_GPL(hwrng_register);

void hwrng_unregister(struct hwrng *rng)
{
- int err;
-
mutex_lock(&rng_mutex);

list_del(&rng->list);
if (current_rng == rng) {
- hwrng_cleanup(rng);
- if (list_empty(&rng_list)) {
- current_rng = NULL;
- } else {
- current_rng = list_entry(rng_list.prev, struct hwrng, list);
- err = hwrng_init(current_rng);
- if (err)
- current_rng = NULL;
+ drop_current_rng();
+ if (!list_empty(&rng_list)) {
+ struct hwrng *tail;
+
+ tail = list_entry(rng_list.prev, struct hwrng, list);
+
+ if (hwrng_init(tail) == 0)
+ set_current_rng(tail);
}
}
+
if (list_empty(&rng_list)) {
unregister_miscdev();
if (hwrng_fill)
diff --git a/include/linux/hw_random.h b/include/linux/hw_random.h
index 914bb08cd738..c212e71ea886 100644
--- a/include/linux/hw_random.h
+++ b/include/linux/hw_random.h
@@ -14,6 +14,7 @@

#include <linux/types.h>
#include <linux/list.h>
+#include <linux/kref.h>

/**
* struct hwrng - Hardware Random Number Generator driver
@@ -44,6 +45,7 @@ struct hwrng {

/* internal. */
struct list_head list;
+ struct kref ref;
};

/** Register a new Hardware Random Number Generator driver. */
--
1.9.1